Horseshoe Bay Waterfront Properties

Horseshoe Bay Waterfront Properties

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HSB IS A GEOLOGICAL PARADISE! 
Published chapter-by-chapter in the HSB BEACON newspaper, beginning July 2006.
The series attempts to explain why much of the attraction and appeal
people feel for the HSB portion of the Texas Hill Country
is directly attributable to the fact that
HSB is located in and the center of a Geological Paradise.
By Ken G. Martin, M.S. Degree in Geology, University of Texas, 1961.


1) Introduction.

2) Basics.  

3) Still Peering Through HSB’s Geologic Window.  

4) The "Himalayas” of Texas.  

5) The Llano Uplift.  

6) The Rise of the Llano Uplift.  

7) The Central Mineral Region.  

8) Austin's Vanishing Mountain Range.  

9) Why There’s a Marble Falls.  

10) The Canyon of the Colorado, part 1.

11) The Canyon of the Colorado, part 2.  

12) Inundating the Llano Uplift.  

13) Regional Physiographic Map.  

14)
Forming the Texas Hill Country.  

15) Exhuming the Canyon of the Colorado.  

16) Why the Canyon of the Colorado Is So Special.  

17) Granite, Granite Gravel & Granite Boulders.

18) Geologic Map of HSB.
 

19) The Geology of HSB and Its Five Golf Courses.  

20) Summary:  What our Area’s Geological History Tells Us.


1) Introduction:

The Horseshoe Bay (”HSB”)-Marble Falls area is noted for exhibiting more geologic history than practically anywhere else in America.  For reasons such as this, western Burnet and eastern Llano counties - deep in the heart of Texas - are among America’s most exceptionally appealing areas.  Young and old, male and female, visitor and resident find our laid-back and friendly area especially attractive. 

Most don’t dwell on the whys.  They know a gift from God when they see one, and let it go at that – satisfied just having the privilege.  They come as often as they can, pleased to simply partake of the offering.

Many find that as their technical understanding improves, so does their appreciation.  Since most of the important technicals are geologically related, the Beacon arranged to have this series published – hoping to enhance appreciation and in-site by explaining the area’s geologic history and principal geological features. 

To begin with, the closely-tied, next-door communities of HSB and Marble Falls, Texas, are located in a Geological Paradise of the highest order.  However, HSB has a substantial geological advantage over the larger Marble Falls community in that it is Texas’ only significant city that is located in all four of central Texas’ favored provinces:  the Texas Hill Country, the Llano Uplift, the Central Mineral Region and the Highland Lakes.   Marble Falls lies just outside the Central Mineral Region.      


Factors such as collisions between the North and South American continents, world-class folding, faulting and mountain building, changing world-wide temperatures and sea levels, and unusually favorable local erosion came together here to produce an extraordinary look into the Earth's past that is unsurpassed in North America.  

This “window” is centered on the northeastern edge of HSB.  It sheds exceptional light on why our area has such wonderfully broad appeal – aesthetically, physically, environmentally, and spiritually.

The Earth’s oldest rocks are about 4 billion years old.  The oldest rocks in Texas, which are only 1/3rd as old, are present here.   

Within a mere 10-mile radius of the window’s center, one can look backward 1.36 billion years.  Considering that the area’s youngest rocks are 100 million years old, it follows that one can examine about 1.26 billion years of geologic history.

The window is among the world’s best known and most heavily studied.  During the past 80 or so years, practically every geology student within a two-day’s drive of our area has come here to study roadside outcrops on a geology field trip.
 
The Booklet covers all of the subjects listed below, as well as several others, though not necessarily in the order shown:

A)  Our Area’s Geologic History, among the country’s most diverse.
C)  Our Geological Window - one of the Earth’s best and most revealing.
D)  Exhibiting 1.26 billion years of Earth history.
E)  The Llano Uplift - home to the oldest rocks in Texas.
F)  The Central Mineral Region – an unfulfilled promise.
G)  The Edwards Plateau – Texas’ dominant geologic feature.
H)  Continental Drift - its impact and effect on central Texas.
I)  Compression vs Tension;  Folds and Thrust Faults vs Normal Faults and Rifts.
J)  The Ouachita Mountains - once Six Miles High at Austin.
K)  The Balcones Escarpment – a super-regional fault system.
L)  The Texas Hill Country – can you say headward erosion.
M)  The Highland Lakes: controlled by LCRA; not the Corps of Engineers, as typical.
N)   HSB occupies Texas' "Sweet Spot" -  solely and uniquely.
O)  Lake LBJ:  America's lake of choice, its largest constant-level lake.
P)  We've got every rock type - Sedimentary, Igneous and Metamorphic.
Q)  We've got all the structures - Faults & Folds; Hogbacks & Cuestas.
R)  “Marble” Falls ain't Marble!
S)  The Marble Falls Limestone - toughest rock in Texas.
T)  The "Marble" and Pedernales Falls – same cause; same effect.
U)  Marble Falls First Waterfall – far higher and far older than today’s.
V)  The HSB-Marble Falls Fault Block.
W) The HSB-Marble Falls Fault - up to one mile of vertical movement.
X)  Our Colorado River Canyon - 50 times older than the Grand Canyon.
Y)  Funneling Billions of Tons of Detritus from the Llano Uplift to the Gulf.
Z)  The Canyon was buried a half-mile deep for 100 million years.
AA) Limestone stands higher than Granite in our area.
BB)  Enchanted Rock - a great example of a monadnock and of exfoliation.
CC) Granite Mountain - a smaller example of both.
DD)  Lago Escondido’s Foundation - twice as old as the oldest fossilized life.
EE)  Dikes and the Like.
FF)  Granite, Granite Gravel and Granite Boulders.
GG)  Flash Floods, Aquifers, Clean Water, Et Cetera.
HH)  Why the area surrounding Lake LBJ is practically Mosquito-Proof.
II)  Limestone stands higher than Granite in our area.
JJ)  Relating local topography to climate and geology.
KK)  Certain sectors of our area look like they did 550 million years ago.
LL)   Certain other sectors look like they did 200 million years ago.
MM)  Our area hasn't has an earthquake in 200 million years.
NN)  Oceans have covered our area for 235 of the past 550 million years.
OO)  Fredericksburg’s Peaches and Wildseed Farm thrive in a 120-million-year-old Sea Bed.
PP)  Don’t let anyone sell you oil and natural gas rights beneath Lake LBJ.
  
Hope you enjoy!

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2) The Basics.
 
Forming the North American Continent:  HSB’s unique “window” into the Earth’s geologic past has been helpful in unraveling such mysteries as the formation of our Universe.  From what we know today, the Universe formed 13,700 million years ago.  Our Solar System developed from it 8,700 million years later, separating out as a super-hot, gaseous cloud some 5,000 million years ago.  

Over the following 500 million years, the Sun and its nine Planets took shape as the cloud condensed into molten masses.  By 4,000 million years ago, the Earth’s outer surface had crusted over through continued cooling, producing its first rocks, which floated on the Earth’s viscous interior.  
 
By 3,500 million years ago, these first rocks floated together, grouping to become the Earth’s first giant landmasses, called proto-continents.  Powerful convection currents circulating within the Earth’s interior drove these proto-continents slowly together 1,300 million years ago into a single super-continent called Rodinia.  The proto-continents stayed glued together as Rodinia for more than 300 million years.   
 
By the advent of the Paleozoic Era, some 600 million years ago, countering forces had long-ago pulled Rodinia apart, splitting it into several continental-sized landmasses.  One of these large landmasses is known today as the North American Continent.  Significant evidence of all this is present locally.

HSB’s 1.3 Billion Years of Geologic History:
 Geologic formations dating from North America’s Pre-Cambrian continental core, the Paleozoic Era and the Cretaceous Period outcrop within 10 miles of Horseshoe Bay.  These rocks range in age from 100 million to 1,360 million years old.  They are also among the country’s most diverse, including sand and sandstones; gneisses; clay, silt and shales; schists; limestones, dolomites and caliches; marbles; glauconites; cherts (flint); granites; basalts; and serpentines.

These outcroppings afford an exceptional look back into the Earth’s formative year and cover a period of time amounting to 1.7 billion years.  The window provides one of the world’s most complete and penetrating views of this time period.  Here are some important observation points:

Our Continental Core:  Beginning at Wirtz Dam road in Cottonwood Shores and driving west on RR 2147 through HSB to Highway 71, and continuing to Llano and beyond on 71 to Valley Spring, one drives steadily over igneous and metamorphic rocks.  These rocks make up part of the outer portion of the North American continent’s original core and range in age from 1,100 million to 1,360 million years, with the oldest rocks best seen in the Valley Spring.  They are the oldest rocks that outcrop anywhere in the southern half of the country.

The World’s First Fossils:  The initial seas of the Paleozoic Era, which covered the continent some 600 million years ago, were the first to contain life sufficiently complex to leave fossils.  These first fossils are present in the Cambrian sandstone that forms the uppermost rim - the Caprock - on HSB’s Thanksgiving Mountain.  The sandstone serves as the HSB Chapel’s foundation.

Except for this basal sandstone, which blanketed the North American continent, North America’s later Paleozoic seas, lasting from 550 to 280 million years ago, were generally calcium-rich.  These seas, also of relatively shallow depths, deposited thick sections of limestone.  As these seas moved episodically, in and out over the continent, they left behind a sequence of fossils that provides the earliest evidence of evolution.  

These whitish, Paleozoic limestone formations, which are mostly Ordivician Ellenberger in age, are about 4,000 feet thick in the HSB area.  In the well-known and highly productive Permian Basin of west Texas, the Ellenberger is home to prolific oil and natural gas reservoirs from depths as great as 30,000 feet.  In the local area, the limestone sequence can be seen most conveniently just upstream from the Slickrock low-water crossing located immediately above Norman’s well-known, giant waterfall.  

HSB’s Major Fault Zone:  On the south side of this same Slickrock crossing, one can actual see (touch) a rare example of a major fault zone.  Along this plane of weakness, about one-half mile of vertical movement (throw)occurred here some 225 million years ago, +/- 25 million.  The whitish Ellenberger dropped downward relative to the much older, reddish Pre-Cambrian granite.  

The fault plane separates 475 million year old limestone from 1,100 million year old granite.  This fault is truly major.  Where seen on the western edge of Marble Falls, its throw has grown to almost 5,000 feet.  That is the largest seen in the Llano Uplift and most likely the largest in the central U.S. – not only in the amount of its vertical movement but also by representing from 625 to 825 million years of missing geologic time!  
 
Relax – the fault has been inactive for 200+ million years.  See the next section for more on what our geological window offers.

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3) Still Peering Through HSB’s Geologic Window.

We initiated our study of the local geology by beginning with the basics.  We made the following technical observations, working up the geologic scale to ever younger rocks:
1)  About 1.26 billion years of geologic history is represented by the rocks that outcrop within 10 miles of northeastern most HSB.

2)  Diverse sedimentary, igneous and metamorphic rocks ranging in age from 100 million to 1,360 million years old outcrop in the area.

3)  The area’s igneous rocks are typically1.09 billion years old.  They intruded from molten depths into the older metamorphosed sedimentary rocks, which are up to 1.36 billion years old.  These Pre-Cambrian rocks form the south-eastern margin of North America’s continental core, and most had fully accreted to the core by a billion years ago.

These are among the oldest rocks that outcrop in Texas.  The heart of the continent’s original core is in south-central Canada, where the Pre-Cambrian rocks are up to 3.0 billion years old.

4)  The Pre-Cambrian rocks in HSB Proper are all intrusive reddish granites that are around 1,090 million years old.  The exposed top of the granite formation is shaped into gently rolling topography containing buried hills (monadnocks) the likes of Granite Mountain.  Some of these project hundreds of feet above the granite's irregularly eroded surface.  The granite surface was covered beginning 550 million years ago by a transgressing shallow Cambrian sea that deposited a marine sandstone and limestone sequence up to 1,540’ thick in the HSB-Marble Falls area.

5)  Fossilized remains of the world’s first fossils, the first hard-shelled organisms, are present in the immediate HSB area.  These organisms - Trilobites - first developed 550 million years ago, during the Cambrian Period.

6)  The conformably overlying Ellenberger limestone was deposited over a seventy million year period ending 439 million years ago.  It is 1,600’ thick in the immediate HSB area and is the same formation that contains some of West Texas’ most prolific oil and gas reservoirs – at depths as great as 30,000’.

In addition, Huber is currently mining the formation for its high calcium carbonate content at its quarry located at US 281 and Starke Dam Road. Fossilized remains of the earliest fishes are present in the Ellenberger.

Moving on, as we peer further, still looking at the basics, we find that:

A)  Overlying the Ellenberger is the Barnett Shale, of late Mississippian age.  It is another important source of Texas’ oil and natural gas.  The two formations are separated by an unconformity that represents approximately 120 million years of missing geologic time.

B)  The Barnett Shale is the Fort Worth Basin’s most prolific producing formation and where many of today’s Texas oilmen are making huge fortunes.  It outcrops in the immediate HSB area and can be seen at several places on the escarpment that parallels the south side of RR 2147.  It is a dark gray, very petroliferous formation, only 15 feet thick.  More hydrocarbons are trapped in this shale per cubic foot than in any other formation in Texas.

C)  The Barnett is conformably overlain by the blue-gray, 385’ thick Marble Falls limestone.  This exceptionally dense limestone underlies most of HSB South, making delivery of water and sewer services difficult.  It is well exposed along Mountain Dew as the street traverses the northern part of that subdivision.   However, it is best seen in Marble Falls, where the River City Grill and Chili’s rest on the formation’s upper surface.

D)  The Marble Falls limestone is the hardest and most resistant rock in Texas.  It‘s the culprit that forms the Colorado River’s “Marble” Falls and the river’s canyon walls, as seen downstream (east) of the US 281 bridge..

E)  The Marble Falls limestone precipitated from a shallow, early Pennsylvanian sea between 305 and 308 million years ago.  It was deposited at the rate of a mere one inch per 650 years over a three million year period.

F)  The 400’ thick Smithwick Shale is an interbedded shale and sandstone sequence that overlies the Marble Falls formation in conformable fashion.  It was deposited over a 15 million year period of late Pennsylvanian time that ended about 290 million years ago.  It outcrops in the immediate HSB area, but is best seen in road cuts going east along RR 1431 between Marble Falls and Smithwick.

It is the oldest Paleozoic formation that outcrops in the area.  It is reflective of the early stages of a major geologic event which we’ll get into next week.

In summary, the sedimentary formations we’ve just discussed were deposited beginning 570 million years ago with the first marine transgression of the continent and ending 280 million years later.  They all originated in relatively shallow seas that covered a slowly sinking North American continent.

They were stacked on the continent in blanket-like fashion, one formation on top of the other.  The result was that some 4,000’ of Paleozoic marine sediment had been buried beneath the ocean floor by the end of the Pennsylvanian Period.

O.K., you’ve just viewed 1.07 billion years of geologic time – dating back from 290 million years ago to 1,360 million years ago.  We stopped just as all hell was about to break loose!

We are near the end of the Pennsylvanian Period.  The continents have begun to drift again - toward each other this time.  The African and South American continents are in the process of colliding with the North American continent.  The coming crash of these three continents during the late Pennsylvanian and the following Permian Periods will result in a collision of huge proportions.

In our next chapter, we discuss this mountain-building collision and its effect on our area, while also introducing you to the “Llano Uplift” - central Texas’ dominate geological feature.

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4) The "Himalayas” of Texas.
In our prior section, we continued examining the rocks viewable through HSB’s unique geologic window, which has long attracted world-wide interest.  We examined rocks ranging in age from 1,360 million years ago - the age of area’s most ancient rocks - forward to a point 290 million years ago.  That amounted to working through a period of time covering 1.07 billion years.  We gained a lot of insight into what had happened in the HSB area during that time.  

We learned that HSB’s oldest sedimentary rocks were deposited about 550 million years ago.  Continuing thereafter for a 260 million year period ending 290 million years ago, more or less continuous shallow marine deposition occurred on top of the slowly sinking surface of the North American continent’s ancestral igneous and metamorphic core.  During that time, some 4,000’ of sediment was stacked on the somewhat irregular surface in a blanketing fashion.

However, we noticed as we finished looking at the last geologic formation we studied – the Smithwick sand/shale sequence, which was deposited between 290 and 305 million years ago during the late Pennsylvanian Period – showed evidence that mountain building was beginning to develop nearby.  This detrital, sand-shale sequence was eroded off an incipient broad mountain range that was just beginning to emerge along the continent’s outer margin, which was in the Austin area at this time.  

The continents had begun to drift again - toward each other this time.  The South American continent was in the process of colliding with the North American continent.  The developing crash of the two continents would ultimately result in a collision of major mountain-building proportions.

A mountain range was emerging in reaction to compression forces being applied by the South American continent, which had drifted north and was colliding with the North American continent.  The South American continent would continue applying this compression force throughout the Permian Period.  The compressive action would ultimately fold the rocks positioned along the continental margin in accordion fashion into a humongous range which the United States Geological Survey estimates was of Himalayan heights and proportions.

This mountain range, which would eventually grow to heights of up to 30,000’, was centered on what is today Austin – riding on an east dipping thrust plane.  From there, the crest trended northeastward into southeast Oklahoma, where it is preserved today as the Ouachita Mountains.  

Going southwest from Austin, the crest wrapped itself around the exceptionally rigid south flank of what is known today as The Llano Uplift, who’s Pre-Cambrian rocks were buried by the 4,000’ of Paleozoic sediment, as we learned earlier in looking through HSB’s geologic window.  

From Austin, the crest trended through Boerne and Uvalde to just north of Del Rio, from where it turned northwestward to connect with what is exposed today as the Glass Mountains.  It then turned south into Mexico.  Along its entire path, the crest of the range was following the North American continent’s outer, southerly margin.

As South America pushed northward into the North American continent, the compressive stress not only folded the continent’s perimeter rocks into the mountain range, it also fractured the rocks into NE-SW trending fault trends.  These faults often exhibit some lateral, strike-slip movement – like that of California’s well known San Andres fault.

By the Paleozoic Era’s close at the end of the Permian Period, South America’s compressive pressure had completely slacked off, and mountain-building had ceased.  The rate of uplift had greatly exceeded the rate of erosion during the late Pennsylvanian and first half of the Permian.  Once the rate of uplift peaked and began to decline, erosion began to take charge.  Accordingly, the mountain range’s great height began to decline relatively rapidly during the last half of the Permian Period.  The range's decline was probably augment by rift forming that occured during the Triassic and is discussed later.

Evidence of erosional detritus being transported down from the mountain range by rapidly moving, westward-flowing streams.  They carried the fresh sediment westward across what were then relatively flat-lying plains of central Texas - before the rise of The Llano Uplift - to a home in deep sedimentary basins that had formed in west Texas.  These basins would become prolific oil and natural gas producers in the 20th and 21st centuries, 245 to 250 million years later.

Beginning with the Triassic Period, which followed the Permian, the continents began to drift again, pulling apart this time, introducing tensional force – the opposite of mountain-building, compression force.  As tension took over the central Texas domain, releasing pressure, movement along the fault zones switched from largely lateral movement typical of the Permian Period to the more normal, vertical direction – upward on one side, downward on the other.  

This scenario opened up the possibility of block faulting.  Block faulting is one of the principal controlling factors among several that influence the topography of our area.  That subject will be dealt with in detail in a future article.

The introduction of tensional forces into our area also bears on The Llano Uplift’s development.  In our next chapter, we get into what The Llano Uplift - a feature we hear about practically the day we arrive in HSB - is all about.

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5) The Llano Uplift.

In our last discussion, we reviewed how an intensely folded mountain began rising in the Austin area 290 million years ago – around the beginning of the Permian Period.  It was connected to a mountain range of Himalayan proportions, with heights up to 30,000 feet.  Components of this range, which collapsed into a broad and deep rift when North and South America drifted apart some 250 million years ago, are hidden today, buried under younger sediments.  
We know from geophysical data and through core samples from deep oil and gas wells that Austin’s ancestral mountain range is a buried extension of the Ouachita Mountain Range of southeast Oklahoma and that it connects regionally with the Glass Mountains of West Texas.  

Going southwest from Austin, the range wrapped around the exceptionally rigid south flank of what is known today as The Llano Uplift.  The roots of this range are buried in the Austin area by several thousand feet of post-Paleozoic sediment.  The Ouachita Trend developed along what was then the North American continent’s outer, southern margin.

The broad, dome-shaped Llano Uplift did not begin to rise relative to the Ouachita until after the mountain-building ended at the close of the Permian, which also happened to be the end of the Paleozoic Era – 250 million years ago. Pre-Cambrian rocks comprise the heart of The Llano Uplift.  They outcrop across a 65 by 35 mile oval-shaped area whose long axis trends NW-SE.  The Uplift‘s center is located about eight miles southwest of Llano.  

Whereas The Llano Uplift exposed Precambrian rocks of the North American continent’s ancestral core, no similarly ancient remnants of the Ouachita Mountain Range can be seen in central and north Texas.  In fact, if it were not for the core samples from the deep wells immediately east of Austin, and the geophysical data, one would have little reason to believe that such a huge mountain system ever existed in the Austin area.  

The Uplift’s oldest Pre-Cambrian rocks were all metamorphosed under intense temperature and pressure 1.2 to 1.36 billion years ago.  About 100 million years after the metamorphism, a dozen cylindrical-shaped granite plutons intruded into The Llano’s metamorphic rocks and forced their way vertically upward from the Earth’s molten interior, 1.09 billion years ago.  

Enchanted Rock, located between Llano and Fredericksburg, is the Llano Uplift’s best example of such a pluton.  Granite Mountain, located just west of Marble Falls’ city limit, forms the crest of another.  Both are also excellent examples of monadnocks.

During their intrusion, these granite plutons were contained within the older domain of metamorphic rocks - buried within, never reaching the ancestral continent’s surface.  However, by the time Cambrian sandstone deposition began to cover the exposed, eroded surface 550 million years ago, the ancestral surface had undergone 550 million years of erosion, which uncovered the plutons, exposing their granite content.

Granite is harder and more resistant to erosion than the softer host metamorphics.  For this reason, Enchanted Rock, Granite Mountain and the cresto of most other granite plutons of The Llano Uplift area stand above their surrounding host terrain as monadnocks.  Enchanted Rock is the most dominant pluton and the most massive monadnock of The Llano Uplift, standing up to 425’ higher.

The previously described 4,000' thick sedimentary sequence of Paleozoic age rocks, which is so well exposed in the HSB area, outcrops with varying thickness around the entire perimeter of the dome-shaped Llano Uplift.  These sandstone, limestone and shale formations, which were deposited over but subsequently eroded from the top of The Uplift, surround the older Pre-Cambrian rocks that outcrop today in The Uplift’s central portion in ring-like fashion.  

The younger Paleozoic rocks – the Marble Falls Limestone and Smithwick Shale – have been stripped back more distant from The Uplift’s exposed Pre-Cambrian center than the older Paleozoic sediments, which ring The Uplift in tighter fashion.  In total, approximately 4,500 feet of Paleozoic sedimentary rocks and older Pre-Cambrian igneous and metamorphic rocks have been eroded off The Llano Uplift’s crest.

In our next section, we’ll examine how and why The Llano Uplift developed.

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6) The Rise of the Llano Uplift.

The prior chapter discussed the United States Geological Survey’s findings that the heavily folded Ouachita Mountain Range of southeast Oklahoma was once part of a huge, linear mountain range that extended southward around central Texas into Mexico.  The mountains were of Himalayan proportions, with heights up to 30,000 feet.  

The range had emerged in reaction to compressive forces applied by the South American continent, which drifted north and collided with the North American continent during the latter portion of the Pennsylvanian Period, beginning about 300 million years ago.  The South American continent continued to apply compressive force against North America for the next 55 million years - throughout the Permian Period.  

You will also recall that the mountain range was located at what was then the North American continent’s outermost edge.  The crest of the range trended through what we know today as Austin, Boerne and Uvalde to just north of Del Rio, from where it turned northwestward to connect with the Glass Mountains, just west of Marathon, before turning south into Mexico.  

During its growth stage, the Ouachita range had wrapped itself around and was partially thrust over the rigid southern edge of what became The Llano Uplift – holding it down through the partial override.  The two continents began to release and drift apart at the beginning of the Triassic Period, 250 million years ago.

Freed in this manner from being held down by the earlier overriding forces, The Llano Uplift began to rise during the Triassic.  The Llano Uplift’s rise was also related to the fact that the continental core underlying the Llano area is much thicker and lighter than elsewhere in Texas.  

This “bubble-like” phenomena help lift The Llano’s basement rocks from the hugging grasp of the Ouachita range.  The Uplift, whose center is located about eight miles southwest of Llano, exposed The Llano’s Pre-Cambrian and Paleozoic rocks in a dome-like fashion.    

Pre-Cambrian igneous and metamorphic rocks of North America’s continental core are exposed today at The Uplift’s center, comprising what we know tday as the Central Mineral Region.  Younger Paleozoic sedimentary rocks overly the Pre-Cambrian and are exposed around the rim of the uplift.  These Pre-Cambrian and Paleozoic rocks, which outcrop in a 65 by 35 mile oval-shaped area whose long axis trends NW-SE, are deeply buried elsewhere in Texas – typically under a thick sequence of softer and much younger Cretaceous limestone.   

The Paleozoic and Cretaceous sections have been completely removed from the crest of The Uplift through two separate periods of primary erosion.  The first predominately erosional period lasted for 130 million years, beginning with the Triassic Period and continuing through the Jurassic into the early Cretaceous.  The second began about 65 million years ago, when the Rocky Mountains began to rise at the end of the Cretaceous Period , and continues on today.

As the two continents began to drift apart, the compressive environment that prevailed during the mountain-building of the late Pennsylvanian and Permian periods was replaced with a tensional one.  The lateral stress that had occurred along The Uplift's fault zones in response to the early compressive forces ceased.

Instead, as the continents pulled apart, the new forces developed of a tensional nature, causing movement along the faults to change from sliding substantially laterally or horizontal, one side versus the other, to sliding vertically – one side dropping downward relative to the other, as if the bottom were falling out.

The change to vertical movement along the previously established fault zones produced what is known as block faulting.  Block faulting is one of the principal factors that influence the topography of our area today.  Excellent examples of the Llano Uplift’s block faulting are preserved in the immediate HSB area.

The best local example is produced by a major fault - the HSB-Marble Falls Fault - that can be seen at HSB’s Slickrock Creek/Hi Circle South crossing, just upstream from the Falls of Slickrock.  Here, on the immediate south side of the crossing, the white Ellenberger limestone has dropped downward relative to the much older, reddish Pre-Cambrian granite, which moved relatively upward.  Along this plane of weakness, about one-half mile of vertical movement occurred during the Triassic Period, which occurred between 200 to 250 million years ago.  

Now, don’t get alarmed!
 The fault has been inactive for 200 million years.  

The fault separates 475 million year old limestone from 1.09 million years old granite.  This fault is truly major – not only in the amount of its vertical movement but also by representing as much as 925 million years

of missing geologic time.  By these two measures, it is one of the largest faults that can be seen in all of Texas.

Although the older granite block has been faulted upward relative to the limestone along this plane of weakness, the granite outcrop nevertheless forms the low valley to the north that holds Lake LBJ.  The high ridge that rises on the south of the fault is held by the younger but more resistant limestone that dropped vertically downward.  

The view-lots overlooking Lake LBJ just uphill from Hi Circle South in HSB Proper occupy the escarpment formed by the north face of this fault block, which trends northeast into Marble Falls.  In all future discussions, we’ll refer to this fault block, which is a graben, as the HSB-Marble Falls Fault Block.


At first thought, one would think when one object drops relative to another, it should end up lower, not higher.  The reason why the opposite appears to be in effect here, why the limestone holds the higher ridge even though it has dropped downward some 2,500 feet, is that in a relatively semi-arid climate like HSB has apparently had for the past 290 million years, limestone resists the forces of erosion much more than granite.  For this reason, limestone terrains stand higher than granite terrains in our area.

If one were to gaze at the horizon in a NNW direction from the Hi Circle South/Slickrock Creek location, Lookout Mountain is visible eight miles distant, overlooking Kingsland.  Lookout Mountain is another block-fault feature.  It is also capped by limestone.  

Once again, the younger, more resistant limestone that holds Lookout Mountain has been faulted downward relative to the older, less resistant granite, which forms the intervening eight-mile wide valley that underlies Lake LBJ.  The granite valley is a horst fault block, where older rocks have been lifted upward relative to rocks in adjacent fault blocks.  Lookout Mountain and Thanksgiving Mountain are both examples of graben fault blocks – younger rocks dropped downward relative to rocks in adjacent fault blocks.

In a future chapter, we continue our discussion of The Llano Uplift by examining the HSB-Marble Falls Fault Block.  In the process, we’ll learn “Why There Is a Marble Falls.”

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7) The Central Mineral Region.

In the previous section, we discussed how the dome-shaped Llano Uplift developed.  We learned that it is comprised of two large segments – a central core of Pre-Cambrian igneous and metamorphic rocks and a surrounding rim of Paleozoic sedimentary rocks.  As previously noted, the Pre-Cambrian rocks are from 1,360 to 1,090 million years old, whereas the Paleozoic rocks are from 550 to 290 million years old.

The portion of The Llano Uplift where Pre-Cambrian igneous and metamorphic rocks outcrop is referred to as The Central Mineral Region.  A vast array of minerals has been found in The Region over the past 250 years, dating back to the early Spanish explorers.  Nevertheless, although igneous and metamorphic rocks have proven elsewhere in the world to often house a wealth of minerals, The Central Mineral Region has been surprisingly barren – falling far short of justifying its name.  

Although the Region has notable occurrences of many minerals, it is even more noted for its deposits being uneconomic.  The most common/important being Asbestos, Barite, Bismuth, Copper, Dolomite, Feldspar, Fluorspar, Gadolinite, Glauconite (Iron), Gold, Graphite, Hematite (Iron), Galena (Lead), Magnetite (Iron), Magnesite, Manganese, Marble, Mica, Molybdenum, Pyrite (Fool’s Gold), Quartz, Rare Earth Minerals, Serpentine, Silver, Talc, Tin, Topaz, Tripoli, Tufa, Uranium, and Vermiculite.  Although The Region has been overrun with prospectors, especially during the late 1800s and early 1900s, none of these minerals have been found in commercial quantities, save for two – Gadolinite and Graphite.

Graphite was discovered in the southeastern part of The Region in the early 1900s.  Mining began in 1916 at a site at the end of today’s Graphite Mine Road, on the southeast side of Lake Buchanan.  It was a commercial success, being the nation’s largest graphite producer in its prime.  It depleted in the 1980s.

The Region’s most interesting boom involved Gadolinite, which was a critical component of the early light bulb’s filament.  This rare earth mineral’s discovery was made about 12 miles north of Kingsland in 1887.  Thomas Edison and Charles Westinghouse, who’s competing fledgling electric companies were in need of Gadolinite, became deeply involved through Geologist N.J. Badu - of Llano’s Badu House fame.  

Badu had learned of the discovery and sent samples to both men.  Badu, a flamboyant promoter of the kind made famous in Texas, is remembered for being involved in numerous schemes to promote The Region.

The mineral proved extremely valuable, selling in 1887 for $144 an ounce, eight times more than the price of gold.  The deposit eventually supplied both companies, but Westinghouse made the most of it.  But advances in the light bulb made the need for Gadolinite short lived, ending by 1910.  The site went under water when Lake Buchanan Dam was completed in 1937.

Also, large quantities of topaz have been produced in Mason County, located on the western edge of The Region.  The stones have been dug largely by amateur rockhounds, not by commercial enterprises.  The largest gem-quality topaz in North America was found there.  It weighed in at 1,298 carats, amounting to almost three pounds.

In addition, The Region is an important source of granite, which is more than a mineral.  It is a rock, comprised of several minerals – typically quartz (white) and feldspar (pink), with lesser amounts of hornblende and mica (black).  Thirteen 13 different varieties of granite have been mined in The Region, including a very limited amount of Llanite – found nowhere else in the world but Llano County.

Mineral exploration of The Region began in the 1750s when Spanish explorers first found gold and silver along the Llano River.  They reported the presence of great quantities and Spanish authorities followed up.  They established a mission to convert the local Indians, hoping to “civilize” them into being miners, as they had done successfully in Mexico and South America.  

Spain’s precious mineral’s venture failed abysmally because great quantities were not present.  Nevertheless, people have believed great mineral wealth lay hidden in The Region ever since.  And several other unsuccessful “booms” followed.  The three subsequent best known failed booms involved gold, iron ore, and uranium ore.  Although gold and iron ore has been produced in The Region, no such effort has proved commercial.  

The Central Mineral Region’s biggest boom was in iron ore.  Iron is found in considerable quantities scattered around The Region. The most notable deposit was discovered at Iron Mountain in northwestern Llano County in 1886.  It induced Llano to begin thinking of itself as an iron-mining center of Pittsburg proportions.  This Pittsburg of the West fever lasted until 1893, when the Iron Boom’s end was signaled by a series of unexplained fires believed set to collect insurance on failing ventures.  Llano establishments could not get fire insurance coverage for several years thereafter.

The Region’s most recent mineral boom began in January 1954, at a time when Americans were concerned about the rise of the Soviet Union and the possibility of nuclear war.  A promoter representing himself as a metallurgist leased property in the Baby Head area, about ten miles north of Llano, then announced a major uranium discovery on the lease.  He supported his claim with rock samples that prospective customers could see measured “hot” by Geiger counter.  

He attracted much attention, including getting an Austin newspaper to report a possible great uranium find, but skeptics developed quickly.  He slipped out of town a few weeks later.  No uranium of commercial value has been found to date.

So it has been with The Central Mineral Region – a land of bent picks and dashed dreams, of snake-oil salesmen and false promises.  But it is the Deer Capital of Texas!

In the next chapter, we look at what was happening to Austin’s six-mile high mountain range while The Llano Uplift was rising.

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8) Austin’s Vanishing Mountain Range.


In the previous section, we learned that the inner portion of The Llano Uplift, where Pre-Cambrian igneous and metamorphic rocks outcrop, is referred to as The Central Mineral Region.  A vast array of minerals has been found in The Region over the past 250 years, dating back to the early Spanish explorers.  Nevertheless, although igneous and metamorphic rocks have proven elsewhere to often house a wealth of minerals, The Central Mineral Region has been surprisingly barren – falling far short of justifying its name.

The Llano Uplift began to rise as the South American continent disengaged and drifted apart from North America at the end of the Paleozoic Period, 250 million years ago. The Uplift’s Pre-Cambrian foundation had been buried by almost a mile-thick cover of Paleozoic sediments prior to the rise of the Ouachita Mountain Range.

The crest of this mountain range, which eventually grew to six-mile heights, was centered on what is today Austin – riding on the east-dipping plane of a huge, regional thrust fault system.  From Austin, the crest trended northeastward into southeast Oklahoma, where it is preserved today as the Ouachita Mountains.  Going southwest, the crest wrapped itself around the exceptionally rigid south flank of The Llano Uplift before trending into West Texas.

This disengaging action released the compressive pressure that produced the mountainous belt on the North American continent’s outer margin when the two continents first drifted together 40 million years earlier, 290 million years ago.  During the Triassic Period that followed the close of the Paleozoic, the drifting apart of the two continents introduced an opposing extensional environment.  The tension led to two geological events of major significance.

As discussed a couple of chapters back, this release of pressure freed the Llano area from entrapment by the Ouachita Range, allowing The Llano Uplift to rise and develop.  And as the two continents continued to drift apart, the segment of the Ouachita Mountains located between the Red River and West Texas, trending through Austin, collapsed into the widening abyss - a rift zone - created by the parting continents.  

This rift zone was a graben of huge proportions.  It developed at and parallel with the outer edge of North America’s ancestral continental margin.  If one placed a pancake on a table and gradually pulled it apart into two pieces, the resulting intervening gap takes on the form of a rift.  With this insight, one can readily picture how the mountain segment dropped or slid eastward into what amounted to a bottomless pit that formed between the two continental land masses as they drifted apart.  They've no collided into one another since.  

The kind of feature we’re talking about resembles the Earth's giant rift valleys of today, such as the Persian Gulf and Red Sea.  The Red Sea and Persian Gulf rift valleys are both approximately 1,300 miles long and up to 200 miles wide.  Both were created when the African and Asian continents began trying to drift apart millions of years after the American continents separated.  

As one can see by examining today’s geographic maps of Asia and Africa, although they have clearly begun to separate, the two continents never completely unhinged.  Nevertheless, upon seeing that rift valleys of Red Sea and Persian Gulf proportions can form in mere partial openings between continents still attached to one another, as maps demonstrate, one gains a better perspective of just how huge the rift we’re discussing must have been.

Since we know North and South America disengaged completely from one another, the intervening rift that swallowed the Ouachita was likely substantially bigger and far more commanding.  A rift of such huge proportions was clearly more than sufficient to absorb even a mountain range of Himalayan proportions, as were the Ouachita, because it vanished far too quickly to have merely eroded away.

You may ask, how is it we know that components of this range either fell into, or slid eastward along the mountain range’s east-dipping basal thrust plane, a buried rift valley?  The consequences of whether it fell or slide is of little practical significance; what is significant is that the mountain range vanished into the rift.

One might also ask, since no remnants are exposed today of the section that collapsed, being the portion extending from the Red River, through Austin, around The Llano Uplift, to the Glass Mountains of West Texas, how do we know the range ever existed?

We’re able to answer these questions from geophysical data and from information gained in the drilling of deep oil and gas wells along the buried segment’s trend.  From these sources, we know when it happened – at the end of Permian and beginning of the Triassic periods – and that it is indeed buried today under thousands of feet of younger sediments.  

In the next section, we return to our discussion of The Llano Uplift by examining the Thanksgiving Mountain-Marble Falls Fault Block in more detail.  It has had an interesting geologic history.  In the process, we’ll learn “Why There’s a Marble Falls.”

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9) Why There’s a Marble Falls.
In our last discussion, we discussed how as North and South America drifted apart, beginning early in the Triassic Period, tensional forces replaced the compressive environment that had prevailed during mountain-building that occurred throughout the Permian Period, lasting for 45 million years, during which the two continents continuously bumped against each other.
  
We also saw how as the continents pulled apart, the new tensional forces caused movement along the fault systems of The Llano Uplift to change from lateral, strike-slip, to vertical in direction.  Instead of the opposing sides of a fault sliding horizontally or laterally with respect to one another, as had occurred during the 45 million years of compression, once the tensional regime set in vertical, up versus down movement prevailed, with one side dropping down relative to the other.

The change to vertical movement along the previously established faults, beginning 245 million years ago, at the start of the Triassic Period.  The correspondingly tensional regime produced what is known as block faulting.

We learned earlier that block faulting is a principal factor in controlling the topography in The Llano Uplift.  We discussed the two best examples of block faults in our area – Lookout Mountain overlooking Lake LBJ at Kingsland and Thanksgiving Mountain overlooking Lake LBJ in HSB.

The HSB example is produced by a major fault, the HSB-Marble Falls-Mormon Mills Fault, the largest in The Llano Uplift.  It can be observed at HSB’s Slickrock Creek/Hi Circle South crossing, just upstream from the Falls of Slickrock.  As seen on the immediate south side of the crossing, the white Ellenberger limestone has dropped downward relative to the much older, reddish Pre-Cambrian granite, which rose upward relative to it.  

The Fault cuts across RR 1431 near Marble Falls’ western city limit and can be seen in the outcrop on the north side of the road.  Here, vertical displacement on the Fault is twice that seen at its exposure on HSB’s Slickrock Creek, now being on the order of 5,000 feet.  

Once again, the terrain on the southeast comprises the Thanksgiving Mountain-Marble Falls Fault Block, which is a graben.  This block has dropped downward approximately one mile relative to the adjacent block immediately northwest, being the granite horst fault block that underlies Lake LBJ and Lake Buchanan.  

This fault, the HSB-Marble Falls-Mormon Molls Fault, has been inactive for over 200 million years, as have all other faults in The Uplift area.

And you ask, how does all this bear on “Why There’s a Marble Falls?”  Answering a question with a question, “Have you noticed the most fundamental geological difference between Marble Falls and Granite Shoals?"  

We just observed that the Fault separates the two towns.  Vertical movement along the fault plane has positioned grossly different rock types beneath to two communities.   The simple difference in the underlying terrain's resistance to erosion explains why Granite Shoals and Marble Falls have their respective names?”  

Marble Falls is located east of the Fault, situated atop the Thanksgiving Mountain-Marble Falls Fault Block.  This graben fault block is capped with the highly resistant Marble Falls Limestone.  Although the graben portion dropped
downward, the terrain is topographically high because its limestone cap has been slow to show effects of erosion.  

On the other hand, Granite Shoals is located west of this major fault, on the exposed surface of the comparatively soft granite horst block.  Although this fault block moved relatively upward, its terrain is topographically low because the soft granite degraded faster.  

Granite Shoals took its geologically-derived name from the granite shoals that were well developed in the bed of the Colorado River near its juncture with Sandy Creek.  The shoals were caused by rounded granite boulders and cobble stones produced as the river flowed over the relative soft belt of granite bedrock. Though present in less dramatic fashion, this shoaling occurred elsewhere in the river bed between Marble Falls and Llano, where the river flows over a continuous outcrop of granite.  

Marble Falls took its geologically-derived name from the “marble” falls that developed as the river traversed downstream from the fault and across the much harder Marble Falls Limestone’s outcrop, which created a much greater impediment to flow.  

The Falls of the Colorado, i.e. “marble” falls, have been covered by Lake Marble Falls since its completion in 1951.  Similarly, the granite shoals have been covered by the waters of Lake LBJ since it was completed that same year.

The Pedernales Falls, located a few miles downstream from Johnson City on the Pedernales River, is also a product of the Marble Falls Limestone.  This same cause/effect relationship holds true for the small falls present on Cypress Creek at Cypress Mill, east of Round Mountain.

As the Colorado River cut ever so slowly from west to east, from upstream to downstream, across the HSB (Thanksgiving Mountain)-Marble Fall’s Fault Block, the Marble Falls Limestone’s extremely resistant outcrop belt left the “marble” waterfalls as a residual impediment in the river bed, clear evidence of the formation’s exceptional resistance to erosion.  In the process, the hard limestone was polished in sandblasted fashion by river-borne silt and sand-size particles, causing it to resemble marble – thus the name.  

Marble is the end result of limestone that has undergone metamorphism, produced by great heat and pressure that comes largely through burial at great depths.  The Marble Falls Limestone has never been under such stress, it has therefore never been metamorphosed, and it therefore is not marble.

However, although hard enough to take on a polish, the Marble Falls Limestone, named for its outcroppings along the river at Marble Falls, is a mere sedimentary rock.  It is indeed rare that a sedimentary rock proves hard enough to show polish.

This dense limestone is harder and more resistant to erosion than any other sedimentary, igneous or metamorphic rock in The Uplift area.  The observation is proven by the fact that the two aforementioned falls are the most pronounced impediments of any left in the river’s path as it traversed across The Llano Uplift’s entire width.

Being such a major impediment, the falls provided an obvious potential source of hydro-power for the area’s early settlers, and the rest is history.  However, if the HSB-Marble Falls Fault Block weren’t capped with the most resistant rock in The Llano Uplift, there would be no Marble Falls!

There is one other story that needs to be told regarding the Marble Falls Limestone.  With invaluable assistance from the river, another landmark stands as further evidence of how the formation slowed and impeded the denudation of The Llano Uplift like nothing else.  

That landmark stands just downriver from the Highway 281 Bridge at Marble Falls.  Have you heard of The Canyon of the Colorado?

Up next, we look at this exceptionally significant Canyon.  You’ll be surprised at the important role it has played in the post-Paleozoic geological history of our region.  

You’ll be doubly surprised to see the strong evidence indicating The Canyon is older than North America’s most famous - the Grand Canyon.  And if that is an grabber, you’ll shocked at the prospects of it being up to 50 times older?

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