A Window into the Past: Geology & Paleontology of Willowbrook Reach

By Erich Rose 

Austin, and the entire Central Texas region are well known for its readily accessible geology.  From the time of the earliest European settlers people have been collecting and commenting on the abundance of fossils and other evidence of ancient life to be found in our streams, road cuts and wherever the bedrock becomes exposed. 

We all know Austin is divided into basically two halves.  The uplifted, western "Hill Country" side and the low-lying relatively flat eastern half.  The rocks and fossils are easily found to the west in the big road cuts and quarries.  But to the east where the rocks are both less disturbed and often softer, we need to look in streams and the occasional construction site.  Willowbrook Reach is one of those spots.

I first visited the stream in the mid 90's.  My in-laws live in the neighborhood and, knowing my interest, it was suggested that I check out the creek bed.  Well to my pleasant surprise fossils were present, and in certain spots, abundant.  Now there are fossil collectors who bring picks and shovels and feel certain the best stuff must be below the surface.  

That's not my style.  

The first thing I did was scramble down to a gravel bar, got down on my knees and started scanning for specimens.  Quickly I spotted chunks of oyster shell.  Those oysters, all of the genus Exogyra, along with the light tan marl, quickly let me know I was probably in the Austin Chalk Group.  The Austin Chalk is the name given to a large group of formations of Upper Cretaceous age (approx. 82 - 85 million years old).  These rocks were deposited in a quite warm sea that covered much of Texas at that time.  This body of water was known as the Western Interior Seaway and at points reached all the way north to the arctic.  It teamed with life.  

Most of the fauna would have seemed quite familiar: oysters, clams, snails, corals, fish, sharks, etc.  But there were some exceptions, big and small.  And the big ones would have been rather awesome.  Mososaurs and plesiosaurs many meters in length swam through these waters.  And there were strange reef-like areas not made up of corals but instead huge tube-shaped clams, known as rudists.  And probably one of the most interesting groups would have been the ammonites.  These now extinct relatives of squid and octopi came in all sorts of shapes from the coiled forms whose remains we see in the creek bed today, to strangely twisted and distorted shells, but all defined by the tentacled creatures that lived within.

As I continued to pick up oyster shells of various sizes I also came across other fossils: a few shark teeth, sea urchin spines, some interesting small worm tubes and eventually fragments of ammonites.  All this stuff in the gravel bars was pointing to what might be found in the actual bedrock itself.  And sure enough as I explored and found the scattered spots where the tan chalk was exposed I started to see shells and other fossils "in situa."  Now I had collected the Austin Chalk at other locations and knew a bit about what to expect but what made my day was coming across one rather large ammonite exposed in the bottom of the creek bed.  Ammonites are not uncommon in the Austin Chalk but this one was large and although it was not collectible it was fun to find.

As I identified the fossils I was finding I did a bit more research on the geology and quickly realized the stream exposed a rather specific section of stratigraphy.  There is a hard layer rich with two species of the oyster Exogyra that typically indicates the top of the Dessau Formation and its contact with the overlying Burditt Formation.  The two species are easily identified: Exogyra tigrina has a wrinkled surface and often shows stripes while Exogyra laeviuscula is rather smooth.  They only occur together in the Austin Chalk at the top of the Dessau.  The hard layer represents a "reef" that these oysters formed over a wide area.  In amongst those oysters I found several other oysters and bivalves (clams & scallops), a few gastropods (snails), several ammonites and one well-preserved sea urchin.  To this day I go back to that spot and look for newly exposed specimens. 

But the streambed holds more than Cretaceous fossils.  The thick layer of rounded stones, gravels and caliche we see just above the tan limestone is still plenty old.  One of my favorite days at the reach was punctuated by my nephew Henry's question "Hey uncle Erich, what’s this?"  Well the "this" he handed me was a rather large tooth.  Not a shark or reptile tooth I might expect from the creek's bedrock but a horse's molar.  

When asked where exactly he found it he pointed to a spot just above the tan limestone.  Now this was no farm animal, it was much older, not millions of years but ice age, maybe 10 thousand years or more.  Horses evolved in North America migrated into Asia and beyond, but then went extinct here.  This was a fossil tooth.  These gravels and sediments that lie above the bedrock are known as the Upper Colorado River Terrace Deposits.  They were set down long before the Spanish or any other westerner brought horses back to Texas.  

My nephew had a real cool fossil.  

Henry went on to find another one but I have yet to find one myself.  So maybe one day Willowbrook Reach will share another secret of it's past with me. Every time I go back cross my fingers.