INTRODUCTION ========================
Ah, the Glen Rose Formation!
Tread where dinosaurs walked
Imagine the lapping of ancestral seas
Ok, let's Geo-Talk.
CONGRATULATIONS HiDude_98 -- FTF 5/13/09
INTRODUCTION ========================
You are standing here because this is one of the prettiest spots in the Austin Metro (in my humble opinion). This beauty is not only skin deep, it runs deep through the bedrock. The convergence of time, water, and geology - with a side of distinctive vegetation - make this not only a sight worth beholding, but worthy of an Earthcache.
Paperless cachers have asked that the ‘test’ questions be moved to the beginning of the narrative. This doesn’t excuse you from the ‘required reading’ – indeed, you may just find the answers you require in the text that follows. [Besides, I worked *really* hard on it. =smile=]
TO LOG THIS CACHE ====================
You *must* EMAIL me the answers to the following (do not post them in your log). Logs without accompanying emails that at least attempt to answer the following six questions will be regretfully deleted (after due notice). One email per party is fine.
1> Define any TWO (just 2 required) of the following: "marl/marly" "arenaceous" "argillaceous" "wackestones" "calcarenites" "micritic limestones" "boundstones" (NOT a village in Surrey, England) "dolomite" "oolitic" "fossiliferous" "stylolitic"
2> What is a key visual aspect of the Glen Rose formation? Do you have an explanation of why that may be?
3> What is the age range of the Glen Rose deposits?
4> What sort of fossils, if any, would you expect to find? Are there any other distinguishing fossil features associated with Glen Rose? If you find a fossil, include a picture with your log! [Good labeling applauded!]
5> Observe the waterfall. What are its notable features? What factors contribute to that sort of erosion?
6> Look closely at the vegetation around you. Do you notice anything particular? Why should this be?
Logs with actual content encouraged! “TFTC”? BORING!!!! Tell us at LEAST a bit about your visit and impressions!
Not REQUIRED, but you’re ENCOURAGED to take a picture at GZ and post it with your log.
Now, to the required reading…
EARTH CACHE GROUND ZERO ===================
The most obvious feature at the coordinates is the undercut ledge over which Bull Creek picturesquely tumbles, literally, into a crack in the earth. This image is a microcosm of the larger Bull Creek Canyon around you. Water always takes advantage of the weakest rock and fractures, dissecting this stretch of Edwards Plateau to expose a fine display of geology and landforms. In addition, the diverse vegetation tells a further story regarding soil depth, composition, availability of water, and the microclimates of sun, heat, and shadow stretching from the low point of the creek bed up the canyon walls.
On the edge of the Balcones Canyonlands Preserve, this setting provides habitat for a number of rare and endangered plant and animal species, some found nowhere else on earth. Above ground are unique woodlands, wetlands and grasslands. Below ground is a honeycomb network of caves, sinkholes and springs containing highly specialized animals adapted to these unique environments. Still deeper are a series of aquifers, including the Edwards Aquifer that is the primary drinking water source for over 1.5 million central Texas residents. [1]
There are many aspects, right here, that are worthy of a thesis, let alone an Earth Cache. But for this smiley we shall focus essentially on the geology.
CENTRAL TEXAS GEOLOGY =====================
The most dominant geological feature in the Austin Metro is the Balcones Fault Zone. Attributed to subsidence along the margin of the Gulf Coast basin, it separates the Edwards Plateau to the west from the Blackland Prairie to the east. I-35 from Georgetown to San Antonio follows the BFZ, crossing and re-crossing its numerous individual faults. All the rocks in the area are of Cretaceous age, with the younger rocks dropped down on the east side against the older rocks to the west. The rock was deposited as limy muds and oozes with rare sands on the sea bottom during a period roughly 180 to 80 million years ago. [2]
The BFZ is visually defined by the harder rocks to the west: hills rising noticeably above the more deeply eroded, softer and younger rocks and alluvium to the east. It is this topographic expression, with its various levels of plateauing, that inspired the early Spaniards to call the region "Los Balcones" or "the balconies."
The Edwards Plateau bedrock is primarily limestone with isolated volcanic (Pilot Knob) and granitic (e.g. Enchanted Rock/Llano Uplift) areas. Elevations range between 100 and 3000 feet. The topography is typical karst with numerous caves, vugs, and voids. Especially in wet years, springs and seeps are common along the fault zone, with rain water from the exposed aquifers in the higher country to the west forced upward and outward by the fracture zones of the Balcones Fault.
This portion of the Edwards Plateau adjacent to the Balcones escarpment is called the Balcones Canyonlands, highly dissected by fast-moving streams through steep-sided canyons, a function of fracturing radiating from the BFZ.
AUSTIN METRO UNDERFOOT ====================
When dealing with the ground and underground of the greater Austin Metro, one can expect to encounter several geologic units. These include (in order of youngest to oldest): alluvium, then, after a 70-million year (plus) gap, the Taylor Formation, layered over the Austin Group (also called Austin Chalk), the Eagle Ford Formation, Buda Formation, Del Rio Claystone, Georgetown Formation, Edwards Limestone (which comprises the important aquifer and karst formations), Comanche Peak Limestone, Walnut Formation, and the oldest formation at this site, the Glen Rose Formation.
MEET THE GLEN ROSE FORMATION ===============
As you stand here appreciating the broad sheet of Bull Creek diving over this stair-stepped falls to continue, winding around the roots of sycamores, along its canyon-carving way, you are viewing a classic example of Glen Rose topography. Named for the city of Glen Rose the formation is a rich source of Dinosaur tracks and various fossils.
Deposited during the periodic retreat and advancement of the Lower Cretaceous Gulf of Mexico, Glen Rose Formation deposits represent low subtidal to supratidal environments which have been preserved as alternating hard and soft layers of marl (limy clays and shales), dolomite, dolomitic limestone, and limestone. With a total formation thickness of approximately 600 feet, bedding is near horizontal and relatively thin, ranging from less than one inch to about two feet in thickness, with a gentle regional dip to the southeast. Rock hardness and susceptibility to weathering may vary widely between adjacent strata. When exposed in outcrops, the Glen Rose typically forms a relatively gentle slope, though 'stair-stepping' is common. [3]
Although the lithology of the Glen Rose Formation is typically described as "limestone," the unit is variable, ranging from quartz siltstones and sandstones to marls, micritic limestones, dolomites, and boundstones. Or even more specifically: wackestones; calcarenites; argillaceous, arenaceous, chalky limestones; and marly, arenaceous clay. [4]
While Glen Rose Formation beds are of different thicknesses, each individual strata is remarkably uniform (e.g. "even-bedded") throughout its extent when traced along the outcrop. Although beds do alternate between limestone and clay, the Glen Rose formation is predominately calcareous, with clays and sand described as "as a minor accessory" and these intervening layers tend to be quite thin.
The Glen Rose Limestone formation is part of the Trinity Group of Lower Cretaceous Rocks. The famous beds containing dinosaur tracks in the Glen Rose, Texas area are near the base of the Glen Rose formation, at or near the Aptian/Albian boundary, dated at approximately 110 million years. Cretaceous sea animal fossils are the most common findings in the Glen Rose formation. [6] In fact, "Cretaceous beach combing" is our favorite distraction from/while finding caches!
REFERENCES ==================================
[1] Travis County: The Balcones Canyonlands Preserve (https://www.traviscountytx.gov/tnr/nr/bcp)
[2] Roadside Geology of Texas; Robert A. Sheldon; Mountain Press Publishing Co; 2nd printing 1982
[3] Russell L. Jernigan, Ph.D., P.E. P.G.; Brierley Associates, LLC (www.brierleyassociates.com)
[4] The University of Texas at Austin: Virtual Landscapes of Texas, Glen Rose Formation
[5] Common Fossils in the Glen Rose Limestone compiled by Glen Kuban (http://paleo.cc/paluxy/gr-fossils.htm)
Excellent further reading on the various formations can be found in the documents at The University of Texas at Austin: Virtual Landscapes of Texas (https://repositories.lib.utexas.edu/handle/2152/69304) , particularly in the Fourth annual report of the Geological Survey of Texas Publication 5235917-4 (https://repositories.lib.utexas.edu/handle/2152/77954) (their search function is very good).
More Further Reading:
Atchison, Dick E., 1954, Geology of the Brushy Creek quadrangle, Williamson County, Texas. Austin, The University of Texas at Austin, M.A. thesis (aka "Atchison (1954)")
Todd B. Housh; Bedrock Geology of Round Rock and Surrounding Areas, Williamson and Travis Counties, Texas (https://repositories.lib.utexas.edu/handle/2152/22932)
CONCLUSION ===================
Hope you enjoyed your visit to this nifty Glen Rose site! Remember: logs without verification email will be regretfully deleted (though not without notice), and pictures are highly encouraged!
Cache on!!!