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EarthCache

CTU - GIVE THAT GLEN A ROSE!

Hidden : 5/7/2009
In Texas, United States
Difficulty:
3.5 out of 5
Terrain:
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The first of my CTU "Central Texas Underfoot" caches sits beside Bull Creek Falls on the broad limestone of the greenbelt trail.
Fully public, dogs ok off leash, parking, wheelchair possible FINALIST!!! 2009 Austin Cache Awards -- BEST EARTHCACHE

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.

While the ‘test’ questions are right up front, you’re not excused from the ‘required reading’ that follows. You may just find the answers down there. [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 EMAILS THAT AT LEAST ATTEMPT TO ANSWER THE FOLLOWING SIX QUESTIONS WILL BE DELETED!!! (Regretfully; after due notice.) One email per party is fine. Quit panicking. I’m an easy grader – I just want you to leave with a greater appreciation for what lies underfoot.

1> Define any TWO (just 2) 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? (Come on, give me a best guess.)

3> What is the age range of the Glen Rose deposits?

4> What sort of fossils would you expect to find? Are there any other distinguishing fossil features associated with Glen Rose? If you find a fossil (this spot is pretty picked over, alas), include a picture with your log! [Good labeling applauded!]

5> Observe the waterfall. What are its notable GEOLOGIC features? What factors contribute to that sort of erosion?

6> Look at the vegetation around you. Do you notice anything particular? (Like, maybe the differences between what grows THERE versus THERE.) Why do you think this is?

Logs with ACTUAL CONTENT encouraged! 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 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, water availability... even the microclimates of sun, heat, and shadow 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. Above ground are unique woodlands, wetlands, and grasslands. Below ground is a honeycomb network of caves, sinkholes, and springs harboring 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, 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 the subsiding margins of the Gulf Coast basin (put a piece of cardboard half off a table top and push down on the unsupported edge – that’s the nutcase–er, shell), the BFZ separates the Edwards Plateau (aka Texas Hill Country) 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 by erosion to the east against the older, exposed rocks to the west. The rock was deposited as limy muds and oozes with occasional sands on the (then) sea bottom across 180 to 80 million years ago. [2]

The BFZ is visually defined by the harder rocks to the west: hills rising above the more deeply eroded, softer and younger rocks and alluvium to the east. It is this topography with its various blocky exposures (“plateauing”) that inspired the early Spaniards to call the region “Los Balcones” or “the balconies.”

Regionally, Edwards Plateau bedrock is primarily limestone with isolated volcanic (e.g. Pilot Knob) and granitic (e.g. Enchanted Rock) 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: where rain water flowing through the aquifers from the west is forced to the surface when it hits the fracture zones of the Balcones Fault. [The mechanism for this can take hundreds of years, and is subject of other area earthcaches, such as GC26PT8 CTU - SEIDER SPRING FLING.]

This particular area is part of the Balcones Canyonlands: known for its many fast-moving streams cutting through steep-sided canyons; a function of BFZ fracturing.

AUSTIN METRO UNDERFOOT ====================

The ground and underground of the greater Austin Metro features several geologic units. These include (in order of youngest to oldest): alluvium, then, after a 70-million-plus year 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 (the important aquifer and karst formations), Comanche Peak Limestone, Walnut Formation, and the oldest rock at this site, the Glen Rose Formation.

[Excellent further reading on the various formations can be found in the documents at The University of Texas at Austin: Virtual Landscapes of Texas ( (visit link) ) , particularly in the Fourth annual report of the Geological Survey of Texas Publication 5235917-4 ( (visit link) ) (their search function is very good).]

MEET THE GLEN ROSE FORMATION ===============

As you stand here, appreciating the (hopefully) 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 advance and retreat of the Lower Cretaceous Gulf of Mexico, Glen Rose Formation deposits represent low subtidal to supratidal environments 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 nearly horizontal and relatively thin, ranging from less than one inch to about two feet in thickness, with a gentle regional dip to the southeast. [Remember the cardboard analogy.] 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]

Think in terms of an artistic layer torte – many layers of various kinds of cakes and frostings. The frosting is the softer rock, from buttery to jam to more set icing and even stickier ganache, it is still more easily eroded away, undercutting the ‘cake’ layers (which are also different hardnesses & densities). When undercutting goes deep enough, the cake layer snaps. It can be big slabs (think gentle slope like a sofa-table with broken legs on one side), or the stair-stepping you see here – the smaller rubble carried away by erosion.

Although Glen Rose Formation rock is typically described as limestone, content varies, 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. [3]

While individual Glen Rose Formation beds are of different thicknesses, as in that chef-artist’s torte, each individual strata’s height is remarkably uniform ("even-bedded"). 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." Thus, these intervening non-limestone 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 old – though the formation itself is dated across a 10 million year period (115-105 million years ago). Cretaceous sea animal fossils are the most common findings in the Glen Rose formation. [4] In fact, "Cretaceous beach combing" is our favorite distraction from/while finding caches!

REFERENCES ==================================
[1] Travis County: The Balcones Canyonlands Preserve ( (visit link) )
[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] North Texas Fossils: Glen Rose Formation ( (visit link) )
[4] Common Fossils in the Glen Rose Limestone compiled by Glen Kuban ( (visit link) )

More 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 ( (visit link) )

Wikipedia Glen Rose Formation ( (visit link) )

CONCLUSION ===================

Hope you enjoyed your visit to this lovely Glen Rose site!
REMEMBER: logs without verification email WILL BE DELETED (don’t make me delete your log – I’m an easy grader and will talk you through the ‘test’). Pictures are highly encouraged!

Cache on!!!

Additional Hints (Decrypt)

Sbe gur Grfg: ERNQ GUR PNPUR CNTR!!!! Naq nsgre? Gur yvaxf va gur Ersreraprf naq Tbbtyr ner lbhe sevraq!

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)

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Inventory

There are no Trackables in this cache.

 

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118 Logged Visits

Found it 111     Write note 6     Publish Listing 1     

View Logbook | View the Image Gallery of 148 images

**Warning! Spoilers may be included in the descriptions or links.

Current Time:
Last Updated: on 4/12/2014 7:31:15 AM Pacific Daylight Time (2:31 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum