When Moses brought forth water from the rock
Could be country like here was where he stalked
Edwards Limestone forms an aquifer renowned
Springs, seeps, and grottos squirt water from underground
Here’s a place to see it, and keep your feet quite dry
Read this page of history, exposed to earthcacher eye.
((Remember, you can't log this without passing THE TEST!!! End of section below))
There are 3 sets of benches at GZ -- a singleton, a pair, and then another singleton. Best viewing is from the eastmost (away from Great Oaks) singleton bench.
===FTF!!! SuperDave66, BackroadsGal, & SlalomDude April 13,2010!!!===
INTRODUCTION============================================
This is the 2nd in my series of earthcaches: CTU – “Central Texas Underfoot.” The first is GC1R2D8 CTU - GIVE THAT GLEN A ROSE! which introduces the intrepid Earthcacher to the stairstepping Glen Rose Formation, famous for Dinosaur Prints. Here we discover an outcropping of the Edwards Formation, which is inclusive of the VERY important “Edwards Aquifer.”
The coordinates bring you to an overhang near Brushy Creek along the Brushy Creek Regional Trail (http://www.wilco.org/CountyDepartments/ParksandRecreation/BrushyCreekTrail/tabid/795/language/en-US/Default.aspx) in Williamson County. You will not have to leave the trail to do this Earthcache.
There is room on the side of the road for parking at the coords, or bike or stroll the trail from the more ‘defined’ parking areas at O’Connor and Hairy Man Road (CO 174, also called “Brushy Creek Road”), or further east along Hairy Man Road at the current trail terminus – there are several caches along your route, and it’s a beautiful stretch of CenTex greenbelt.
There are 3 sets of benches at GZ -- a singleton, a pair, and then another singleton. Best viewing is from the eastmost (away from Great Oaks) bench.
THE TEST ==================================
Paperless cachers have requested that the questions be placed early in the cache page copy. Happy to oblige – but this does NOT excuse you from the required reading!!!
To log this Earthcache, please *email* me the answers to the following questions:
1> Brushy Creek is some yards behind you, well across the road. Why do you suppose this cliff has been exposed, with that particular undercutting?
2> Describe the texture of the rock above the undercut. Is this illustrative of a distinguishing feature of Edwards Formation Limestone?
3> Is the ‘seep’ of this ‘grotto’ flowing, dripping, seeping, or dry?
4> Observing the undercut walls, note particular rock deposit/formations. If we were in a cave, what would they be called?
5> Looking at the cliff face, at roughly eye-level, these deposits show distinctive horizontal as well as vertical features. Describe them and give me your theory as to why they look the way they do.
6> Where does the Edwards get its name? [HINT: answer below!]
Logs without verification email will be regretfully deleted, though not without forewarning. If you visit as a group, a single representative email for the group is acceptable – but I expect good logs in exchange. [This is an Earthcache, you need to EARN your icon. Bwahahaha.]
If feasible, when you log, please include a picture taken at/from ground zero – with or without yourself and your party. [GPS not required in picture.]
TEXAS GEOLOGY============================================
The geologic history of Texas is recorded in its rocks. Mountains, seas, rivers, volcanoes, and earthquakes, all add their chapters to this unfolding story, and the keen observer can read the ground like a history book, as the various strata document origins in billions of years of changing geography.
AUSTIN/ROUND ROCK METRO UNDERFOOT====================
The Austin/Round Rock metro bedrock is predominately limestone under a varying layer of ‘alluvium.’ If your yard is like mine, the alluvial layer goes from none at all, to only a few inches. But while we’re all familiar with ‘limestone,’ is all limestone created equal? Of course not.
Though all limestones are sedimentary rock composed mostly of calcium carbonate derived from marine fauna, some are punky as rotted wood, while others are impermeable as marble. [Marble, after all, is the metamorphic form of limestone – just add pressure, heat, and a few millennia]. And we won’t even get into dolomites (yet).
True, when roaming around the area, the observant layperson may note changes in rock texture (fine to coarse grained, hard rock to chalky), weathering , color (yellows through whites to grays, with some mineral staining thrown in), inclusions (fossils, chert)… but, other than a couple, easily defined formations (like the stairstepping Glen Rose) even experts rely on (or at least confirm their estimation via) geological mapping, to be sure of their formations.
And Formations there are, each with reasonably distinct physical characteristics -- but mostly divvied by age.
The ground and underground of the greater Austin Metro is comprised of several geologic units. These include (in order of youngest to oldest): the aforementioned alluvium, then, after a 70-million year (plus) gap, the Taylor Group (Taylor Marl), 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 Glen Rose Formation.[1]
What you’re looking at here, at ground zero, is an exposed section of Edwards Limestone.
EDWARDS LIMESTONE==================================
In brief, the Edwards Formation consists of massive limestone beds with bands of chert nodules and rudistid biostromes (e.g. fossilized reefs, especially the corals). Particularly susceptible to “chemical weathering processes” (dissolving by water), the Edwards Formation is “typically vuggy where exposed,” meaning, it’s as holey as Swiss cheese – and the nice slice of sourdough bread you serve it on. These holes vary from the microscopic to full-on cavern systems, and karst features are typical throughout the Formation.
The rocks of the Edwards Formation were deposited by shallow, early Cretaceous seas, showing evidence of the range of marine possibility: reefs and shoals to lagoons, tidal zones to open water sea-bottom. Dolomitization (replacement of calcium by magnesium) and the presence of chert (silica) nodules are locally important. [2]
In 1898-9, pioneer geologists Robert Hill [http://en.wikipedia.org/wiki/Robert_T._Hill ] and T. Wayland Vaughan [http://gsahist.org/gsat/gt95nov21_22.pdf] applied the name “Edwards” to the cherty, rudistid (reef critter) bearing limestone between the Georgetown (or “Kiamichi” as they called it) and the Comanche Peak Formations in north-central and central Texas. This name replaced the earlier "Caprina limestone" of Benjamin Franklin Shumard (1860) [http://www.tshaonline.org/handbook/online/articles/SS/fsh33.html] and the "Barton Creek limestone" described by Hill ten years earlier. Not to lose this reference, the type locality was designated as “Barton Creek, near Austin,” by Walter Scott Adkins (http://www.tshaonline.org/handbook/online/articles/AA/fad18.html ) in 1933.
The name Edwards was taken from its physiographic region, the Edwards Plateau, which takes *its* name from Edwards County, named in honor of Hayden Edwards, an early colonizer. Turns out, however, this was an unfortunate name selection for the "Caprina Limestone" because the assumed relationships between the Edwards Limestone of central Texas and the Edwards Plateau, vague even then, have now been proven, well, WRONG. Ah well.[3]
As noted Edwards Limestone is a highly ‘perforated’ limestone, with many vugs, voids, and other ‘conduits’ which enable excellent water flow (but not particularly long-term retention [4]). Some of these holes are actually fossils: “pholad borings,” e.g. Cretaceous worm and clam burrows; others are caused by weathering, typically the percolation of acidic water through rock – the processes which excavate, then decorate limestone caverns.
This entire region is described as karstic – you will find many sinkholes and cave entrances around and about (cachewise, visit GCWP7Y Father's Day Series #9 - Karst Park, GCZJDB Knucklehead, and/or GCV5AA Unfortunately, No Karst Spelunking Allowed! – and that’s just off the top of my head). On the large scale, Inner Space Caverns, just to the North, is well worth a visit (and has its own Earthcache (GC1D6GF Father's Day Series 2008 #1 - Inner Space Caverns Texas). But I digress.
Bounded on either side by more impervious rocks, the holey Edwards Formation is like a Brillo™ pad sandwiched between two plates. Here, in the Austin Area, it runs into the Balcones Fault Zone, which cuts and exposes these conduits to the open air, enabling springs and seeps. Many of the local streams come ‘out of nowhere,’ and some subsequently disappear, the water draining back down the network of ‘pores’ and ‘pipelines’ in the rock layers.
[If you’re a glutton for TECHNICAL detail, I point you to [5] BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TEXAS 78712 Peter T.Flawn, Director Geologic Quadrangle Map No. 38: Austin West, Travis County, Texas http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-6105648/txu-oclc-6105648.pdf ]
Famous as “the Edwards Aquifer” – feeding such local treasures as Barton Springs Pool and San Marcos (“Aquarena”) Springs – the Edwards Formation actually has two faces when it comes to water. The local fresh-water, artesian system that we locals know and love, and, east of the Balcones Fault Zone, where the Edwards Limestone dives abruptly downward, a stagnant, briny water system (with local accumulations of hydrocarbons – can you say “Texas Tea”?) from deep in the Gulf Coast Basin.
The local, fortuitously sweetwater version is the Jollyville Plateau, an outlier of the Edwards Plateau. Here the Edwards Limestone survives in an unbroken belt, straddling the Brazos/Colorado drainage divide. To the south the tributaries of the Colorado River have incised deeply into the landscape, giving rise to remarkable scenic vistas. Here, on the drainage divide’s northern side (which coincides with the Travis/Williamson County Line), the land is flatter, the waters not as powerful. So the tributaries of Brushy Creek (itself the southernmost tributary of the San Gabriel River within the Brazos drainage basin), take a generally level route across the gentle slopes. [6]
In 1901 Hill reported that the Edwards Formation was 230 feet thick along Brushy Creek. In 1953 Atchison determined a minimum thickness of 210 feet for the Edwards Formation in the Round Rock area. By contrast, logs of two representative water wells (state well numbers 5828711 and 58353175) indicate thicknesses for the Edwards Formation between 138 and 160 feet. [7]
DEFINITIONS =================================
ALLUVIUM: Sediment deposited by flowing water, as in a riverbed, flood plain, or delta. Also called alluvion. [http://www.answers.com/topic/alluvium]
AQUIFER: underground bed or layer yielding ground water for wells and springs etc (wordnetweb.princeton.edu/perl/webwn)
BIOSTROME: A horizontally bedded stratum of fossilized remains of sedentary organisms (en.wiktionary.org/wiki/biostrome)
CHERT: variety of silica containing microcrystalline quartz (wordnetweb.princeton.edu/perl/webwn)
CRETACEOUS: Latin for "chalky", usually abbreviated K for its German translation Kreide (chalk), a geologic period and system from circa 145.5 ± 4 to 65.5 ± 0.3 million years ago (Ma). …The Cretaceous was a period with a relatively warm climate and high sea levels. The oceans and seas were populated with now extinct marine reptiles, ammonites and rudists; and the land by dinosaurs. At the same time, new groups of mammals and birds as well as flowering plants appeared. The Cretaceous ended with one of the largest mass extinctions in Earth history, the K-T extinction, when many species, including the dinosaurs, pterosaurs, and large marine reptiles, disappeared. [http://en.wikipedia.org/wiki/Cretaceous]
DOLOMITE: a kind of sedimentary rock resembling marble or limestone but rich in magnesium carbonate rather than calcium carbonate (wordnetweb.princeton.edu/perl/webwn)
KARST: terrain formed by the dissolving of bedrock; generally is characterized by sinkholes and caves that channel water underground. Texas caves and karst aquifers are important economic, scientific, and recreational resources. (further reading on Texas caves: http://www.esi.utexas.edu/outreach/caves/texascaves.php; http://www.utexas.edu/tmm/sponsored_sites/tss/cavesandkarst/index.html; http://www.tshaonline.org/handbook/online/articles/CC/rqc3.html; Texas Cave Conservancy (texascaves.org))
PHOLAD BORINGS: biologically created holes in rock, mostly attributed to extinct mussels [http://tafoni.com/Quandary.html]
RUDISTS are a group of box, tube or ring shaped marine heterodont bivalves that arose during the Jurassic, and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean. (http://en.wikipedia.org/wiki/Rudist); elongate, conical bivalves (clams) common in Cretaceous rocks but now extinct (Rodda et al., 1966, pp. 5-8). (http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-6105648/txu-oclc-6105648.pdf)
TAFONI: ellipsoidal, pan- to bowl-shaped, natural rock cavities http://tafoni.com/Definition.html (spend time with the entire article, it’s worth it!)
VOID: cavities inside bedrock, rule of thumb: ‘larger than a breadbox’ smaller than a cavern. [R.L. Jernigan PE PhD Brierley Associates LLC]
VUG: small to medium-sized cavities inside rock that may be formed through a variety of processes [en.wikipedia.org/wiki/Vug]
ENDNOTES============================================
[1] Excellent further reading on the various formations can be found in the documents at The University of Texas at Austin: Virtual Landscapes of Texas: http://www.lib.utexas.edu/books/landscapes/ , particularly in the Fourth annual report of the Geological Survey of Texas Publication 5235917-4 http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-5235917-4/txu-oclc-5235917-4-a001a.html (their search function is very good).]
[2] Bedrock Geology of Round Rock and Surrounding Areas,
Williamson and Travis Counties, Texas; Todd B. Housh: http://repositories.lib.utexas.edu/bitstream/handle/2152/413/Bedrock%20Geology%20of%20the%20Round%20Rock%20area.pdf?sequence=3
[3] Stratigraphy of the Fredericksburg Division, South-Central Texas; Clyde H. Moore, Jr.: http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-2601138/txu-oclc-2601138.pdf
[4] Edwards Aquifer Website: http://www.edwardsaquifer.net/geology.html
[5] Bureau of Economic Geology The University of Texas at Austin; “Geologic Quadrangle Map No. 38: Austin West, Travis County, Texas”: http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-6105648/txu-oclc-6105648.pdf
[6] Moore [3]
[7] Housh [2]
ADDITIONAL READING====================
EDWARDS AQUIFERNORTHERN SEGMENT, TRAVIS, WILLIAMSON, AND BELL COUNTIES, TEXAS C. M. Woodruff, Jr., Fred Snyder,Laura De La Garza, and Raymond M. Slade, Jr., Coordinators GUIDEBOOK 8 AUSTIN GEOLOGICAL SOCIETY 1985 http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-13014589/txu-oclc-13014589.pdf
Edwards Aquifer Website: http://www.edwardsaquifer.net/geology.html
“Edwards Formation (Lower Cretaceous), Texas; dolomitization in a carbonate platform system”; W. L. Fisher, and Peter U. Rodda: http://aapgbull.geoscienceworld.org/cgi/content/abstract/53/1/55
EDWARDS LIMESTONE OUTCROP ALONG BARTON CREEK, Sue Hovorka; GROUNDWATER RECHARGE IN TEXAS, Bridget Scanlon; BARTON SPRINGS: WATER QUALITY AND ENDANGERED SPECIES, David A. Johns http://www.beg.utexas.edu/mainweb/services/pdfs/edwards.pdf
Encyclopedia > Geology of Texas; http://www.statemaster.com/encyclopedia/Geology-of-Texas
Geologic maps of Texas: http://www.aeg-tx.org/geo.asp
Geology of the Brushy Creek quadrangle, Williamson County, Texas. Austin, The University of Texas at Austin; Dick E. Atchison, 1954, M.A. thesis (aka "Atchison (1954)"
Symposium on Edwards Limestone in Central Texas, E.Lozo, H. F. Nelson, Keith Young, B.Shelburne, and J. R. Sandidge http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-744434/txu-oclc-744434.pdf (Check out the chapter “Edwards Fossils as Depth Indicators”!!!)
Texas Geology - Map of Texas http://geology.com/states/texas.shtml
Zilker Park Walking Tour Guidebook: A Recreational Visit to the Edwards Limestone http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-32363355/txu-oclc-32363355.pdf
And for something completely different:
Geology at the University of Texas Coloring Book
http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-244294995/txu-oclc-244294995.pdf
CONCLUSION ========================================
Hope you enjoyed this visit with Texas Geology!
Remember! Logs without verification email will be regretfully deleted, though not without notice. Pictures and logs with actual content highly encouraged!!!
Cache on!