CTU – WATER FROM A STONE (Edwards Aquifer)
In Texas, United States
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Come see the watery pipeline of the Edwards Aquifer in dynamic action!
This is a technical hike, with steep terrain, and several creek crossings. Be gentle to the land. Follow trails or stick to bare rock. ****DO NOT TRY THIS IN STORMY WEATHER!!! **** If the creekbottom is impassible, approach close as possible from above and make your observations from there.
== CONGRATS! FTF April 11, 2010 by ChiefEagleBear ===
The Israelites wandered out of Egypt’s land
MapSourced or sinners -- led by misdirecting hand
Came to Rephidim with a powerful thirst.
Whining and complaining, Moses they cursed.
Moses said, “God? What we gonna do?
These people make me CRAZY – and they’re gonna stone me, too.”
The Lord said, “Be easy, Son. Get your staff and take a walk.
Go where I tell you, and rap a certain rock.”
Moses did as told, with some witnesses along
Gave that rock a good ol’ knock, and the water poured out strong.
Clearly ol’ Moses and his stick have been through here:
Right out of the Cliffside the water’s pouring clear.
Chalk Ridge Falls Park, just downstream of the Stillhouse Hollow Dam, is a fascinating place of rugged terrain and flowing water managed by the Army Corps of Engineers (visit link) This is a free area that is open from 8:00 am - Sunset. Dogs, glass containers, alcoholic beverages, bicycles, nor firearms ARE NOT ALLOWED in the park. There are over 5 miles of hiking trail and some wonderful caches in the Park.
You can approach the earthcache from downstream --GC1V1M9 The Chalk Ridge Fallen and GC11GQM Guardian's Treasure -- or upstream -- GCQ5WJ Mark's Cave II. In the stretch between these points you’ll encounter a fascinating little creek carved ravine with more than one earthcache-worthy note.
This requires a technical hike, with steep terrain, several creek crossings, and potentially wet and algae-coated, “slicker ‘n snot” rocks. Please avoid cutting your own trail and keep to the bare rock and/or established routes.
PRACTICE CITO – and don’t leave any of your own.
WARNING: do NOT try going up/down the stream bed in stormy weather – flashfloods are not to be messed with, and there is NOWHERE to run. If the creekbottom is impassible, approach close as possible from above and make your observations from there.
THE TEST ================================================
Paperless cachers have requested that the questions appear early on the cache page – so here they are. This does not excuse you from the REQUIRED READING!!!!
To log this cache, *email* me answers to the following questions:
1> Is the water from the ‘deep grotto’ (N31 00.749 W097 31.869) seeping, flowing, or rushing?
2> Given that the average garden hose full on puts out 5 gallons per minute, what do you estimate this flow to be? [Look, if math makes you blanch whiter than the limestone of this canyon, just describe the flow: straw, garden hose trickle, garden hose blast, fire hose, broken water main….]
3> As you continue upstream, does the amount of water coming out of the rocks increase or decrease? Any theories why? [No right answer, I just want you to think about it.]
4a> When was the last measurable rain?
[For actual precipitation see (visit link) and click on the ‘Yesterday’ tab, navigate as necessary from there. Another link for local weather can be found at (visit link) ]
4b> How do you think precipitation affects the water flow?
4c> Do you think the presence of the Lake might have any effect on the water table?
4d> What is the current lake level? [see: (visit link) ]
4e> Do you think the spring & seep activity along this ravine is a good indicator of water table level?
5> What is the average residence time for water in the aquifer? [Note: you don’t need to google for the answer, just read on!]
LOGS WITHOUT VERIFYING EMAIL WILL BE REGRETFULLY DELETED (though not without due notice).
I don’t demand it, but pictures with your logs are encouraged (of you and/or the scenery … whatever strikes you as NEAT!), as are logs with actual content, telling us about your adventures in the park and what you thought about this nifty spot.
Now, to the instructional portion of this earthcache!
According to the cache page for GC11GQM Guardian's Treasure, this creek in its scrubbed bare-rock state has a relatively short history. Choked with generations of sediment and vegetation, the creek was barely visible. However, in 1992, a spectacular event occurred: Stillhouse Hollow Lake filled to capacity (maximum level reached 667.97 msl on March 4, 1992 (visit link) and overflowed. An enormous volume of water cascaded over the spillway and into the valley below.
“You really had to see it to understand the enormity of the event," writes benclem on the cache page. "A vast amount of soil, and all that grew in it was wiped away to reveal the ancient creek bed you will be hiking.”
But all that history just makes this earthcache *possible!*
Except for a little topsoil, the exposed rock of Chalk Ridge Falls Park is comprised of Edwards Limestone. In brief, the Edwards Formation consists of massive limestone beds with bands of chert nodules. The limestone is greatly attributed to “rudistid biostromes” (e.g. fossilized reefs, especially the corals). Various marine fossils are common.
Particularly susceptible to “chemical weathering processes” (e.g. this rock dissolves in water), the Edwards Formation is, in a word, perforated. These holes vary from the microscopic to full-on cavern systems, and karst features are typical throughout the Formation.
More importantly to THIS site: all those holes create a network of pipes and straws, which water LOVES to travel THROUGH rather than over.
“The key to why the Edwards is a good aquifer lies in the fact that the limestone was exposed, extensively eroded, and then covered over again with new limestone formations. When the Edwards limestone was exposed, erosion created cavities and conduits and made the limestone unit capable of holding and transmitting water. When it was covered over again, the new sediments were relatively impermeable and formed a confining unit.” [Edwards Aquifer Website: (visit link) ]
In other words, the Edwards Aquifer is like a Brillo™ pad between two plates. And where the edges of this rocky sandwich are exposed, say, by a creek canyon, the water leaks on out.
Again, quoting the Edwards Aquifer Website (visit link)
“Because the movement of water in the Aquifer is highly complex, the waters we pump from the ground and drink are a mixture of waters of many different ages. In some places water moves only a few feet a day, but in other places water may move 1000 feet a day or more…. The average residence time for water in the aquifer is around 200 years, so much of the water that San Antonians drink today probably went underground around the time of the American Revolution.
“In all karst aquifers, most of the water storage occurs in the matrix, and most of the water movement occurs in conduits. In the Edwards, there are many large caverns, but one should not picture the underground reservoir as a vast pool. Rather, think of it as a saturated sponge with pipes. The rock matrix has many pore spaces similar to the holes in a sponge, and some of them are connected by well defined conduits through which water can readily flow. Pores that are not connected to other pores or to a conduit cannot provide much water. The measure of pores that are connected and can provide water is called effective porosity.
“Water enters the Aquifer easily in the recharge zone, but the subsurface drainage is generally inadequate to hold all the water that falls in large rain events. Recharge conduits and sinkholes quickly become filled up with water. This is one reason why the region floods so easily.
“The honeycombed rock matrix stores 95% of the water in the Aquifer. To move long distances, water leaves the matrix and enters a well-defined conduit, where it may be transmitted very far rather quickly.“
EEP! NO CREEK! ==========================================
So as you move up the canyon from the crossing between GC1V1M9 The Chalk Ridge Fallen and GC11GQM Guardian's Treasure you will, providing we’re not in a full-on drought, notice an increasing number of seeps, springs, and, if you’re lucky, rather nice flows right out of the rock. Waymarks were taken at two “Spring Caves”: N31 00.754 W 097 31.849 and N31 00.749 W097 31.869, ground zero for this earthcache.
As you approach the head of the canyon -- N31 00.756 W 097 31.897 -- you’ll find the headwaters of this little tributary. Rise 6 feet, and the streambed is… gone. Like the waters of Rephidim, this stream comes right out of the rock.
Pretty cool, huh? That’s why you’re here, after all.
Thanks again for visiting this Earthcache – hope you enjoyed this spot as much as we do!
Remember: LOGS WITHOUT VERIFYING EMAIL WILL BE REGRETFULLY DELETED (though not without due notice).
PS: the earth is one amazingly interlocked system. Did you know that the Edwards Aquifer responded to the Great Japan Earthquake of 2011? It's true! In response to the energy released by the quake, the water level of Texas' Edwards Aquifer was displaced about a foot. Amazing! See (visit link) or google "edwards aquifer response to japan quake" for more specifics.
(No hints available.)
Last Updated: on 9/15/2013 5:35:44 PM Pacific Daylight Time (12:35 AM GMT)
Coordinates are in the WGS84 datum