Aledo Caprock Formation & Erosion -- EarthCache
This EarthCache demonstrates the power of water to build up and tear down in geology. Water was essential in the formation of this caprock layer more than 100 million years ago and water is now slowly destroying it with steady erosion.
Please park in the parking lot at the Aledo Community Center. Do not park on the road. The exposed caprock area is just west of the parking lot and easily viewed from the Community Gardens area. Do NOT drive or walk over the caprock itself.
(Image by Odysseus2000)
Aledo Caprock Formation
During the Cretaceous Era, most of Texas was under a shallow ocean that reached as far north as Canada, but sometimes retreated south to only Nebraska or Oklahoma. These changes in levels sometimes put the area that would become Aledo at the shore of a beach or mudflat, and sometimes covered it with a shallow warm ocean.
During the Lower Cretaceous Era, 112-108 million years ago, this land was near the shores of an ancient ocean and the sediment built up from beach sands and clays washed down in streams. Early mammals were small creatures. The layers deposited here formed from the relatively soft sands and clay.
Later, the shallow sea that covered this area was rich in life. The oysters and mollusks proliferated and deposited thick layers of their hard calcium shells. The hardened shells layer protected the softer sand layer the way crunchy Royal Frosting protects soft moist cake inside. At the caprock erosion, you can see inside the cake.
(image courtesy of Joshua Paul, wikiCommons http://upload.wikimedia.org/wikipedia/commons/6/64/94_mya_Texas_Geology.JPG)
The hard caprock is what remains of the Walnut Shell Formation (Kwa), a hardened layer of marine fossils that resist erosion. The Walnut Shell Formation is from the Lower Cretaceous Era, about 108-106 million years ago. The ocean was deeper here, then, and covered reefs and colonies of oysters and mollusks. The actions of the tides and storms would grind them up and then cement them together into limestone. Temperature and chemical changes turned limestone into a hard and weather-resistant caprock conglomerate. This caprock in Aledo is made up of hard beds with sparry calcite and massive beds of common Gryphaea fossils. It is identified in the soil surveys as the Aledo-Brackett-Maloterre Soil Association.
(Aledo fossil bed, image by Odysseus2000)
Aledo Caprock Erosion
But, here at the Aledo Park, the caprock has broken and started to erode away by water. From the west edge of Aledo Park, you can see how the softer material under the caprock is eroding away. This erosion has been going on for a long time, but you can see the effect of water erosion as it washes across the hard fossil layer and then tumbles into the crack in the caprock to wash away the softer layer underneath.
The water source may not be apparent while you are there, but an occasional stream, named Crawdad Creek, runs under the roadway and across the hard caprock. A normal rain does not cause much change, but the rare Texas gully-washer can send a tremendous amount of water across this shelf. The water dropping off the ledge erodes the ground under the shelf, and roils the rocks with great force to dig back under the overhang.
(Aledo caprock erosion, image by Odysseus2000)
Your Mission: From the picnic area in the park you can see the edge of the shelf and under the overhang. Observe the effect of the erosion and draw some conclusions about the formation and erosion in this area.
Q1: What do you see embedded in the caprock? How big is it?
From the parking lot you can actually see two shelves of caprock. One small step where the water floods a wide plain and drops a small way -- this is safe to walk on. The other shelf is the edge of the deep drop -- do not walk on this area. The water erosion is wearing away the matrix of the caprock and freeing the original source of the limestone for you to see. Look closely at the shapes embedded in the caprock. Where else have you seen these shapes? You might even find whole pieces scattered around. What are they and how big are the ones laying loose on the ground?
Point to Ponder: The Walnut Shell Formation Caprock can be up to 28 feet thick in Fort Worth. But as you can see yourself, it is not that thick here. Why do you think it is not as thick in this particular area?
Q2a: How far back from the edge has the erosion dug under the caprock? What is that backwall digging force called?
Q2b: How deep back do you think the water has to dig before a piece of the front edge falls off? Can you see slabs of caprock that have recently fallen off the leading edge? If so, how many and what size are they?
Remember that the water that runs off the edge does not just fall straight down. When it hits the ground, it churns up rocks and soil and splashes backwards, digging into that back wall of softer rock. How deep back from the falling edge did it dig? Can you use the size of plants growing under the ledge as a guide? Can you even see the back wall under the shelf? Estimate how deep back from the front edge does the open area under the ledge reach. Study the diagram above to find the name of that process.
Point to Ponder: How many years do you think it will take for this caprock cliff to reach the road? Will drought and flood years speed or retard the process? Why?
Q3: Compare the chunks of caprock on top of the ledge with the type of rock exposed in the gully. Describe three differences that running water would have on each.
You can safely enter the gully from the picnic/playground area away from the rock shelf, or go around the building to the lower level stairs by the tennis courts for easier access. Examine the two types of rock in terms of color, hardness, particle size, cohesion (how well it holds together), water absorption, how easily roots penetrate it, etc. Pick three traits that are different in how it would react to running water.
Point to Ponder: How much water would it take to overflow this area? The roadway has been underwater twice in the last ten years. What effect do you think very large amounts of water flowing here has on the caprock erosion?
To score this EarthCache find, please email or message your answers to the cache owner:
Q1: What do you see embedded in the caprock? How big is it?
Q2a: How far back from the edge has the erosion dug under the caprock? What is that backwall digging force called?
Q2b: How deep back do you think the water has to dig before a piece of the front edge falls off? Can you see slabs of caprock that have recently fallen off the leading edge? If so, how many and what size are they?
Q3: Compare the chunks of caprock ontop of the ledge with the type of rock exposed in the gully. Describe three differences that running water would have on each.
Sources:
- Map of Earth 94 million Years Ago - Joshua Paul, WikiCommons: http://en.wikipedia.org/wiki/File:94_mya_Texas_Geology.JPG
- Lance L. Hall, NorthTexasFossils.com: http://www.northtexasfossils.com/ also personal email information and guidance, guidance reading the "Geologic Atlas of Texas, Dallas Sheet" - Bureau of Economic Geology, 1987 map .
- Fossil defintion from TXESS: http://www.txessrevolution.org/Microfossil_Activity1
- USGS National Geologic Map Database, Walnut Unit: http://ngmdb.usgs.gov/Geolex/NewRefsmry/sumry_11044.html
- USGS info page: http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=TXKwa%3B0
- http://en.wikipedia.org/wiki/Caprock
- Geology of North Central Texas: http://www.nhnct.org/geology/geo1.html
- Geology of Texas 1992 Bureau of Economic Geology: http://www.lib.utexas.edu/geo/pics/texas92a.jpg
- Walnut Clay formation: http://northtexasfossils.com/walnut.htm
- caprock erosion: http://birleycc.files.wordpress.com/2007/05/the-formation-of-waterfalls.doc
- Parker Co. Soil Survey: http://soils.usda.gov/survey/online_surveys/texas/TX367/Parker.pdf