Toenails Under Your Feet EarthCache

This EarthCache will take you on a brief stroll (less than 1,000 feet round trip on flat ground) across an ocean floor from 100 million years ago. Your mission will be to discover what this caprock layer is made from, what cements it together, and how thick it is right now. You will also get to see back in time another 25 million years just by observing from the overlook.

Please park in the parking lot, and walk to the Caprock Trail trail-head behind the Hardwicke Building to start. Go left (west) along the trail to the overlook.

At the geocache coordinate points, you are standing high up on a ridge overlooking the Trinity River. The top of the Caprock Trail is at an elevation of 685 feet above sea level. But the Trinity River flowing a few hundred feet northeast is at an elevation of only 595 feet. Not quite the Grand Canyon, but the Trinity River and its precursors cut that almost 100 foot drop.

If you hike down the slope to the Trinity River, you will be traveling back in time to layers of the Earth's surface that was laid down 120 million years ago. That record of the geology of this spot, from the top of the caprock down to the sands along the river spans about 25 million years. 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 along a beach or mudflat, and sometimes covered it with a shallow warm ocean.

Much of the Fort Worth Nature Center & Refuge is in the Paluxy Sand (Kpa) geological layer, made up of lots of grey and brown sand and clay, compressed into soft sandstone layers. The Paluxy Formation was laid down during the Lower Cretaceous Era, 112-108 million years ago. At that time, this land was near the shores of an ancient ocean and the sediment built up from beach sands and clay washed down by streams. The dinosaurs were already extinct at this time, and early mammals were small creatures.

Above the Paluxy Sand Layer, the caprock of Caprock Trail is what remains of the Walnut Shell Formation (Kwa), a hardened layer of marine fossils that resist erosion. The Walnut Shell Formation is also from the Lower Cretaceous Era, but 108-106 million years ago. That thick limestone cap was formed in only 2 million years, and is relatively thin in the Fort Worth area, up to only 28 feet thick. 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.

As you hike along the Caprock Trail, you can see how the softer sandstone under the caprock is eroding away, and blocks of limestone break off and slide down the cliff toward the river. This erosion has been going on for a long time, but you can see recently detached blocks of limestone as they have cracked free, and some just starting to slide. Stay on the trail!

Your First Mission: As you hike along the Caprock Trail, estimate the thickness of the Walnut Shell Formation Layer. Do not leave the trail. From the trail you can see deep narrow cracks as blocks of limestone break free. From some curves in the trail, you can see the ledge and erosion underneath. You can get good views of the thickness from several points.

Q1: Estimate how thick the caprock layer is. Also, give the GPS coordinates of where you took this reading.

Point to Ponder: This formation 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?

Now, walk back toward the trail-head where you started.

Look closely at the ground under your feet -- that lumpy-bumpy stuff are fossils; the shells of clams, mollusks and oysters that thrived in the shallow ocean that used to cover Texas. Fossils (from Latin fossus, literally "having been dug up") are the preserved remains or traces of animals, plants, and other organisms from the remote past.

Please leave all fossils, rocks and pieces of nature in place for the next person to observe.

Most of the fossils you can see in the Caprock Trail are two types of ancient oysters: Cerastostreon texanum and Texigryphaea mucronata. But you can also find some ancient snails, clams and sea urchins, and sometimes an ancient ancestor of the nautilus, called an ammonite. The most common fossils along the Caprock Trail are gryphaea, the shells of ancient oysters. These oysters lived in large colonies in shallow waters and the shell had two parts -- a flattened "lid", and a larger toenail-shaped dome that sat in the ocean-floor mud.

Your Second Mission: What, exactly, is "gryphaea"? You may find the answer along the trail, by observation, or you might have to look it up.

Q2: What is "gryphaea"?

Point to Ponder: Look closely at the fossil shells you are walking on. Can you imagine why they are called "Devil's Toenails"?

To complete your last mission, walk past the building in the other direction. Here the caprock enters forest cover, but you can still see the fossils. Stay on the trail until you come to an educational display about the Paluxy Sandstone and Walnut Shell Formation. Here you can read more interesting facts about the area.

Your Final Mission: Find out what mineral cements together the quartz in the soft Paluxy Sandstone and what cements together the shells for the hard Walnut Shell Formation limestone.

Q3: What cements together the Paluxy Sandstone? What cements together the Walnut Shell Formation limestone?

Point to Ponder: Do you think the "glue" or the base material has more to do with how soft or hard the layer is? Justify your answer from observations.

To score this EarthCache find, please email your answers to the cache owner:

Q1: Estimate how thick the caprock layer is. Also give the GPS coordinates of where you took this reading.

Q2: What is "gryphaea"?

Q3: What cements together the Paluxy Sandstone? What cements together the Walnut Shell Formation limestone?

Parking: 32.846626,-97.475496 -- Park here. A nice, large, rainwater-permeable parking lot.

Trailhead: Caprock Trail 32.847596,-97.47592 -- Start on trail here. Please use the Caprock Trail to get to the cache. Stay on the trail and be respectful of the ecosystem.

Overlook: 32.84744, -97.477078 -- The overlook of the Trinity River Valley from Caprock Trail

Sit Rock: 32.847744, -97.476281 -- A good place to sit and think about the trail questions

Education Display: 32.84715 -97.47535 -- Read about the geology here

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 .
• Geology of Fort Worth Nature Center, publication of the FWNC&R
• 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
• 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
• Geology of Tarrant County, Univ. of Texas Bulletin: http://northtexasfossils.com/geologytarrant24-39.htm
• Gryphaea fossils: http://en.wikipedia.org/wiki/Gryphaea