This Earthcache will bring you to two locations in Fort Worth built by two very different looking stones that were nevertheless mined in the same location, and tell us the prehistory of our state. Both locations are about 2.5 miles away from each other and both have free parking on site for short term visits to their business.
Location 1: Fort Worth Post Office Downtown N32 44.791 W097 19.745
This location is open Monday-Friday from 8-5, but there is no need to enter this building, so free to come during daylight hours. Instead, walk along the sidewalk at take a look at the columns in front of the building to answer the following questions.
At this location, please answer the following questions:
- Was the stone before you deposited in the upper or lower shoreface? How do you know?
- The columns have an interesting pattern, called crossbedding. What created this pattern?
- What are the small grains in the limestone called?
Location 2: the Amon Carter Museum of American Art N 32° 44.872' W 97° 22.105'
The museum is open Tuesday - Sunday and is free, and I highly suggest you check it out while you are here, however, everything you need to see for this Earthcache is available on the outside, so feel free to come any time during daylight hours. Please look but do not touch the walls.
At this location, please answer the following questions:
- Was the stone before you deposited in the upper or lower shoreface? How do you know?
- Do you see any whole shells, or just shell imprints? What happened to the original shells in the walls?
- Take a picture of two different shell molds and Identify which shell in the table below made that mold.
During the Mid Cretaceous Era (about 105 million years ago), Most of Texas was covered by a shallow ocean called the Western Interior Seaway. About 200 miles south of here, in Cedar Park, Texas is a limestone quarry, where the ocean tides left limestone deposits in an offshore bar about 40 miles long and 5 – 20 miles wide.
From the limestone left behind, we can tell that this was a very active tidal area with constant movement stirring up the silt and organic particles. During this movement, bigger marine life would settle down to the ocean floor in calmer areas, where smaller grains of limestone called ooids would drift in the waves. In the higher energy upper shoreface, these ooids would eventually settle in a crossbedding pattern, solidifying in a wavy pattern in the stone.
There are two distinct types of limestone that formed from this movement. Both are mined in the same quarry and sold under the name Cordova Limestone.
Cordova cream is a Oolitihic limestone and does not contain fossils. Often, the ooids can be seen in a crossbedding pattern in the stone, showing the traces of historical waves tossing these particles around.
Formed below the Cordova cream is the Cordova Shell limestone, this is the stone that you see before you. It is a fossiliferous limestone that bears the imprint of many Cretaceous sea creatures.
These imprints are a type of fossil called a mold. As the limestone silt buried the sea creatures and hardened over time, the shells dissolved, leaving internal and external molds behind. There are two very common shell molds here:
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Trigonia - An extinct genus of saltwater clams, fossil marine bivalve mollusk in the family Trigoniidae.
The genus Trigonia is the most readily identifiable member of the family Trigoniidae, having a series of strong ribs or costae along the anterior part of the shell exterior
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Turritella - a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.
They have tightly coiled shells, whose overall shape is basically that of an elongated cone.
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Resources:
https://www.geologyvirtualtrips.com/whitestone-quarry-tx
https://www.stoneworld.com/articles/85236-a-long-time-producer-of-texas-limestone
http://dcfossils.org/index.php/gallery11/