My next post about the Geology of the National Parks Through Pictures is from my move back from Texas to live in Buffalo for a few years.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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The Alibates Flint Quarries National Monument is nestled up right next to the Lake Meredith National Recreation Area, so it is an easy two for one National Park stop.
Located in the panhandle section of Texas, this area is known for it's extensive plains and the hard caprock that covers most of the region. The Red Beds of the region were deposited during the Permian Period, approximately 260 million years ago. The rocks deposited in these two parks include sandstones, siltstones, and mudstones, as well as gypsum and dolomite deposits. The red beds are made up of the Whitehorse Sandstone, Cloud Chief Gypsum, and Quartermaster Formation. The red in these red beds are produced by small amounts of iron oxide, AKA rust, mixed into the rock units.
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| Upper geology layers found at Alibates Flint Quarries. Image courtesy of Quigg et al., 2011. |
The red beds, ending with the Whitehorse Sandstone on the top, are all very soft deposits where any significant amount of rain can easily erode away the rocks. However the arid environment of the region helps preserve these softer beds, as well as the harder, more erosion resistant "caprock" that was deposited on top of the red beds.
Over time the sea level slowly rose, eventually inundating this region under a shallow sea. Once the sea transgressed into the region, deposition of the Alibates Dolomite started and was produced from the plant and animal life living in the water (plankton, shelled animals, algae, corals). Dolomite is a variety of limestone where the primary mineral is dolomite (CaMg(CO3)2) instead of calcite (CaCO3). While calcite and dolomite for the most part are nearly indistinguishable to the naked eye, dolomite is generally less dissolvable than calcite, has a slightly higher hardness, and the crystals of dolomite typically have a slight curve to them while calcite surfaces as generally flat. The deposition of the dolomite layer created a "caprock" to the softer red beds below and can be seen in the photo above towards the top of the red slope surface.
The Alibates Dolomite caprock creates flat areas known as mesas (Spanish for table), which are periodically broken up by cracks in the dolomite and stream erosion. Over time some parts of the dolomite had slowly been altered to flint. Flint is a rock where the host mineral, here dolomite, has slowly been replaced molecule by molecule by a microcrystalline quartz known as silica. Flint, chert, and jasper are all microcrystalline quartz rocks that are produced in similar ways. The naming differences are often subjective to the uses for the particular rock where rocks with known archeological significance are called flints, as in this instance.
The source of the silica for the flint is partially a mystery though. There are several hypotheses for where it could have come from. One theory is that an eruption from the Yellowstone supervolcano dropped ash in the area 675,000 years ago. Ash is predominantly silica (quartz) and has been found in several areas of the park in beds up to three feet thick. It is also possible silica was brought in during deposition of the overlying Ogallala Formation which brought in sediment from the Rocky Mountains of Colorado and New Mexico around 10 million years ago. And a third theory is that immediately after the formation of the dolomite, the dolomite was then replaced by the silica nearly instantaneously in a geological sense.
The quality of the flint is heavily dependent on the size of the quartz crystals. While in modern day, large quartz crystals may be more visually appealing, large crystals were considered "garbage" and were thrown away by Indigenous people because it reduced the effectiveness of the flint as a cutting tool. Larger crystals were produced along cracks of the dolomite, similar to a geode, where the space allows the crystal to grow with the groundwater. The finer the quartz crystals, the sharper the edge that was possible in the flint. Flint pits, as is seen here, were dug up to try and harvest the fresh flint that had not been weathered.
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