Logging Requirements: (My preference is that you send the answers through the new "Messenger" feature)
1. Compare and Contrast the TWO distinct layers. Include
A. Colors
B. Hardness
C. Direction of the rock planes
D. Texture of rock
2. What would happen to this part of the High Plains if the top layer was eliminated?
3. What caused the unconformity at this roadcut?
Geology:
"The edge of the plains to the west comes to an abrupt hault on the north side of the town of Post, where U.S. 84 drops off the plains into the erosional topography of the Double Mt. Fork of the Brazos River. Sand and conglomerate of the Miocene-Pliocene Ogallala formation lie directly on colorful, varicolored sand and clay beds of the Triassic Deckum group in the roadcut."
What makes this roadcut so interesting so fascinating is more than just the vivid color differential and rock texture. Here you see an unconformity (a significant break or gap in the geologic record). The Deckum group was deposited during the early Mesozoic (208 million years ago according to evolutionary theory), whereas the Miocene-Pliocene Ogallala formation (2-66 million years ago according to evolutionary theory).
Terms:
erosion - the wearing away of the earth's landscape by natural forces of water, waves, ice, wind, or tides.
High Plains - Formed during Tertiary time. Cut through by streams and rivers, exposing lower formations.
Miocene-Pliocene Ogallala formation -- This formation is the "flat" surface of the High Plains and resulted from massive volcanos and a resurging uplift in the Rocky Mountains. Huge amounts of sediment were swept into the panhandle and western Texas.
Triassic Deckum group -- The Triassic Deckum group was laid down when Pangea was still a supercontinent. However, the Gulf of Mexico is starting to appear as the "tearing" begins. Deposits in the Dockum group are bright red and were deposited in wester Texas and eastern New Mexico exculsively. This is a rather thin layer in most exposures and often creates an unconformity with the Anisian group below it.
Unconformity - a significant break or gap in the geologic record, caused by uplift and erosion, or lack of deposition.
Study.com gives us a more detailed definition of Unconformity - "An unconformity is the contact between sedimentary rocks that are significantly different in age, or between sedimentary rocks and older, eroded igneous or metamorphic rocks. Unconformities represent gaps in the geologic record; periods of time that are not represented by any rocks." Erosion (either wind or water) typically removes the "missing" layers. Sometimes quakes or volcanics tilt or break up a rock cycle sequence. These types of unconformities have further definitive terms (disconformity and angular unconformity).
Reference:
Spearing, Darwin. <i>Roadside Geology of Texas</i>. 13th ed. Missoula, MT: Mountain Pub., 2013. Print.
"Unconformities in Geology: Definition & Types | Study.com." Study.com. N.p., n.d. Web. 21 July 2015. .