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Eagle Ford shale of Upper Cretaceous age underlies the Carrollton quadrangle in Dallas and Denton counties. Exposures in the western sector of the quadrangle reveal seams of impure bentonite containing nodules of pyrite and selenite. Near the center of the quadrangle, bodies resembling dasycladacid algae are located in a silty ferruginous bed. Outcrops in the eastern part of the area contain elastic limestone beds, and at one horizon, large limestone concretions. The Elm Fork of the Trinity River bisects the quadrangle, and with the aid of its tributaries, has built terraces parallel with the river. The terraces form four distinct levels, the middle levels east of the river being most extensive and best preserved. After the Eagle Ford shale was deposited and compacted, the area was uplifted and the present dendritic drainage developed. The streams underwent stages of alternate downcutting and filling, thereby forming the terraces. Probably these are related in origin to fluctuations in sea level during stages of Pleistocene glaciation and deglaciation.

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