Field and Laboratory

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In the Cedar Hill Quadrangle, bedrock consists of the Eagle Ford shale and Austin chalk, both of Upper Cretaceous age. The shale averages about 430 feet in thickness, with only the upper 140 feet exposed. This is predominantly a dark gray, fissile mudstone with limestone concretions and several thin limestone beds in the upper part of the section. A stratum of elastic limestone six inches thick lies 107 feet below the top and serves as a marker bed in areal mapping. The shale is disconformably overlain by the Austin chalk, at the base of which is a bed of conglomerate a foot or less thick. The Austin is characterized by massive beds of chalk separated by thin layers of calcareous shale. Joints are common throughout, and numerous small faults may be due to differential compaction within the underlying shale. Local unconformities within the formation probably represent scour-and-fill structures. Terrace remnants in the quadrangle are largely correlated with the Love Field terrace along the Trinity River to the north. This terrace stands 30 to 40 feet above the present floodplains and consists of gray clay with lenses of gravel and, locally, of foraminiferal sand.

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