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The Austin chalk and Taylor marl of Upper Cretaceous age underlie the Pleasant Grove area. These units strike north-northeast and dip at approximately 55 feet per mile toward the east. Both the middle marly and the upper chalk members of the Austin are present. Within the upper 40 feet of the chalk there are several local unconformities. In cross section these appear as buried depressions as much as 9 feet in original depth, with their bottoms truncating beds of massive chalk. The basal beds filling these depressions contain much conglomeratic material abounding in fossil fragments. These structures vary in age from place to place. The Austin-Taylor contact zone is of a transitional character marked by a 12-foot sequence of marls intermediate in lithology between the two formations. After withdrawal of the seas toward the Gulf of Mexico, erosion of the land began. The Trinity River, diagonally bisecting the area, formed floodplains at three successive levels. Most of the hills in the area are dissected remnants of terraces made of sand and gravel that obscures much of the bedrock, especially the Taylor formation.

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