Geology
The Glen Rose is the uppermost, thickest and most extensively exposed formation of the Trinity Group, a series of shallow-water marine formations deposited on a southeastward flank of the Llano Uplift, through a number of sea regressions and transgressions. Wells drilled in eastern Travis County have encountered over 1,000 feet of the Glen Rose. In the northern part, the Glen Rose is laterally continuous with the Paluxy Formation. The Glen Rose overlies the Hensel Sand and is overlain in turn by formations of the Fredericksburg division. In 1974, Keith Young concluded, based on ammonite zonation, that the formation ranges from late Upper Aptian into the Lower Albian, about 115-105 million years old.
The formation consists mostly of hard limestone strata alternating with marl or marly limestone, but is replaced by sandy facies shoreward (to the northwest). Because of the differing strengths of the layers, the limestone weathers to form a staircase profile on hills. These strata were originally referred to as the "Alternating Beds", which term included the overlying Fredericksburg formations.
The Glen Rose has been divided into upper and lower portions, separated by a one foot layer of Corbula shells, a small bivalve.
The formation was named in 1891 for the town of Glen Rose, Texas, by paleontologist Robert T. Hill. The type locality is a near shore section exposed in the Paluxy River near the town of Glen Rose. The stratigraphy of the formation was most recently revised in a 1971 study.
Read more about this topic: Glen Rose Formation