The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas. The formation is most widely known for the dinosaur footprints and trackways found in the Dinosaur Valley State Park near the town of Glen Rose, Texas, southwest of Fort Worth and at other localities in Central Texas.
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. Individual steps that form this distinctive stair-step topography extend for many miles without any apparent change in expression. 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.
A stratigraphic column at the Mount Bonnell location starts with the Lower Cretaceous Trinity Group overlain by the Edwards Group. Upper Cretaceous formations follow, starting with the Del Rio Clay, Buda Limestone, and then the Eagle Ford Group. Formations within the Trinity Group include the Hammett Formation, Cow Creek Formation, Hensel Formation, and Lower and Upper Glen Rose Formation. The Hammett and the lower portion of the Upper Glen Rose act as confining units (or aquitard) for the Middle Trinity Aquifer. The Upper Glen Rose contains the Upper Trinity Aquifer, which appears to have inter-aquifer groundwater flow with the Edwards Aquifer as water levels are at the same elevation.
Caves and caverns
There are a number of caves in the Glen Rose, some of which are open to the public, including Cascade Caverns and Cave Without a Name, both in Kendall County, Texas and Natural Bridge Caverns in Comal County, Texas, the lower parts of which are in the Glen Rose.
Fossil content
A variety of fossils are found in the Glen Rose, more abundantly in the lower Glen Rose than the upper, including numerous gastropods, clams and echinoids. Many species in the lower portion are not found in the upper portion. Dinosaur tracks have been found in many localities, as well as isolated vertebrate remains. Microfossils also are present, including one of the largest foraminifera ever found.