Most Texas dinosaur tracks are in limestone that formed in shallow water at the edges of an
ocean. Limestone is a type of rock created mostly by living things, such as calcareous algae, corals,
oysters, and clams. Shells and hard limey stems and fronds were broken into sand-sized grains and even mud-sized particles.
The sediment was deposited in the dry or
shallow-water coastal environments where dinosaurs roamed. Storms would blow in and deposit new layers of
sediment on top of the dinosaurs' footprints, burying the prints and preserving them.
Dinosaur tracks are covered and uncovered by erosion and deposition of gravel, sand,
silt, and clay. Fossil hunters love rivers because they cut down through rock layers and expose rock surfaces
that have been buried for millions of years-although once they have exposed the fossils, rivers will continue
to erode, destroying what they have exposed, eventually revealing the layer beneath.
The gravel, sand, silt, or clay that is deposited by rivers is called alluvium.
Alluvium is classified first by grain size and second by what the grains are made of. Gravels have the largest
grain size and in Texas are most commonly made of quartz, chert, and limestone pebbles. Sand and silt are
composed mostly of quartz, and clay is composed mostly of clay minerals.
Limestone and mudstone. One key to good exposure of trackways is layers of soft mudstone on top
of relatively hard limestone.
Photo courtesy of Texas Parks and Wildlife Dept.