The Dinosaur Valley track impressions were made on broad, shallow tidal flats of sticky, limey mud. As tides acted on the imprints, they slowly filled with clay and silt from a different river-borne material brought down from upland areas. The track layer became hard limestone, while the filler layer became either soft shale or a brittle rock called lime marl. Over time, erosion removed the softer filler layer and exposed molds of the animals' feet. The process began when an animal walked across soft, moist, cohesive sediment, continued as the imprint filled with sediment of contrasting texture without distorting the original and both layers hardened, and ended as differential erosion removed the softer rock type.