Geologic Setting

The red star indicates the location of Waco Mammoth National Monument between the Brazos and Bosque Rivers.

Drawing courtesy of City of Waco and Dava Butler.

We can better understand what the Ice Age environment was like by studying the geology of the site today. Geology is the study of Earth’s soils, rocks, and minerals. Geologists study three different types of rocks: igneous, metamorphic, and sedimentary. Igneous rocks are formed by lava or magma. Intense pressure or heat makes metamorphic rocks. Sedimentary rocks are created by water and wind. Geologists have only found sedimentary soils at Waco Mammoth, meaning the soil was moved here by water during the Ice Age. The site is located between the Bosque and Brazos Rivers.

The dig site at Waco Mammoth National Monument is a partly excavated riverbed with soils that erode easily. Erosion means “to eat away.” Geologists think the two rivers moved soils from other places and deposited, or dropped them, at the site over time.

This riverbed is located between two ecosystems separated by the rivers. One side of the riverbed has sedimentary soil from the Brazos River and the other side has soil from the Bosque River. These are called paleosols. “Paleo-“means old or ancient and “sol” means soil. Therefore, paleosols are ancient soils and they are different from the soil that covers the ground today. These sedimentary paleosols are four to five meters thick (13 to 16 feet).

The soils from the Bosque River are mostly Blackland Prairie soils and contain a lot of calcium. Soils from the Brazos River side are Blackland Prairie soils as well. However, these Blackland Prairie soils are made of silica, like sand, and have quartzite and chert. Geologists think the two rivers used to meet closer to the site than they do today because minerals from both riverbeds are found in the same place at the site.