The park is known for producing myriad mammal fossils dating from the last glacial period. While mammal fossils generate significant interest, other fossils, including fossilized insects and plants, and even pollen grains, are also valued. These fossils help define a picture of what is thought to have been a cooler, moister climate in the Los Angeles basin during the glacial age. Among these fossils are microfossils, which are retrieved from a matrix of asphalt and sandy clay by washing with a solvent to remove the petroleum, then picking through the remains under a high-powered lens.