[1][2] The San Timoteo Formation represents Pliocene and Early Pleistocene sediments, mostly sandstone.
[2] Fossils of Irvingtonian prehistoric mammals were found in the San Timoteo Formation (El Casco, Mammoth and Shutt Ranch localities).
[2][3][4][5] These include: Fossils have also been recovered from the El Casco Substation locality, which was dated to 1.4Mya.
Recovered specimens include Smilodon gracilis, Homotherium, two ground sloths, two types of camels, a llama, horse, and deer.
Plant fossils include birch, pine, sycamore and oak trees.