It is of nonmarine origin, consisting predominantly of sandstones and conglomerates laid down in a riverine, shoreline, and floodplain environment between the upper Eocene Epoch (around 40 million years ago) through the lower Miocene.

Since many of its sandstones are more resistant to erosion than many other regional sedimentary units it often forms dramatic outcrops and ridgelines in many local mountain ranges.

[13][17] Numerous vertebrate fossils have been found in the Sespe, with the principal locations of the finds north of Simi Valley in Ventura County.

[18] In some places, certain high-porosity sandstones within the Sespe unit have been deformed into anticlinal structures, and contain considerable amounts of petroleum and has produced 400 million barrels (64,000,000 m3) of oil in Ventura County, California.

Within Southern California, the combined Sespe-Vaqueros coarse-grained clastic rocks form the second-most important petroleum-bearing unit, second only to the Pliocene-age Pico and Repetto Formations, which are usually much nearer the surface.