Unlike any other Northern Channel Island, Santa Rosa Island is blanketed predominantly with gentle rolling hills and grasslands. High mountains with deeply cut canyons are limited. The northeastern shore consists of well developed marine terraces where cliffs abut the ocean. The southern sector of the island is steeper and more rugged. The island's coastline is quite variable, with broad sandy beaches on the northwest, northeast and southwest.

The structure of Santa Rosa Island is that of an uplifted block deformed by folding and faulting. This island has a larger area of Tertiary sedimentary non-volcanic rocks than any other Channel Island. Tertiary sandstones, siltstones, shales and volcanics on the northern half of the island are covered with a thick layer of Quaternary (Pleistocene Era) deposits. The oldest Cretaceous rocks are on the south side, along with a complex series of Eocene, Oligocene, and early Miocene shale and sand stones interbedded with early Miocene volcanics, consisting of basalts, breccias, and conglomerate rocks.