The second largest park island is Santa Rosa. Cliffs on the northeastern shore rival those of Santa Cruz Island. High mountains with deep cut canyons give way to gently rolling hills and flat marine terraces. Vast grasslands blanket about 85 percent of the island, yet columnar volcanic formations, extensive fossil beds, and highly colored hill slopes are visible. Rocky terraces on the west end provide superb habitat for intertidal organisms including astounding concentrations of black abalone.
Harbor seals haul out and breed on the island's sandy beaches.
On the eastern tip of the Island, a unique coastal marsh is among the most extensive freshwater habitats found on any of the Channel Islands. The entire island is surrounded by expanses of kelp beds. Consequently its surrounding waters serve as an invaluable nursery for the sea life that feeds larger marine mammals and the sea birds that breed along the coastal shores and offshore rocks of all the Channel Islands. Beneath Santa Rosa's non-native grasslands are the remains of a rich cultural heritage. More than 180 largely undisturbed archeological sites have been mapped.