LANDSCAPE EVOLUTION OF SANTA ROSA ISLAND, CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA

The intriguing landscapes of Santa Rosa Island, one of five islands comprising Channel Islands National Park in southern California, are largely a product of the combined effects of its climate and tectonic histories. Santa Rosa Island is a hilly, dissected terrain with elevations ranging from sea level to 484 m at Vail Peak in the south-central part of the island. Most visitors arriving at Bechers Bay, in the northeast part of the island, note its broad, elevated coastal plain, an alluvium- and eolian-sand-mantled, uplifted remnant of a last-interglacial marine terrace. Remnants of this and older marine terraces can be found around much of the island’s coastline. The Santa Rosa Island Fault (SRIF), an east-west-trending, left-lateral strike-slip fault with a complex dip-slip component, divides the island into two geomorphically distinct regions. South of the SRIF, the terrain is more rugged, with higher elevations, more local relief, and deeper dissection of the steep-walled, V-shaped valleys. Several of the north-draining streams cross the SRIF and are deflected westward, some by more than 1 km. North of the SRIF, an uplifted early Pleistocene marine terrace is dissected by broad, low-relief, U-shaped stream valleys. These valleys are filled with alluvium as much as 32 m thick that has been incised to form near-vertical-walled arroyos, or barrancas. The aggradation may have been a response to rising base level as sea level rose from its last glacial low of approximately -120 m to its present level. Arroyos that cut through Chumash shell middens and burial grounds indicate that reincision of the streams likely occurred sometime in the past few hundred years. Overgrazing by as many as 100,000 sheep in the 1860s and 1870s, followed by several years with above-average rainfall between 1878 and 1884, is the most likely cause of the incision of the alluvial fill. Extensive dune fields occur at Sandy Point, Carrington Point, and Skunk Point. At present, they are mostly stabilized by vegetation. The dune sands contain a large proportion of carbonate minerals derived from sand-sized skeletal debris of marine invertebrates that was blown onshore during the last glacial period, when submarine shelves were exposed to subaerial erosion.