By Sam Flanagan
The San Andreas Fault is one of the world’s best-known faults, and it passes right through our own backyard. Traversing much of California, the San Andreas forms the boundary between two of the earth’s crustal plates: the North American Plate on the east, and the Pacific Plate to the west. Evidence of its existence is clearly visible along its length, where it offsets streams and gouges out long, linear valleys. In some places, such as the Carrizo Plain in southern California, Hollister and Tomales Bay, it appears as a great fissure in the Earth. Even near Shelter Cove, the fault is visible as a north-south trough cutting through the hillside. Shelter Cove Road drops into this small hollow before turning west and dropping down onto the coastal terrace. At Deadman Gulch, about a mile or two in from the coast, the fault heads south from Shelter Cove Road into the ocean before re-emerging on land near Point Arena. However, north of Shelter Cove, the picture is not as clear.
Efforts to map the extent of the San Andreas began shortly after the great San Francisco earthquake of 1906. Working northwards along the fault, a state-appointed earthquake commission documented spectacular evidence of freshly ruptured ground due to movement along the fault. In all, the fault ruptured along 477 kilometers (296 miles) of its length from Shelter Cove in the north, to near San Juan Bautista in the south, just east of Monterey Bay. At its northern end, the commission noted surface rupture near Shelter Cove to near Kahluna Cliff that was formed by a massive landslide in the southern King Range. Further north, ranchers noted fissures and landslides in the King Range, but the sites were not visited by the commission and could well be a product of the shaking rather than localized fault movement. Beyond that, no evidence of rupture was found, although the terrain becomes nearly impassible in places. So, the question remains, where does the fault go north of Shelter Cove?
Several hypotheses exist on the ultimate fate of the San Andreas north of Shelter Cove. Some researchers have suggested it extends offshore south of Big Flat, running along the Lost Coast, before turning to the west. Others have suggested that the San Andreas splinters out - dispersing its movements along many smaller faults that finger into the rugged King Range. Still others have suggested that the fault extends inland, wraps around the east side of the King Range before turning west and entering the sea near Cooskie Creek in the northern section of King Range, six miles south of the mouth of the Mattole River.
Whatever the case, the result is a region highly fractured, deformed and faulted from complex crustal movements. Odd linear features such as Bear Creek in the upper Mattole River watershed and Whale Gulch suggest crustal movements parallel to those seen on the San Andreas. If the fault extends offshore, it is too close to shore for traditional ocean survey vessels to observe. Similarly, an inland extension of the San Andreas would traverse some of the area’s most rugged terrain. Until more evidence is uncovered, the San Andreas remains lost along California’s Lost Coast.
Sam Flanagan is a Geologist who works with the Bureau of Land Management out of the Arcata Field Office.
This article is part of a series about Natural Life of the Lost Coast, a project of the Lost Coast Interpretive Association. If you are interested in contributing an article to this bi-weekly column about the plants, animals and human history of the Lost Coast Region, contact Cathy Miller at firstname.lastname@example.org.
Shown here is fault scarp passing through Wood Gulch Shelter Cove. Following the April, 1906 San Francisco earthquake, the State Earthquake Investigation Commission documented surface rupture along the San Andreas fault, shown in this photo at the head of Wood Gulch near Shelter Cove.