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M any coastlines around the world have stair-step landforms, known as marine terraces. Marine terraces make up a large part of coastal California’s landscape—from San Diego to Crescent City. Find out how these landscapes form, why marine terraces are of interest to scientists, and where you can explore these landscapes. Marine terraces result from the interaction of two geologic processes: uplift of the land surface and the natural rise and fall of sea level over hundreds of thousands of years. As sea level rises, waves move underwater sediment—sand and gravel—back and forth against bedrock, acting like sandpaper to hone bedrock into flat, broad platforms. When sea level falls, wave-cut surfaces are exposed above water. Earthquakes on California’s coastal faults, along with other processes that deform the Earth’s crust, cause uplift of the land surface. When uplift is fast enough, it causes wave-cut surfaces to be preserved as marine terraces above the influence of the ocean. Where land is not uplifted high enough, wave-cut surfaces will be flooded by the rising sea. It is important to note that high marine terraces seen along the coast do not represent past shorelines at that elevation, rather, they record ongoing uplift that occurs simultaneously with fluctuating sea level. The crust steadily moves upwards and high sea-level stands erode notches into the uplifting terrain.

The northern segment of the San Andreas fault last ruptured in 1906, producing the great San Francisco earthquake. This study involves the collection and interpretation of geologic data from the segment of the northern San Andreas fault near Point Arena, California, to determine the recurrence interval and slip rate of this segment of the fault. Holocene sediments deposited on an alluvial fan preserve a record of prehistoric earthquakes near Point Arena, California. Excavations into the fan provided exposures of the sediments across the San Andreas fault zone. At least five earthquakes were recognized in the section. All of these occurred since the deposition of a unit that is approximately 2000 years old. Because deposition in this setting was intermittent and deposition of younger units involved the erosion of underlying units, it is likely that events occurred that were not recorded in the section. Radiocarbon dating of units in the section allows constraints to be placed on the dates of the earthquakes recognized. A buried Holocene (2356-2709 years old) channel has been offset a maximum of 64 ± 2 meters. This implies a maximum slip rate of 25.5 ± 2.5 mm/yr. These data suggest that the average recurrence interval for great earthquakes on this segment of the San Andreas fault is long - between about 200 and 400 years. Offset marine terrace risers near Point Arena and an offset landslide near Fort Ross provide estimates of the average slip rate across the San Andreas fault since Late Pleistocene time. Near Fort Ross, a landslide has been offset approximately 1.7 km across the San Andreas fault. Radiocarbon analysis of charcoal from this deposit indicates that the landslide is older than 43,700 years. This implies a slip rate of less than 39 mm/yr. Correlation of two marine terrace risers across the San Andreas fault near Point Arena suggests offsets of approximately 1.5 and 2.5 km. The U-series age of a solitary coral, altitudinal spacing and corre

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-5. Period covered: July 1, 1977-June 30, 1978.

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-6. Period covered: July 1, 1978-June 30. 1979.

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-9. Period covered: July 1, 1981-June 30. 1982. Wildlife research.

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-8. Period covered: July 1, 1980-June 30. 1981. Wildlife research.

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-7. Period covered: July 1, 1979-June 30. 1980.

Idaho Dept. of Fish and Game Wildlife Div. report W-160-R-4. Period covered: July 1, 1976-June 30, 1977.