Shaded-Relief Image of Santa Monica Margin

[Shaded Relief of Los Angeles Margin]

 Bathymetry was mapped at:

  • 4-m grid spacing from the 20 to 40 m isobath
  • 8-m grid spacing between the 40 and 100 m isobath
  • 16-m grid spacing >100 m depths

The mapping revealed five dramatic submarine canyons. Two of them (Dumeand Redondo Canyons) head in very shallow water (less than 10 meters) whilethe other three (Santa Monica, San Gabriel, and Newport Cantons) head ata depth of about 100 meters.

Santa Monica, San Gabriel, and Newport Canyons are products of riversthat coursed out onto the continental shelf during periods of low sea levelswhen ocean waters were locked up in the great ice caps of North America,Greenland, Europe, and Antarctica. Only 15,000 years ago, sea levels stoodas much as 120 meters lower than today. During these periods of low sealevels, the coastline in the Los Angeles region was located where wateris now 120 meters deep. Consequently, during these periods, coastal riversreached the coast at the same locations as the Santa Monica, San Gabriel,and Newport canyons head on the continental shelf.

The origins of Dume and Redondo Canyons are more problematical, althoughtheir meandering form tells us that they probably do not follow a fault.But, most importantly, the heads of these two canyons intercept littoraldrift (a river of sand that flows towards the east and south on the innermostshelf) and probably divert the sediment downcanyon to the deep basin beyondthe margin. The major sewage outfalls of the Los Angeles area are pipedbeyond the littoral drift and are directed in the vicinity of Santa MonicaCanyon.

The continental slope south of the Palos Verdes Peninsula is carved bynumerous canyons, channels, and slump scars. It appears from the new mapsthat this slope is much more prone to failure than the slope directly westof Los Angeles or south of the Malibu Mountains.

The shaded-relief image of the land surrounding Santa Monica Bay wasconstructed using USGS 7.5 minute DEMs (Digital Elevation Models).

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