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Southern California Geology

 

Surficial Geologic Materials in Southern California

 

Surficial materials--geologic materials that have accumulated at the land surface over the last one million years or so--are widespread throughout southern California. These are relatively unconsolidated materials that mantle the ground surface of valleys and hillslopes, or that form the uppermost fillings of valleys and alluvial fans. Typical examples include:

 

  • sand and gravel deposits that occur in river and creek bottoms or that form valley floors;
  • loose rubble that lies on hillslopes;
  • landslides and other slope-failure materials that occur on hillsides;
  • wind-deposited sand dunes and sand sheets.

The Southern California Areal Mapping Project (SCAMP) is developing a uniform scheme to classify and map the myriad different types of surficial materials that occur in southern California. The provisional classification uses three kinds of information to distinguish among geologic-map units of such deposits:

  • physical properties and lithologic features (including consolidation and induration, depositional fabric, grain size, sorting, bed forms, matrix support versus grain support);
  • geologic age (as determined from pedogenic-soil characteristics, desert varnish and pavement, vegetation, degree of incision, radiometric analyses, paleontology, magnetostratigraphy);
  • genesis and geomorphic setting (physiographic setting) and mode of origin (alluvial-fan, slope-wash, pediment-veneer, estuarine, etc.).

Inclusion of geologic origin (genesis) as a classification factor runs counter to the way most geologic materials are classified. With most classifications, descriptive features are used to define similarities or differences among geologic items; highly interpretive and derivative criteria such as genesis are avoided. However, the use of genetic criteria as a basis for the classification and mapping of surficial materials in southern California is inescapable because the materials are forming right before our eyes: we can observe how the natural setting and geologic origin of the materials determines their physical properties. Thus, it is useful to include genetic factors in the differentiation of these materials.

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URL: http://geomaps.wr.usgs.gov/archive/scamp/html/scg_surf.html
This site last updated September 3, 2004 (ps)

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