7.2 Geology

The geology (lithology) variable identifies generalized rock type and is present for all coastal grid cells and line segments within this data base. The geology data were derived from state geologic maps ranging in scale from 1:250,000 to 1:1,000,000 with publication dates from 1968 to 1992 (maps used are listed in Section 13.2). The geologic data were classified in terms of an ordinal scale based on the relative hardness of minerals comprising the rock. This geologic classification system was adapted in part from one used by Dolan et al. (1975). It contains 5 major groups further subdivided into 21 subgroups (Table 1).

Appendix B contains a glossary of the terms used in Table 1. This ranking scheme is generalized;consequently, a wide range of erodibilities exist for each rock type listed. The erodibility of each rock depends upon the mineral content, cementation (especially for sedimentary rocks), grain size (for unconsolidated sediments), and presence of planar elements (i.e., bedding, schistosity, cleavage, and fractures) within the rock. The key discriminant between the individual classes identified in Table 1 is the relative resistance of each rock type to physical and chemical weathering.

The geology data were assembled as follows:

  1. Enlarged maps of the 1:2,000,000 digitized U.S. West Coastline included in this NDP were plotted in small sections (i.e., approximately 5° latitude by 5° longitude on 3 ft2 paper).
  2. Polygons (boxes) were drawn around each coastal segment as identified by state geologic maps (listed in Section 13.3).
  3. The hand-drawn polygons were digitized into ARC/INFO using the 1:2,000,000 digitized U.S. West Coastline coverage included in this NDP as a backdrop.
  4. These polygons were then overlaid onto the backdrop coverage with the ARC/INFO IDENTITY command whereby the coastal segments took on the values of the polygons.
  5. For the gridded data groups, a 0.25° latitude by 0.25° longitude grid was overlaid onto the 1:2,000,000 digitized line coverage using an additional ARC/INFO IDENTITY command.

Each grid cell took on the geology code from the line segment with the greatest total length -as illustrated in Fig. 4 (e.g., four line segments in a cell with lengths and geology codes of 100 km, 350; 80 km, 300; 120 km, 310; and 50 km, 300, would yield a cell value of 300).

Appendix C gives a breakdown of the geology codes that occurred within each 0.25° by 0.25° coastal grid cell. The geology codes listed in Appendix C are identical to those found in the line segment-based data groups within this NDP.

In general the bed rock geology of the West Coast consists of five distinct zones. The coastal areas of Washington and Oregon consist of exposed basalts and sedimentary rocks that have been folded and metamorphosed. These rocks were thrust beneath less disturbed Tertiary and sedimentary and volcanic rocks during the Cenozoic uplift. In southern Oregon and northern California four overlapping thrust sheets of volcanic and sedimentary rocks have been intruded by granitic and ultramafic rocks. These sheets are upwarping and in some areas are volcanically active due to the collision of the Pacific and Farallon Plates with North America. This collision is one of the driving forces that has produced the uplift and deformation of the Coast Ranges in northern California. The San Andreas fault, and several others, traverse the Coast Ranges in the north, through the Transverse Ranges in central California, and into the Peninsular Ranges in the south. The Peninsular Ranges are composed of Paleozoic and Mesozoic granitic and metamorphic rocks while the Transverse Ranges are composed of Tertiary sedimentary rocks. The Los Angeles basin is filled with Quaternary sediments (Muhs et al. 1987)