GEOLOGIC MAPPING
A geologic map shows the distribution of materials that underlie the
surface of the Earth. These materials are classified by their physical
character (kind of rock, size of sediment, and so on), the process that
created them, and their age. They are not the same as soils maps,
which were originally developed to identify suitable areas for growing
crops. Instead, geologic maps “look below” the upper few feet of
vegetation, buildings, and the soil to describe the underlying layers.
A geologic map—especially here in tree-covered western Washington—is
rarely a straightforward description of what the geologist sees. One
simply can't see enough! Most geologic maps combine observations and
highly educated inference.
One of the most important aspects of
understanding geologic conditions, and identifying and mitigating geologic
hazards in the Seattle area, is being able to determine the distribution,
character, and age of the various glacial and nonglacial deposits and
bedrock. In our heavily developed and lushly vegetated landscape this is
a formidable task. Yet obtaining a good understanding of this information is critical for
determining the recurrence intervals for earthquakes, predicting zones of
groundwater occurrence and contamination susceptibility, identifying areas
of particularly high landslide or erosion hazard, and recognizing sites
where low bearing strength or liquefaction may be problematic.
