This geologic map database is compiled from previously published data and new mapping, that represents the general distribution of bedrock and surficial deposits in the Des Moines, WA 7.5-minute quadrangle. The map delineates units that are identified by general age and lithology following the stratigraphic nomenclature of the U.S. Geological Survey. The scale of the source map limits the spatial resolution (scale) of the database to 1:24,000 or smaller.
BASE MAP-The base map for the digital compilation is a Digital Raster Graphic (DRG) of the U.S. Geological Survey, 1:24,000-scale topographic map of the Des Moines 7.5' quadrangle (1995), which has a 25-foot contour interval. The image inside the map neatline is georeferenced to the Universal Transverse Mercator projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map. The base map layer is a digital image, but no information other than location is attached to the lines. The base map is provided for reference only.
We note that on the steep bluffs of the Des Moines quadrangle, map features have commonly been located by elevation. They are thus shown here with a positional accuracy that corresponds to the circa +/- 20 meter (vertical) reproducibility of routine elevation determinations with a pocket barometer. In the upland regions of the quadrangle, contacts between till (Qvt) and recessional and advance outwash (Qvr, Qvrl, Qva) are commonly quite difficult to observe, due to limited outcrop and extensive regrading and paving. These contacts are in many places located with a horizontal precision no better than 100-200 meters.