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USGS Geology in the Parks

Geologic story of the North Cascades button

North Cascades National Park:
Patterns in the Rocks: Geologic Maps

North Cascades geologic map
North Cascades geologic map. Click on image to open to full size (170 kb).

The major domains and faults in the North Cascades could not be recognized until geologists had plotted their observations on at least a rudimentary geologic map. To make a geologic map, a geologist must look at the rocks and note where they are, as well as determine what they are. He or she uses simple tools: compass, hammer, hand lens, pencil and map. He must look at as many rock outcroppings as possible and may travel by car, by horse, by helicopter or, most commonly, by foot, often cross country. He looks for fossils; he looks for the contact between different rock types; he notes deformation in the rocks: have originally planar sedimentary beds been folded? Or are the rocks crushed by fault movement? He collects samples to study back in the laboratory.

Geologist marking observations
Geologist marking observations on the map.

Marking observations on a map, as the geologist goes along, is essential because the geometric relationships revealed on the map express the time relationships among the various rock units being studied, relationships that are the essence of their life story. In the process of piecing together the geologic story, geologists can use a partly completed map as a useful guide to areas most critical to visit next. Conversely, the geologic history developed in the course of mapping is usually needed to complete the map, for large parts of most map areas are typically covered with soil, trees, buildings, and other elements that obscure the underyling rock.Or a region is inaccessible, and the geologic history is used to extrapolate beyond the small fraction of the map-area that has been directly observed. The extrapolation from isolated outcrops to a complete map usually depends strongly on the geologist’s understanding of general geologic principles and the Earth history. As a result, maps become obsolete as our understanding of geologic principles evolves.

Of great importance to the map-making endeavor is a good topographic map of the area. Although Reginald Daly had a limited topographic map along the 49th parallel, made by the Boundary Commission, other early workers in the North Cascades did not have adequate maps. Peter Misch and his students had a few good maps on the edge of the mountains, but good topographic maps of the whole range were not available until the early 1970s, after Misch and his students had finished their work.

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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