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North Cascades Geology

Sedimentary Rocks

Bedded argillite and thin beds of sandstone of the Methow Ocean, north of Devil’s Pass.
Sedimentary rocks (bedded argillite and thin beds of sandstone) of the Methow Ocean, north of Devil’s Pass.
spacer image Debris carried off the continents and into the oceans by streams and rivers is the most common material that makes up sedimentary rocks. Mud becomes mudstone or shale, sand becomes sandstone, and gravel becomes conglomerate (evidently the term "gravelstone" never caught on).
spacer image Some sedimentary rocks, however, are not of continental origin but are mostly derived from the skeletons of ocean animals. Accumulations of sea shells or the bodies of tiny calcareous plankton become limestone, made up of the mineral calcite (calcium carbonate). Siliceous plankton (known as radiolarians) become chert, a significant North Cascades rock made up of the mineral quartz. Plant debris becomes coal. Sediments become rock when buried, subjected to pressure, and cemented by mineral precipitates. Geologists, somewhat casually, refer to any hard, dark fine-grained sedimentary rock like shale or mudstone as argillite. Parts of the North Cascades display almost all the named varieties of sedimentary rocks.

On to Metamorphic Rocks


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This page was last updated on 11/30/99
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Material in this site has been adapted from a new 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