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North Cascades Geology

Extension of the crust

Unconformity.
Unconformity between older folded beds and younger flat-lying beds, deposited on top.

On top of, and in strong contrast to, the strongly deformed and metamorphosed rocks found in the Western and Metamorphic Core Domains are younger (55 to 40 million years old), unmetamorphosed, sandstones and conglomerates. Most of these sedimentary rocks were deposited by streams or rivers flowing across the older rocks.

Sandstone deposited by streams during the Eocene extensional event.
Sandstone deposited by streams during the Eocene extensional event. Geologist is looking at fossilized tree trunk in Chuckanut Formation on Bacon Peak.

Where young bedded rocks overlie eroded older rocks, geologists call the contact an unconformity; such a break generally indicates a time break in the geologic record-some missing tape. Simple uplift and erosion of sediments before deposition resumes may produce an unconformity-only a few million years of sedimentation missing-or the older beds may have been deeply buried, folded, metamorphosed, uplifted, eroded, and then submerged in the ocean for burial under younger sediments-a profound unconformity with maybe tens of million years of record missing.

Experts who study sedimentary rocks (sedimentologists) have concluded that the younger sandstone and conglomerate found unconformably on the older rocks of the North Cascades were deposited very rapidly, probably in down-dropping fault valleys or basins. To create these depressions, geologists believe the crust must have been stretching or extending. For this reason we call these sandstone and conglomerate accumulations extensional deposits. The extensional deposits erode rapidly and are only preserved in somewhat special circumstances. In the area of this guide, extensional deposits of sandstone and conglomerate overlie older rocks southwest of the town of Glacier and in a few other places. (Visit the geologic map to see where).

Something extra: Forming depressions along faults

On to Caulking the Cracks
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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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