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North Cascades Geology

Domains of The North Cascades

The early studies of Peter Misch and his students established a general architecture for the North Cascade’s foundation. The range is sliced by two major faults separating the rocks into three distinct blocks. Faults are significant crustal cracks where the rocks on each side have moved relative to each other. The western fault is the Straight Creek Fault; the eastern, the Ross Lake Fault. Because the Ross Lake Fault consists of many fault strands, it is sometimes called the Ross Lake Fault System.

Volcanic cover Methow Domain Metamorphic Core Domain Western Domain Three major geologic domains of the North Cascades
Three major geologic domains make up the North Cascades: the Western, the Metamorphic Core, and the Methow Domains. Heavy lines are major faults.

For convenience in this website, we call the three faulted blocks domains. From west to east they are the Western Domain, the Metamorphic Core Domain, and the Methow Domain.

The Western Domain mostly consists of sedimentary and volcanic rocks and, although these rocks are complexly faulted and folded, they still retain textures and structures, such as sedimentary bedding, that are typical of their origins on the surface of the Earth. Many of these rocks contain fossils.

The central, Metamorphic Core Domain comprises highly squeezed and recrystallized metamorphic rocks which were once at great depths in the Earth’s crust. Their story is complex. If they ever had fossils, the fossils have been destroyed by the metamorphism, and how these rocks originally formed can only be inferred from their compositions and vague relicts of their original structures and textures.

The Methow Domain is essentially unmetamorphosed sedimentary and volcanic rocks. They are less complex than rocks in the other domains and have fossils, in some places abundantly.

Something extra: Patterns in the Rocks: Geologic Maps

On to Plate Tectonics and Tectonic Terranes
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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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