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USGS Geology in the Parks

Terranes of the North Cascades Terranes of the North Cascades: Little Jack Terrane

Location of Little Jack Terrane rocks
Location of Little Jack Terrane rocks shown in green.

Summary: Metamorphosed sediments deposited at the toe of a submarine fan derived from a volcanic arc. Contains blocks of metamorphosed mantle.

Another mystery terrane is the Little Jack terrane, which is mostly exposed in the Ross Lake Recreation area. Fine-grained mica schist and layers of fine-grained amphibolite make up the Little Jack terrane. Most of the rock probably began as deep-sea sediments at the toe of a submarine fan. In these metamorphosed sediments are blocks of ultramafic rock; that is, metamorphosed mantle. This peculiar assemblage suggests that as the sediments accumulated, blocks of mantle material, perhaps previously uplifted at a subduction zone, slid off into the ocean. The Little Jack terrane might be metamorphosed rocks of the Methow Domain, except that the Methow rocks contain no ultramafic blocks.

Visit Western Ross Lake Overlook to catch a glimpse of the Little Jack terrane.
On to the Methow Domain

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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