Terranes of the North Cascades

Little Jack terrane

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

| North Cascades geology home | North Cascades National Park home | | Geologic & Field Trip Maps | Geology field trip | Site contents |

| USGS Geology in the Parks home | NPS Park Geology Tour home | This site is a cooperative endeavor of the US Geological Survey Western Earth Surface Processes Team and the National Park Service. Please share your comments and suggestions with us! parkgeology@den.nps.gov http://www.nature.nps.gov/grd/usgsnps/noca/t8littlejack.html This page was last updated on 11/30/99

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