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Geology of the National Parks


folded slate and phyllite
Fig. 13. Folded slate and phyllite.

Events Deep in the Earth

Drawings of metamorphic rocks
Fig. 15. Metamorphism changes sandstone to seimischist and shale to slate and phyllite.

Soon after the basalt, sandstone, and shale accumulated, and probably even while they were accumulating, slow movements of the earth's crust began to squeeze and break the rocks. The bedded sandstone and shale were folded and then folded again. In thick masses, bedding was broken, stretched apart, and squeezed along in softer sediments. The thick, strong basalts resisted folding to some extent and they broke in many places. Thin flows of basalt that trailed out into the pile of sediments were broken off in whole chunks and squeezed along in a mush of shale and soft sandstone. Thick masses of hard sandstone were also mixed in†like nuts in pudding. At times when squeezing waned, solutions rich in silica deposited quartz in cracks. The rocks were again folded, and when folding ceased, new cracks formed and once more filled with quartz. Quartz-veined rocks are conspicuous in some areas (fig.14).

sandsone with quartz veins
Fig. 14. Light-colored quartz veins in sandstone west of Graywolf Pass.

During this disruption many of the rocks were becoming harder. The heat and pressure deep in the earth made the old minerals in the sandstone and shale and basalt react with each other and with solutions in the rocks. This metamorphism produced new minerals. The squeezing elongated sand grains, made pebbles into rods, and filled the shales with minutely spaced cracks. As individual sand grains were smeared out, the sandstone developed streaky layering, and some of it became semischist (fig.15). Shale became slate and phyllite (figs.13, 15), and basalt recrystallized to greenstone which, although it is greener, still looks like basalt.

Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

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