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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Rocks of the Olympic Mountain Range - Sandstone, Shale, and Conglomerate

photo of bedded sandstone with structures
Fig.6. Well-bedded sandstone and shale, with stuctures showing lumpy settling of heavy sands into muds and worm borings (circular light-colored blobs in topmost shale bed).

Sandstone

Sandstone forms from sand†small grains of mineral and rock eroded from pre-existing rocks (fig. 5). Usually the grains can be seen easily: glassy grains of quartz, white grains of feldspar, shiny black or clear flakes of mica, and green or black grains of rock or iron- and magnesium-rich silicate minerals. It is obvious that sand grains are carried to the sea by streams and rivers, but for a long time geologists puzzled over how sand could travel through quiet water into the deep ocean. The answer came from careful study of deep-ocean sands and laboratory experiments. Features in sandstone beds indicate that the sand was swept into deep water in a slurry of sand, silt, and water that flowed along the ocean floor. Such slurries, called density currents, maintain their identity and flow downhill under the ocean just as streams of water on land maintain their identity and flow under a sea of less dense air. Some sandstone beds deposited by density currents are so uniform and structureless that they look as though they were delivered to their resting places all at once. Other beds are graded, the coarsest sand at the bottom, the finest at the top. Gentle ocean-bottom currents may pick up and redeposit upper layers of the sand, producing still other varieties of bedding. In addition, marine worms and other boring creatures may leave their tracks on and in the beds. A variety of deep-water events are recorded in these sand beds and can be recognized millions of years later (fig. 6)

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Shale

Shale is rock made from mud. Olympic shales are commonly made up of clay minerals, quartz, feldspar, and micas (fig. 5), although the particles are usually too fine to be seen easily. In contrast to coarser sand, these fine particles were swept into deep ocean water and suspended by ocean currents. The particles settle out of suspension slowly and can drift far from shore before deposition. Whether or not this fine material becomes a shale bed†and if so, how thick it is†commonly depends on how much material rains down and how much time passes between slurries of sand (fig. 7). In some places the sand flowed out at very regular intervals, giving rise to a rhythmic alternation of shale and sandstone beds. Transformation of the mud and sand into shale and sandstone takes millions of years of deep burial. The weight of overlying sediments squeezes out water and presses the grains tightly against each other. Minerals such as calcite, limonite, and quartz precipitate from water between the grains to cement them together. Eventually, the sediments become rock.

Sandstone beds with thin shale interbeds on Windfall Peak
Fig. 7. Sandstone beds with thin shale interbeds on Windfall Peak.

Conglomerate and sedimentary breccia

Occasionally while Olympic sands and muds were accumulating on the ocean bottom, a particularly thick slurry would carry rounded pebbles and cobbles originally from the continent into the deep ocean.. Time and pressure have bound these gravels into conglomerate (fig. 5). At other times, as a slurry rushed along the ocean bottom, it ripped off chips of partly consolidated mud and deposited them nearby. Shale-chip and slate-chip breccias, as these deposits of angular fragments are called, make striking spotted outcrops in many areas of the Olympics.


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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