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Geology of the National Parks


Glaciers: The Heavier Hand (cont.)

At least six times during the Ice Age, the Puget lobe of the Cordilleran ice sheet crept down into the Puget Sound area. The ice dammed up against the Olympics and split into two branches: one branch flowed out the Strait of Juan de Fuca to the sea, and the other flowed down the east side and part way around the southern end of the mountains (fig, 34). The ice was at least 3,500 feet thick on the northern edge of the mountains, where it smoothed down the foothills and gouged out the peripheral valleys. Furthermore, the Cordilleran ice, loaded with rock debris picked up in Canada, dropped foreign rocks or erratics, such as granite, gneiss, and schist, all around the northern, eastern, and southern sides of the range.

Canadian Ice Sheet in Puget Sound
Fig. 34. The Puget Lobe damed up against the Olympic Mountains on the north and east. After Ralph Haugerud and Harvey Greenberg, "Reconstructing the last (Vashon) continental glaciation of the Puget Lowland". Click here to see quicktime animation.

In the earlier part of the Ice Age, the Olympic massif may have been almost covered by its own cap of ice. That part of the geologic record is unclear. It is known that while the Cordilleran ice sheet was stretching southward, snow piling deep on the high peaks of the Olympics grew into glaciers that descended most Olympic valleys at least four times. These glaciers gouged out the stream valleys, and melt water deposited tremendous amounts of outwash gravels around the west and south sides of the mountains.

The alpine glaciers on the northeastern side of the range melted back before the last retreat of the Cordilleran ice. The valleys, thus dammed by the ice sheet, were filled with fiordlike lakes. (CLICK for a view of ancient Lake Elwha). Icebergs that broke off the ice sheet floated into the interior of the mountains. When these bergs melted, they dropped erratics far up the drowned valleys.

Deciphering  the complex history of alpine glacial advance and retreat employs many scientific disciplines. Geologists such as Dwight Crandell of the United States Geological Survey have sorted out moraines and glacial outwash on the Hoh, Queets, and Quinault rivers. Old moraines can be told from younger ones because their boulders and cobbles are more deeply weathered. The ages of young moraines may also be determined by counting the annual growth rings of the trees on them. Experts examine fossil pollens in layers of mud and silt that were deposited in ancient bogs. Professor Calvin Heusser of New York University has been able to estimate the abundance of different plants present on the western Olympic Peninsula at various times of glacial advance and retreat. By comparing these abundances with those of presentday plant communities at sites ranging from the Olympics to the Arctic, he has been able to describe the changes from tundra to forest and back again as the glaciers waxed and waned and has estimated the average July temperatures, which have been only three to eleven degrees (Fahrenheit) colder than today. Students from the University of Washington have detailed the advance and retreat of the glaciers of the Queets, Quinault, Humptulips, and Wynoochee, as well as other drainages.

>Olympic alpine glaciers are not only noted for their beauty; they have also contributed much to man's understanding of his environment. Glaciologists have been watching changes in the Blue Glacier on Mount Olympus and its relationship to climate for many years. Professors Robert P Sharp, Barkley Kamb, and their associates have measured the pulse, form, and flow of the lower Blue Glacier. Professor Edward LaChapelle and his group have dissected the Snow Dome on the upper Blue. The research teams have determined the ways in which snow accumulates and ice melts, how the glacier flows, and even variations in the types of oxygen (isotopes) making up the ice. For instance, the Blue Glacier moves by internal flow†that is, slippage on planes in the ice crystals themselves†and by slippage along the bottom. There is more flow in the center of the glacier than at the bottom and along the edges. Differences in flow velocity stretch the brittle ice near the surface and it cracks, forming crevasses (fig. 35).

Black Glacier
Fig. 35 Crevasses on the Black Glacier, north side of Mount Olympus, where the ice flows down a steep gradient.

The world became warmer than it is today after the last ice advance in the Puget Sound area about thirteen thousand years ago. Some geologists say that this warm period was a minor fluctuation and that we have not yet emerged from the Ice Age because the earth is still somewhat iced, as shown by the Antarctic and Greenland ice caps, the frozen North Polar Sea, the numerous small glaciers sprinkled about many mountain ranges, and the wide expanses of permanently frozen ground in Canada, Alaska, and Siberia. During the warm period the northern ice sheets and most of the alpine glaciers disappeared, although the largest and highest†such as the Blue and Hoh glaciers†may have survived. Present-day Olympic glaciers apparently were born about twenty-five hundred years ago and reached their maximum growth only about two hundred years ago, evidence of a current cool episode. Since then they have been steadily retreating, but whether this present warming is a long-time trend or whether a full-scale ice age will reoccur is unknown.

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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