Glaciers

Glaciers have been an important feature of the earth's landscape during the present Cenozoic era, which spans about 65 to 70 million years. Cooling near the end of the era began in the Pliocene epoch, and mountain glaciation in the western United States occurred somewhere between 1 and 2 million years ago. The Cenozoic era is divided into the Tertiary and Quaternary periods, with the most recent glaciation occurring during the Quaternary period, which covers approximately one million years before the present (BP). This period is divided into the Pleistocene and Holocene or Recent epochs. During the Pleistocene, four major glacial advances and three interglacial stages occurred. The Holocene or Recent epoch spans a period of approximately 11,000 years BP. During this time, the fourth inter-glacial stage or a temperature increase has been occurring, including melting ice and a rise in sea level. This may last for tens of thousands of years, and may be followed by another future period of cooling and the growth of glaciers.

Prior to this present age of Pleistocene glaciers, there were at least five other known glacier periods, dating back to two occurrences of glaciation in Precambrian times. Toward the end of the Permian period glaciers appeared again, and they [Page 592]were rather widespread. During the early and late portions of the Cretaceous period glaciers appeared once more. The occurrence of glaciers is evidently a cyclical phenomenon, but the precise cause of their appearance is not known.

During the Pleistocene glaciers were widespread, covering both polar regions and extending outward a considerable distance, covering approximately 30% of the earth's total land area. In the present Holocene epoch, the earth is witnessing a global warming as well as a global dimming: Both of these appear to be antagonized by increased human interference. All glaciers are receding, and have been for thousands of years, but at seemingly a much faster rate now than when they were flowing forth. Consequently, the sea level rose at an average rate of about 6 inches per 100 years during the Holocene. A noticeable increase has been noted during the last several hundred years, however, and this is expected to continue.

In the 19th century, Louis Agassiz, like James Hutton and other scientists in the century before, asserted that the large boulders in the valleys were not placed there by Noah's flood, but were once plucked by erosion out of the advancing ice, sometimes a mile or more deep, and then deposited by a receding ice flow. Agassiz's views were initially met with skepticism and hostility by most of the audience at a meeting of the Swiss Society of Natural Sciences; widespread acceptance came only gradually, later on.

Glaciers are commonly defined as either continental or mountain types. Continental glaciers begin in an area with polar temperatures and extremely large accumulations of snow. There were three areas in the northern hemisphere where this most strikingly occurred during the Pleistocene; in Europe over the northern Baltic Sea and much of Norway, in North America over Hudson Bay and north central Canada, and over northern Siberia. Not to be overlooked are the general expanse of the Arctic icecap including Greenland and other parts of the northern hemisphere, as well as the massive Antarctic continent in the southern hemisphere. These continental glaciers appear to be located on or very near shields consisting of Precambrian igneous and metamorphic rock. with regard to mountain glaciers, the

...