37 Volume 16 – 1950


Volume 16, 1950

All material courtesy of the National Park Service.These publications can also be found at http://npshistory.com/
Nature Notes is produced by the National Park Service. © 1950

A Return of the Ice Ages

By Franklin C. Potter, Ranger-Naturalist

Mt. Mazama and Its Glaciers
from a Painting by Paul Rockwood

In the flatlands of eastern North America continental glaciation occurred on a large scale in recent geological times. Snow fell in such quantities that summer melting failed to keep pace with winter accumulation and eventually glaciers resulted. Moving in all directions from their source in central Canada, they invaded the area of what is now the United States as far southward as the present location of the Ohio and Missouri Rivers. A time of lighter snowfall or increased temperatures resulted in a wasting away of the glaciers until they had entirely disappeared. This cycle was repeated a number of times; the last of the four continental glaciers – the Wisconsin – wasted away some 22,000 years ago.

In the higher mountains such as the Cascades local valley glaciers existed rather than the ice sheets which completely mantled the lower lands to the eastward. The evidence of glacial drift separated by layers of volcanic rock indicate, however, the same response to climatic variation. In the mountain areas where valley glaciers still exist the present trend of the glaciers is toward smaller size. Annual measurements in Glacier and Mount Rainier National Parks demonstrate that summer wastage exceeds winter accumulation so that year by year the glaciers decrease in size.

Supporting data of decreased precipitation or increased evaporation is contributed by numerous lakes in the western areas of the United States. During recent geological times a larger Great Salt Lake covered much more area than its shrunken remnant. At its maximum size, its predecessor, “Lake Bonneville,” was 1000 feet deeper and overflowed its basin on the north into the Snake River: it was undoubtedly a fresh water lake. At the same time Lake Lahontan in Nevada covered a large area; its existing remnants are Walker, Pyramid, and Humboldt Lakes. Death Valley also supported a lake in recent geological times.

Accurate weather records have been kept for such a short time that they can not reveal long time trends. Short cycles of temperature and precipitation, influenced no doubt by sun-spots, are known, but we must turn to the geological record for the longer trends. If the present trend continues, we should expect our valley glaciers to continue to decrease in size until most of them have disappeared. Likewise the semiarid end arid portions of the western United States would increase in size and aridity. How far this cycle will continue is, of course, problematical. Higher areas such as the Cascades should continue to receive more precipitation than the lowlands even though there might be some decrease of winter snowfall. At least a few of the mountain glaciers should persist. The prediction of the future is further complicated by the fact that transition from the culmination of one age to the next is not constant but has many irregular variations.

Although 22,000 years seems a long time, in the geological story it is but a moment. The climate seems to be becoming warmer and drier, but may not there be a reversal toward colder and wetter conditions again sometime in the future? Each of the intervals between the ice sheets of the Ice Age was greatly longer than the 22,000 years that have elapsed since the Wisconsin sheet withdrew. From interglacial deposits in the Don River valley near Toronto fossils of both plants and animals that now are found no farther north than Missouri and Kentucky have been found. The climate of the northern United States and southern Canada may well continue to ameliorate for thousands of years to come.

Before we postulate a possible continuance of the Ice Age with the formation of another continental glacier in the East and numerous valley glaciers in the mountains, it is well to inquire into the causes of Ice Ages. Each of the times of extensive glaciation in the geological past has coincided with a time of great mountain building although the exact mechanics and the explanation of the four separate ice advances of the Pleistocene Ice Age are unknown.

One of the great mountain-making epochs of the earth is the present. Perhaps the lands of the earth still are high enough for another reoccurrence of glaciation on a large scale. If so, however, our climate undoubtedly will continue to get warmer, and in some areas drier, for some tens of thousands of years before a reversal of conditions occurs. And even if a fifth stage does occur, the Crater Lake National Park area and Mount Mazama will not be affected as much as they were in the past. Mount Mazama was high enough to support glaciers comparable to those of Mount Rainier of today. But now Mount Mazama has lost its higher elevations and the return of glacial conditions could produce no more than a few small glaciers. Mount Scott supported a large enough glacier to excavate the northern segment of the mountain and with a return of extensive glaciation it undoubted!, would support another. The only other possibilities seem to be for small, isolated glaciers that would occupy very limited areas since most regions in the Cascades with elevations no higher than those in the park did not support glaciers in the past and there is no reason to suppose that they would in the future.