Snow which fills crevasses and wells during the winter often melts out from below, leaving thin snowbridges in the early part of the summer. These constitute real hazards to travel on a glacier because the thinner ones are incapable of supporting a person’s weight. This is one very good reason why the inexperienced should never venture onto the surface of a glacier without a guide.
It is probable that the Park glaciers are not remnants of the large glaciers present during the Ice Age which terminated approximately 10,000 years ago, because it is known that several thousand years after that time the climate of the Glacier National Park region was somewhat drier and warmer than now. Under such conditions it is probable that most, if not all, of the present glaciers could not have existed.
Shrinkage of Park Glaciers
Prior to the beginning of the present century all glaciers in the Park, and most of those in the rest of the world, began to shrink in response to a slight change in climate, probably involving both a temperature rise and a decrease in annual snowfall. From about 1900 to 1945 shrinkage of Park glaciers was very rapid. In other words these glaciers were not in equilibrium with the climate, for less ice was added to them each winter than disappeared by melting and evaporation during the remainder of the year.
Over a period of several years such shrinkage is apparent to the eye of an observer and is manifest by a lowering of the glacier’s surface, and more particularly by a “retreat” of the lower edge of the glacier. This part of the ice is generally referred to as the ice front. When sufficient snow is added to the upper part of the glacier to cause the ice at the front to move forward equal to the rate at which it melts away, the glacier is in equilibrium with the climate. When the yearly added snow decreases in amount the ice front seems to retreat or move back, whereas the mass of the glacier is merely decreasing by melting on top and along the edges, just as a cube of ice left in the kitchen sink decreases in size.
The National Park Service initiated observations on glacier variations in 1931. At first the work consisted only of the determination of the year by year changes in the ice front of each of the several glaciers. From 1937 to 1939, inclusive, the program was expanded to include the detailed mapping of Grinnell, Sperry, and Jackson Glaciers to serve as a basis for comparisons in future years. Aerial photographs were obtained of all the known Park glaciers in 1950 and 1952 and again in 1960. Maps have been compiled and published of the Grinnell and Sperry Glaciers based on the 1950 and 1960 aerial photography. The 1950 and 1960 maps of each glacier are shown on one sheet for convenience in comparison.
Since 1945, the glacier observations have been carried on in cooperation with the U. S. Geological Survey. The work has included the periodic measurement of profiles to determine changes occurring in the surface elevation of Grinnell and Sperry Glaciers and also the determination of the rate of annual movement. Some of the more important data yielded by surveys on Grinnell and Sperry, the two largest glaciers in the Park, are summarized in the following tabulations:
| GRINNELL GLACIER | ||
|---|---|---|
| Year | Area (Acres) | Remarks |
| 1901 | 525 | From Chief Mountain topographic quadrangle map. |
| 1937 | 384 | From mapping by J. L. Dyson and Gibson of lower portion of glacier plus area of upper glacier (56 acres), as shown on 1950 USGS map. |
| 1946 | 336 | As above. |
| 1950 | 328 | From USGS map compiled from aerial photography. |
| 1960 | 315 | As above. |
The Grinnell Glacier originally consisted of an upper and lower portion connected by an ice tongue. This tongue disappeared in 1926 and since then the two portions have been separate. The area of the upper portion of the glacier was essentially the same in 1960 as in 1956—56 acres. The upper section is known as Salamander Glacier because of its shape as viewed from a distance.
The terminal recession of the Grinnell Glacier is somewhat difficult to determine accurately as a part of the terminal portion ends in a lake, the shore of which varies from year to year. The recession for a half-mile section extending southeast from the lake is shown below: