Glaciers and Glaciation in Glacier National Park

by James L. Dyson

One cannot remain long in Glacier National Park without noticing the varying colors of its lake waters. In fact this feature is so striking that ranger-naturalists probably are questioned more about it than about any other feature or phenomenon. To find the answer we must go again, as in so many instances, to the glaciers. As the ice moves it continually breaks rock fragments loose. Some of these are ground into powder as they move against each other and against the bedrock under the glacier. Most types of rock, especially the limestones and shales on which the Park glaciers rest, when ground fine enough yield a gray powder. All melt-water streams issuing from glaciers are cloudy or milky from their load of this finely ground “rock flour.”

Water from Grinnell Glacier is so laden with rock flour that the small lake along the edge of the ice into which the water pours is nearly white. Much of the silt is deposited in this lake, but enough is carried downstream to give Grinnell Lake a beautiful turquoise hue. Some of the very finest sediment which fails to settle in Grinnell Lake is carried a mile farther to Josephine Lake to give it a blue-green color. Even Swiftcurrent Lake, still farther downstream, does not contain clear water.

The rock flour which colors these as well as other Park lakes can also be seen in the streams. Baring Creek at Sunrift Gorge (see p. 13 in Motorist’s Guide) is milky with powdered rock from Sexton Glacier. Cataract Creek along the trail between Josephine and Grinnell Lakes is noticeably milky, extraordinarily so in mid-afternoon on very warm days. At such times melting of the glaciers is accelerated and more silt is then supplied to the streams.

Part of Sperry Glacier, in contrast to Grinnell, rests on a bright red shaly rock (known to the geologists as argillite) which yields a red-gray powder when finely ground. Hence the water in several small lakes adjacent to the glacier has a pinkish tint.

Although a large number of Park streams are fed by glaciers there are many others, particularly in the south and west sections, which have no ice as their source. On a trail trip from Sunrift Gorge to Virginia Falls, one is certain to be impressed by the extreme clarity of the water in Virginia Creek. For half a mile below the falls the trail follows this cascading torrent from one crystal pool to another. So clear is the water that we are apt to mistake for wading pools places where the water may be five or more feet deep. Snyder Creek near Lake McDonald Lodge nearly rivals Virginia Creek in clarity. The sources of these two streams obviously are not melting glaciers.

From the foregoing discussion, it is evident that glaciers constitute one of the principal controlling factors in the color of the water in Park streams and lakes. Where there are no ice masses streams are clear, and where glaciers occur the water possesses many shades varying from clear blue through turquoise to gray, and in rare cases even pink.

MORAINE NEAR GRINNELL GLACIER IS 120 FEET HIGH. THE GLACIER EXTENDED NEARLY TO TOP OF MORAINE 50 YEARS AGO. (DYSON PHOTO)

Although the former large glaciers of the Ice Age transported huge amounts of rock debris down the valleys of the Park, the moraines which they deposited are, as a rule, not conspicuous features of the landscape. The Going-to-the-Sun Road, however, crosses several accumulations of moraine in which road cuts have been made. The road traverses a number of such places along the shore of Lake McDonald. Because of the large proportions of rock flour (clay) in these accumulations, the material continually slumps, sometimes sliding onto the road surface. One of these cuts has been partly stabilized by a lattice-like framework of logs. The largest excavation in moraine along the highway is located about three miles east of Logan Pass just below the big loop where the road crosses Siyeh Creek. The surfaces of many boulders in this moraine are marked by grooves and scratches, imparted to them as they were scraped along the side of the valley by the glacier 10,000 or more years ago.

A small moraine is exposed along the exit road from the parking lot at Many Glacier Hotel. It contains a number of small red boulders, the sources of which are the red rock ledges in the mountains several miles up the Swiftcurrent Valley, plainly visible from the hotel.