No less conspicuous than the mountains themselves are the lakes. In most instances glaciers have been either directly or indirectly responsible for the origin of the several hundred in the Park. In general, these lakes may be divided into five main types, depending upon their origin.
(1) Cirque lakes. This type of lake frequently is circular in outline and fills the depression plucked out of solid rock by a glacier at its source. Some of the most typical examples are listed in the foregoing discussion of cirques.
(2) Other rock-basin lakes. This type, referred to above, fills basins created where glaciers moved over areas of comparatively weak rock. In all cases the lake is held in by a bedrock dam. A typical example is Swiftcurrent, which lies behind a dam of massive Altyn Limestone layers. The highway, just before it reaches Many Glacier Hotel, crosses this riser of the glacier stairway.
(3) Lakes held in by outwash. Most of the large lakes on the west side of the Park fall in this category. The dams holding in these lakes are composed of stratified gravel which was washed out from former glaciers when they extended down into the lower parts of the valleys. Lake McDonald, largest in the Park, is of this type.
ST. MARY LAKE FROM GOOSE ISLAND OVERLOOK
(4) Lakes held by alluvial fans. St. Mary, Waterton, Lower St. Mary, and Lower Two Medicine Lakes belong in this group. These bodies of water may have been rock-basin lakes, but at a recent date on their history streams entering the lake valley have completely blocked the valley with deposits of gravel; thus either creating a lake or raising the level of one already present. St. Mary and Lower St. Mary Lakes probably were joined originally to make a lake 17 miles long. More recently Divide Creek, entering this long lake from the south, built an alluvial fan of gravel where it entered the lake. This fan was large enough to cut the lake into the two present bodies of water. The St. Mary Entrance Station at the eastern end of Going-to-the-Sun Road, is located on this alluvial fan, the form of which can readily be distinguished from a point along the road at the north side of the upper lake near its outlet.
(5) Moraine lakes. Most lakes with moraines at their outlets are partly dammed by outwash or rock ridges. One of the prominent examples is Josephine Lake near Many Glacier Hotel. The moraine which is partly responsible for the lake is a hill which can be seen from Many Glacier Hotel. Several of the large lakes on the west side of the Park are also held partly or entirely by moraines.
Another type of moraine lake, which occurs only at Sperry and Grinnell Glaciers, has already been mentioned. It differs from all other Park lakes in having a glacier for part of its shoreline. There are two of these lakes at Sperry and one at Grinnell. Despite their small size, they are tremendously interesting, not only because of their relation to the glacier, but also because they are ordinarily filled with icebergs throughout the summer. Their surfaces often remain frozen until mid-summer.
There are several types of minor importance, the principal one of which is that formed by a landslide damming the valley.