Fig. 440.—Diagram to show the manner of formation of pit lakes.
Pit lakes are thus easily recognized by their occurrence usually in groups within a plain of glacial outwash and by their characteristic banks inclined at the angle of repose of such materials ([Fig. 441]).
Fig. 441.—Diagrams to illustrate the characteristics of pit lakes and a sample map from the glaciated region of North America.
Glint or colk lakes.—It has been found to be true of existing continental glaciers that where their mass has been held back by a mountain wall, their current at the portals within this rampart becomes greatly accelerated. Though the upper layers of the glacier in the vicinity may move forward with a velocity of but an inch per day, the current within the outlet may be as much as seven hundred or a thousand times as great. In many respects these conditions are similar to those about the raceway of a reservoir where the near-by surface of the water is lowered by the indraught of the outlet and the current in the raceway is so accelerated that, unless protected, the bottom of the race is carried away and a basin excavated which extends a short distance both above and below the position of the dam. In Holland such basins hollowed out beneath breaks in the dykes are known as colks. Basins which were excavated beneath the glacier outlets by a similar process would not be open to our inspection until after the ice had disappeared from the region; but it is most significant that in Scandinavia, where the Pleistocene continental glacier, advancing westward from the Baltic, was held in check by the escarpment at the Norwegian boundary (the glint), lake basins have been excavated in hard rock whose walls show the abrading and polishing which are characteristic of glacial sculpture, and whose positions are such that they lie beneath the former outlets partly above and in part below the line of the escarpment. Their position in reference to the rampart and to the former outlets is brought out in [Fig. 442]. The largest of the glint lakes of this series is Torneträsk in northern Lapland (see [p. 277] and [Fig. 443]).
Fig. 442.—Diagram to show the manner of formation of glint or outlet lakes where the continental glacier of Scandinavia issued from the Baltic depression through portals in its mountain rampart.
Fig. 443.—Map showing a series of glint lakes which lie across the international boundary of Sweden and Norway.