Fig. 429.—Moraines and drumlins about Lake Constance upon the site of the earlier piedmont glacier of the Upper Rhine. The white area outside the outermost moraine is buried in glacial outwash (after Penck and Brückner).
Behind the recessional moraines within the glaciated valley are found the valley moraine lakes ([Fig. 448], [p. 413]), in association with the rock basin lakes due to glacial sculpture ([Fig. 447], [p. 412]). After the glacier has vacated its valley, the precipitous side walls become the prey of frostwork and are the scenes of disastrous avalanches or landslides. Within the cirques, drifts of snow are nourished long after the ice has disappeared, and as a consequence the amphitheater walls succumb to the process of solifluxion ([p. 153]).
Diversions and reversals of drainage, which are so characteristic of the work of continental glaciers, are hardly less common to glaciated mountain districts. Many of our most beautiful waterfalls have resulted from either the temporary or permanent obstruction of earlier valleys above the falls. The famous Yosemite Falls offers an interesting illustration of the shifting of an earlier waterfall, itself no doubt due to ice blocking in a still earlier glaciation ([plate 22 B]).
Marks of the earlier occupation of mountains by glaciers.—It is well that we should now bring together within a small compass those evidences by which the existence of earlier mountain glaciers may be proven in any district. These marks are so deeply stamped upon the landscape that no one need err in their interpretation.
MARKS OF MOUNTAIN GLACIERS
High-level sculpture. The grooved upland with its cirques, or the fretted upland with its cirques, cols, horns, and comb ridges.
Low-level sculpture. The U-shaped main valley, the hanging side valleys with their ribbon falls, the glacier staircase with its rock bars and gorges, the rounded, polished, and striated rock floor.
Deposits. The recessional moraines of till and the valley trains of sand and gravel, the soled erratic blocks derived always from higher levels of the valley.