Ch. XI.] EXCAVATIONS MADE BY CASCADES. . 141 



merits left by a receding glacier, so are they the most liable to oblit- 

 eration ; for violent floods or debacles are often occasioned in the Alps 

 by the sudden bursting of what are called glacier-lakes. These tem- 

 porary sheets of water are caused by the damming up of a river by a 

 glacier which has increased during a succession of cold seasons, and 

 descending from a tributary into the main valley, has crossed it from 

 side to side. On the failure of this icy barrier, the accumulated waters 

 are let loose, which sweep away and level many a transverse mound of 

 ■gravel and loose boulders below, and spread their materials in confused 

 and irregular beds over the river-plain. 



In addition to the polished, striated, and grooved surfaces of rock 

 already described, another mark of the former action of a glacier is, 

 the " roche moutonnee." Projecting eminences of rock so called have 

 been smoothed and worn into the shape of flattened domes by the 

 glacier as it passed over them. 



Although the surface of almost every kind of rock, when exposed 

 in the open air, wastes away by decomposition, yet some retain for 

 ages their polished and furrowed exterior ; and, if they are well pro- 

 tected by a covering of clay or turf, these marks of abrasion seem 

 capable of enduring for ever. They have been traced in the Alps to 

 great heights above the present glaciers, and to great horizontal dis- 

 tances beyond them. 



There are also found, on the sides of the Swiss valleys, round 

 and deep holes with polished sides, such holes as waterfalls make in 

 the solid rock, but in places remote from running waters, and where 

 the form of the surface makes it difficult to suppose that any cascade 

 could ever have existed. Similar cavities are common in hard rocks, 

 sack as gneiss in Sweden, where they are called giant caldrons, and 

 are sometimes ten feet and more in depth ; but in the Alps and Jura 

 they often pass into spoon-shaped excavations and prolonged gutters. 

 We learn from M. Agassiz that hollows of this form are now cut out 

 by streams of water which, after flowing along the surface of a glacier, 

 fall into open fissures in the ice and form a cascade. Here the fallen 

 water, causing the gravel and sand at the bottom to rotate, cuts out a 

 round cavity in the rock. But as the glacier moves on, the cascade 

 becomes locomotive, and what would otherwise have been a circular 

 hole is prolonged into a deep groove. The form of the rocky bot- 

 tom of the valley down which the glacier is moving causes the rents in 

 the ice and these locomotive cascades to be formed again and again, 

 year after year, in exactly the same spots. 



Another effect of a glacier is to lodge a ring of stones round the 

 summit of a conical peak, or a single block on a sharp ridge, which 

 may happen to project through the ice. If the glacier is lowered 

 greatly by melting, these blocks or circles of large angular fragments, 

 which are called " perched blocks," are left in a singular situation at or 

 near the top of a sharp pinnacle or ridge, the lower parts of which 

 mav be destitute of boulders. 



