400 EARTH FEATURES AND THEIR MEANING 



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 water- 

 falls have resulted from either the temporary or permanent ob- 

 struction 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 glacia- 

 tion (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. 



Lakes. The valley moraine lakes and the chains of rock basin lakes. 



READING REFERENCES FOB CHAPTER XXVIII 

 Glacier movement : 

 L. AGASSIZ. Nouvelles Etudes et Experiences sur les Glaciers Actuels, 



etc., Paris, 1847, pp. 435-539. 



H. HESS. Die Gletscher, Braunschweig, 1904, pp. 115-150. 

 H. F. REID. The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp. 912- 

 928 ; Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S ..Geol. 

 Surv., Pt. i, 1898, pp. 445-448. 



