276 



EARTH FEATURES AND THEIR MEANING 



the glacier until " soled " or " shod," first upon one side and then 

 upon another. Accidental contact with some obstruction upon 

 the floor may suffice to turn the fragment and so expose a new sur- 

 face to wear upon the abrading floor. Minor obstructions com- 

 ing in contact with one side of the fragment only, may turn it in 

 its own plane without overturning. Evidence of such interrup- 

 tions can be later read in the different directions of striae upon 

 the same facet (plate 17 A). 



The floor beneath the glacier is reduced by the abrading process 

 to a more or less smooth and generally flattened or rounded sur- 

 face the so-called glacier pavement (Fig. 304). To accomplish 



this all former mantle rock 

 due to weathering processes 

 must first be cleared away, 

 and the firm unaltered rock 

 beneath is wherever suscep- 

 tible of it given a smooth 

 polish although locally 

 scored and scratched by the 

 grinding bowlders. The 

 earlier projections of the 

 surface of the floor, if not 

 entirely planed away, are 

 at least transformed into 

 rounded shoulders of rock, 

 which from their resem- 

 blance to closely crowded backs in a flock of sheep have been 

 called " sheep backs" or "roches moutonnees." Thus the effect 

 of the combined action of the processes of plucking and abrasion 

 is to reduce the accent of the relief and to mold the contours of 

 the rock in smoothly flowing curves, generally of large radius. 



The lifting of the grinding tools and their incorporation 

 within the ice. Wherever the ice is locally held in check by the 

 projecting nunataks, relief is found between such obstructions, 

 and there the flow of the ice has a correspondingly increased ve- 

 locity (Fig. 305 6). If the obstructions are not of large dimensions, 

 the ice which flows around the outer edges is soon joined to that 

 which passes between the obstructions and so normal conditions 

 of flow are restored below the nunataks. The locally rapid flow 



FIG. 304. A glacier pavement of Permo-Car- 

 boniferous age in South Africa. The striae 

 running in the direction of the observer are 

 prominent and a noteworthy gouging of the 

 surface is to be noted to the right in the 

 middle distance (after Davis). 



