LANDSCAPE, POPULATION, AND OCCUPATION 217 



moraines, but if the latter are lofty and massive the water may 

 be ponded up to such a height that it will escape over rock in 

 some other direction, and give the appearance of being entirely 

 confined by a rock bar. If, however, these lakes are really 

 moraine-dammed they will drain out of their sides rather than 

 their ends, through narrow, gorge-like valleys often with rapids 

 and waterfalls. 



Some of these lakes are, however, certainly contained in 

 real hollows in the solid rock, and not held up by moraine 

 material. Their basins appear to have been excavated out 

 of the rock. Now, water could obviously not excavate such 

 hollows, for the still waters which would result would eventually 

 fill up the hollow with transported debris. It is possible that 

 moving ice may have been the excavating agent. But there 

 is another explanation which may sometimes be applied, that 

 of earth-movement. If movement occurs while water is the 

 main denuding agent, it will either cut through the barriers as 

 they are elevated, or fill up the hollows formed behind the bars 

 with the sediment it carries. If, however, the country is occupied 

 by ice it will continue to transport its material as before, and 

 will only fill up the hollows produced with ice without clogging 

 them with detritus. When the ice disappears, and is replaced 

 by water the latter will discover the hollows and fill them with 

 water ; they will be lakes for a time, until filled up with debris 

 carried by the water, when they will be replaced by alluvial flats 

 (Fig. 68). If at any point in a valley system either downward 

 or upward movement occurs while the valley is filled with ice, 

 a lake will be an inevitable consequence. This may in part 

 explain the association of rock basins with glaciation. They will 

 sum up the result of all the earth-movement which occurred 

 during the Glacial Epoch. 



Study of the geological map of any country will bring out the 

 relation of relief to geological structure. In England the oldest 

 rocks are to the west and north, the newer rocks succeeding 

 regularly to the east and south. The two main exceptions to 

 this are the arches of the Pennine Chain and the Weald (Figs. 

 65 and 66), in both of which there is introduced a repetition of 

 the features resulting form the arched and denuded rocks. On 



