6oo 



THE NATURE BOOK 



lines of drainage have not had sufficient 

 time to mature their courses. They are 

 now engaged in grading their channels, 

 hlling up hollows and removing obstacles, 

 and, given time enough, they will again 

 attain perfect adjustment. 



We are now in a position to inquire 

 how the drainage of a country may be 

 disturbed in such a way as to produce 



"ONLY A NARROW CHANNEL INTERVENES BETWEEN THE DELTA 



AND THE OPPOSING SHORE." 

 Straits of Haweswater. 



lakes. Their distribution would lead us to 

 suppose that glaciers were responsible in 

 some or perhaps the majority of cases. 

 Earth movements, too, resulting in up- 

 heaval or depression across the line of a 

 valley would obviously raise barriers 

 which would hold up water to the level 

 of the outflow, or cause depressions in 

 which the waters would lodge. 



A glacier may work in two ways. The 

 ice plough may cut more deeply in one 

 part of its course than another, or it may 

 leave a moraine or other debris in a valley 

 in such a way as to obstruct the course of 

 the river. \\'hen the first of these causes 

 has been at work we get lakes of erosion, 

 and they may 1)e com])ared with reservoirs 

 excavated below the general level of a 

 valley. They nestle under the steep chffs through Asia Minor far into the interior 



of mountain cirques or spread over a 

 valley track, usually where the sides con- 

 verge to form narrows. In its efforts to 

 squeeze a broad mass through a contracted 

 gap the ice became vertically thicker and 

 moved with a velocity greatly in excess 

 of that flowing through the broader parts 

 of the valley. The resultant effect was 

 to overdeepen the floor, and a hollow was 



cut out of the 

 solid rock. If 

 the rim of the 

 basin can be 

 followed all 

 round without 

 a break, and 

 the outflow is 

 seen to be over 

 solid rock, there 

 can be no dis- 

 pute as to the 

 origin of the 

 basin, but \-ery 

 frequently the 

 ground is 

 strewn w i t h 

 glacial debris 

 which hides 

 the true rela- 

 tionship. 



Turning now 

 to those lakes 

 formed by 

 earth m o v e- 

 ments, we must 

 seek for them 

 in places where 

 the crust has been subject to changes of 

 level in recent times. The chain of lakes 

 in the basin of the St. Lawrence river 

 affords conclusive proof that changes of 

 level have taken place since the Glacial 

 Period, and the tilting may still be in 

 progress. Earth movements may result 

 in the production of faults or fractures, 

 and when the land between two fractures 

 has dropped, or, what comes to the same 

 thing, when two areas are raised, leaving 

 a part between unaffected, we have a 

 trough or rift formed. If the floors are 

 inland and below sea level, or at a lower 

 level than the drainage system of the 

 country, water must lodge in them and 

 form lakes. One of the best known rift val- 

 leys runs in a north and south direction 



