288 THE ICE AGE AND ITS WORK. 



a platform across tlie track of a river rarely diverts it from its course, 

 for the stream saws its bed into the rocks as fast as the obstacle rises." 

 Scanty raiu-fall and great evaporation seem therefore to be almost 

 essential to the formation of the larger plateau lakes. Earely, such 

 lakes may have been formed in comparatively -well -watered districts, 

 but the earth movements must in these cases have been exceptionally 

 rapid and extensive, and they are accordingly found most often in 

 countries subject to volcanic disturbances. Such are the lakes of 

 southern Italy, of Macedonia, of Asia Minor, and perhaps those of 

 central Africa. 



Quite distinct from these are the sub-alpine lakes of those mountain 

 groups which have been subject to extreme glaciation. These are 

 cbaracteristically valley lakes, occurring" in the lower portions of the 

 valleys which have been the beds of enormous glaciers, their fre- 

 quency, their size, and their depth bearing some relation to the form 

 and slope of the valleys and the intensity of the glaciation to which 

 they have been subject. In our own country we have in Wales a small 

 number of valley lakes; in the lake district, where the ice sheet can 

 be i)roved to have been much thicker and to have lasted longer, we 

 have more numerous, larger, and deeper lakes; and in Scotland, still 

 more severely glaciated, the lakes are yet more numerous, many of 

 those in the west opening out to the sea and forming the lochs and 

 sounds of the western highlands. Coming to Switzerland, which, as 

 we have seen, bears indications of glaciation on a most gigantic scale, 

 we find a grand series of valley lakes both on the north and south, 

 situated for the most part in the tracks of those enormous glaciers 

 whoso former existence and great development is clearly proved by the 

 vast morames of northern Italy and the travelled blocks of Switzer- 

 land and France. In Scandinavia, where the ice age reigned longest 

 and with greatest power, lakes abound in almost all the valleys of the 

 eastern slope, while on the west the fiords or submerged lakes are 

 equally characteristic. 



In North America, to the south of the St. Lawrence Itiver and of lakes 

 Ontario and Erie, there are numbers of true valley lakes, as there are 

 also in Canada, besides innumerable others scattered over the open 

 country, especially in thejSTorth, where the ice sheet must have been 

 thickest and have lingered longest. And in the southern hemisphere 

 we have, in New Zealand, a reproduction of these phenomena — a grand 

 mountain range with existing glaciers, indications that these glaciers 

 were recently much more extensive, a series of fine valley lakes form- 

 ing a true lake district, rivaling that of Switzerland in extent and 

 beauty, with fiords on the southwest coast comparable with those of 

 Norway. 



Besides these valley lakes there are two other kinds of lakes always 

 found in strongly glaciated regions. These are alpine tarns — small 

 lakes occurring at high elevations and very often at the heads of valleys 



