A MOUNTAIN STREAM 157 



simply mean more or less permanent outflowings of 

 the rain which has seeped into the upper part of the 

 hill and been kept as in a sponge. In keeping up the 

 supply when there is no rain, the literal sponges of 

 Bog Moss play, as we have seen, a very important 

 part. 



In the development of some valleys a glacier may 

 have helped, carving especially by means of the stones 

 which it carries in its grip. What are called " hanging 

 valleys " are especially common in glaciated regions, 

 and glaciers have probably helped to make them. The 

 curious term is applied to side valleys which open, 

 not into the bed of the main valley, but high up on 

 its sides, so that the tributary stream tumbles into the 

 main river by a series of rapids and waterfalls. This 

 is due to the main valley having been eroded more 

 rapidly than the side valleys. As hanging valleys may 

 occur in districts which are not glaciated, the in- 

 equality in the rate of erosion must sometimes be due 

 to causes independent of glaciers. To follow up the 

 enquiry, the student should consult books like Lake's 

 "Physical Geography" (Cambridge, 1915) or Sir Archi- 

 bald Geikie's "Class Book of Geology" (London, 

 1907). 



Just where our stream joins the river there is a 

 thicket of Osiers and small Alder-trees, making a 

 sheltered spot. On the clean gravel there we found a 

 large Trout not long since killed, with a big piece 

 bitten out of it just behind the head. We do not doubt 

 that an Otter had been disturbed at its meal. There 

 is no doubt that the Otter accounts for a good many 

 fish, but there should be enough and to spare ; and who 

 will not wish good luck to one of the most interest- 



