The Development of Vegetation in the English Lakes. 261 



with sand ; (3) a silted (and more or less muddy) zone in deeper water. 

 Generally, the stones or gravel of the terrace extend to about 2 m. in 

 depth, while the finer sands and silts lie below 4 or 5 m. The finest 

 sediments tend to get carried along the shore by the littoral current, and 

 they are deposited in bays or holes, where this current slows down as it 

 passes into deeper water. It follows, therefore, that bays and holes tend to 

 get silted up — points and exposed shores to be kept silt-free. Since the 

 lakes are of different lengths and lie at different angles to the prevalent 

 winds (here S.W.), the strength of wave action varies very greatly, and its 

 results are shown in very different degrees. For instance, wave effects are 

 most severe in Wastwater, Coniston, Windermere and Ullswater, all long 

 lakes lying roughly S.W. to iST.E. 



A second type of modification is produced in any lake by the presence of 

 fluvial sediments. The lateral aSluents of lakes lying in such steep-sided 

 valleys as these are normally rapid, and carry down into the lakes quantities 

 of coarse detritus, chiefly gravel. The mouth of every small stream thus, in 

 time, projects into the lake on a gravel delta, and as a lake increases in age,, 

 its shores tend to become more and more irregular owing to the development 

 of these deltas. It then follows that parts of the shores become sheltered and 

 more silted than they otherwise would be. 



This modification of the lake shores is only one sign of the effects of 

 regional erosion and this group of factors produces other changes in the 

 characters of the lakes. As the surrounding country weathers from crag and 

 scree into gentle slopes and soil, the gradients of the streams tend also to 

 decrease and their carrying power is diminished, particularly where they 

 enter the lake. Hence, they ultimately carry into the lakes, not gravels but 

 silts, and since the surrounding country is developing soil as it becomes more 

 eroded, finer silts tend to iecome available in increasing quantities, replacing the 

 sands and gravels of the earlier stages of lake development. 



Geology of the District. — As the map shows, three main types of strata exist 

 — to the north, the Skiddaw slates ; in the centre, the volcanic ashes and 

 lavas of the Borrowdale series ; and in the south, a group of Silurian slates, 

 grits and flags. In their appearance and general characters, these three 

 types of rocks are very similar, but though all are hard, the central volcanic 

 rocks are harder than the sedimentary rocks of the Silurian and Skiddaw 

 beds, and, moreover, possess larger and more regular joints. Hence they 

 weather more slowly, and also give rise to block screes, rather than to finer 

 products. The effect of this upon the topography of the district is very 

 noticeable. Having weathered least, the central hills tend to be higher, more 

 rocky and steeper, while around them lie the lower and more eroded hills of 



