The Development of Vegetation in the English Lakes. 27 d 



Both features can be accounted for by the fact that Hawes Water has heen 

 heavily silted in the past, shown not only by the condition of the lake floor 

 but also by the size of the gravel alluvia at the mouths of the two main 

 affluents. Moreover, in marked contrast to the gravel deltas in other lakes in 

 this district, those at Hawes Water show no signs of recently deposited gravel- 

 Mill (loc. cit.) also points out, quite justifiably, that the Measand Delta could 

 never have been laid down by the present stream, and he attributes its 

 formation to the " washing out" of a glacial lake. The third point of interest 

 is that the sediments of Hawes Water are fine sands, not silts* such as 

 would be carried into a lake by torrential streams. Very vigorous erosion in 

 the past seems, therefore, to have washed huge quantities of sand and gravel 

 into Hawes Water. The sand helped in the siltiug-up of the shores, it 

 allowed the replacement of Isoetes by Nitella, but it is too coarse to allow the 

 extensive development of Potamogeton, spp. Thus Hawes Water, though 

 abnormal, is not at variance with the views expressed previously. 



Bassenthwaite owes its peculiarities to the brown and peaty nature of its 

 waters, a coloration due to the extensive peaty alluvia at the lake head. 

 The water is so discoloured that the lower light limit of vegetation is only 

 2-5 m., and the fine deeper water sediments cannot be colonised by plants. 

 Owing to wave action, silting is naturally less rapid in shallow water, and 

 moreover the sediments are coarser and become rapidly organic, owing to the 

 slower deposition of silts, and their comparative poverty in potash. Thus, 

 the vegetation of such a lake will be that characteristic («) of wave-washed 

 shores (little silted), {b) of the coarser sediments, (c) of the more organic 

 sub-strata. The plants occurring in such habitats are (a) Isoetes, (6) Callitriche 

 intermedia., (c) Myriophyllum and Ranunculus, all of which are abundant in 

 Bassenthwaite. The absence of tbe finer and softer sediments within the 

 photic zone accounts for the scarcity of Potamogeton and Xitella, spp. 



Coniston and TJllswater resemble Bassenthwaite in possessing a relatively 

 high proportion of Isoetes, yet their light conditions are normal and both 

 mclude abundant Pota/mogeton. An examination of Table I shows that both 

 lakes have a smaller percentage of cultivable land in their drainage systems 

 than is compatible with the degree of silting, i.e., their drainage systems are 

 more primitive than the lake basins. It has already been remarked that 

 their abundant silts are largely due to washings from lead or copper mines, 

 and it is known, of relatively recent times. The coarser silts from such 

 washings smother the lake floor round the mouth of the incoming streams, 

 the finer are deposited all over the lake floor, except in the regions subject to 

 wave-wash, i.e., on most of the lake shores between the water margin and 



* Cf. also Mill, loc. cit. 



