In contrast, the deeper hollows of Derwentwater are floored with dark brown mud which completely masks the stony bottomSpa hotel lake district . In the shallow water towards the southern end of the lake there is often a mass of muddy peat consisting of roots and stems of water plants felted together. Occasionally this rises from the bottom to the surface and forms what has long been known as the 'Floating Island of Derwentwater'. It appears particularly after dry summers between June and September in a position about 300 yards north of the mouth of Watendlath Beck The origin of the island has intrigued many and from time to time fanci¬ful and varied explanations have been put forward. The most likely explanation, however, is that the decaying vegetation liberates a gas which gives buoyancy to the vegetation lying on the lake bed and brings it to the surface for a time.
Bassenthwaite Lake has the simple form of a deep elongated hollow with a mean depth of 70 ft, about the same as for Derwentwater. Its shape suggests that it was scooped out by a glacier as a distinct hollow quite separate from that con¬taining Derwentwater. Along its eastern shore at Bowness and Broadness the ice left behind hillocks of boulder clay which run out into the shallow waters of the lake. At one time Bassenthwaite and Derwentwater formed one large lake but subsequent deposition of alluvium brought down by the River Greta on to the rock bar separating the two basins led to their separation. The difference in level between the two lakes is only 21 ft so that the river joining them is rather sluggish except after heavy rains in Borrowdale. When these occur the level of the lakes can rise by as much as 9t ft. One result of the changing level is that the lake is fringed by a fairly wide storm beach which can be washed by sizeable waves when a strong wind is blowing.
The paired lakes of Derwentwater and Bassenthwaite are repeated on a smaller scale at Buttermere and Crummock Water. Here again the original single lake has been split into two by deposition of sediment on the intervening rock sill. Both lakes are deep, Buttermere descending to 94 ft near its head and Crummock Water having a maximum depth of 144 ft at a point approximately halfway along its length. Their main distinctive feature is not their great depth, for this is exceeded in Wastwater and Windermere, but their abrupt trough ends rising out of a flat floor. Some of the side slopes exceed 45 degrees, but once the bottom is reached gradients fade off imperceptibly to only 1 : 300. In Crummock Water, off Hause Point the slope becomes almost precipitous.
The trough like form of the rock basins which hold the waters of Buttermere and Crummock Water is adequate testimony of the erosive power of valley glaciers. When the Ice Age was at its height, so that even perhaps the highest peaks were submerged beneath a great enveloping ice dome, powerful radiating streams of ice were generated. Some believe that the radial symmetry of the pattern of lakes, which Wordsworth in 1820 likened to the spokes of a wheel, has resulted from these outward flowing glaciers. If this view is correct, such was the power of ice that it completely altered the original river valley system which was made up of mainly northsouth elements.
Not everyone subscribes to this view that the radial pattern of valleys and their lakes has been forged within the last million years of the Ice Age. Opinion, however, is unanimous that ice is a very potent force in over-deepening existing valleys. Wastwater, for example, has a maximum depth of 258 ft between Long Crag and Ill gill Head , which means that its bed is well below sea level in places . The gouging out of the valley floor was often greatest where the valley was restricted in width. The formation of any rock basin which ultimately becomes the site of a lake implies that the main energy of the glacier is expended in over deepening its bed and not widen¬ing the valley. For this reason the lakes are all long and narrow and usually straight.