THE LAKES OF SNOWDONIA AND EASTERN CARNARVONSHIRE. 421 



-compared with the elevations of the surrounding ground." They are seen to be " but 

 shallow grooves at the bottom of the valleys in which they occur, and their depth is 

 small as compared with the thickness of the ice which moved over these spots." From 

 these and other considerations he concludes that " the immediate cause of these lake- 

 basins was the onward movement of the old glaciers, ploughing up their beds to this 

 slight depth, in the way Professor Ramsay's theory suggests." 



As to the mountain tarns, these appeared to him to be due "sometimes wholly to 

 glacial erosion, sometimes to this combined with a moraine dam, and occasionally to the 

 pounding back of rocks by moraines alone, or moraine-like mounds, at the foot of snow- 

 slopes." 



Dr H. R. Mill gives in the Geographical Journal for 1895 (vol. vi. pp. 46, 135) 

 an account of his work in this district under the title of " Bathymetrical Survey of the 

 English Lakes." The paper is illustrated by coloured maps, giving the subaqueous and 

 land-contours and also representative sections of the lakes. 



He found that the lakes conform to two main types : — 



(1) The shallow, exemplified in Derwent-water and Bassenthwaite. They only 

 average 18 feet in depth, aud their average depth is only 25 per cent, of the maximum 

 depth. These two lakes formerly formed one sheet of water. Their beds "may be 

 roughly described as an unduhiting plain, grooved and ridged into shallow hollows, and 

 low shoals running parallel to the long axis of the lake." 



(2) The deep type, " the shallowest of which has an average depth of 40 feet, and in 

 which the average depth varies from 36 to 61 per cent, of the maximum depth, showing 

 a steep-sided character." This type is exemplified in Crummock-water, Buttermere, 

 Wastwater, Coniston-water, Windermere, Hawes-water, and Ullswater. Ennerdale 

 shows the characteristics of the two types, being deep in its upper sections and shallow 

 in its lower part. He adds that " the typical form of this class of lake is a steep-sided, 

 flat-bottomed trough, diversified along the slopes by the still deeper conical mounds of 

 debris thrown down at the mouths of streams." 



The deepest parts of the lakes are, in several cases, found to reach below sea-level. 



In the Introduction to his paper, Dr Mill shows that the mountain and valley 

 system of the Lake District presents a simple radial symmetry bearing no relation to 

 the present geological structure ; on the contrary, the drainage-lines " bear testimony to 

 an earlier and simpler structure, when a dome of vanished rocks spread over the area, 

 the dissected skeleton of which now alone remains." The valleys are arranged like the 

 spokes of an irregular wheel passing out from the centre of the ancient dome ; and each 

 valley possesses one or more lakes, actual or extinct. 



Mr J. E. Marr has devoted much attention to the Tarns and Lake-basins of the 

 Lake country. In 1895-96 he published papers in the Quart. Journ. Geol. Soc. (vol. li. 

 p. 35 ; vol. lii. p. 12) on " The Tarns of Lake-land." He points out that no one doubts 

 the occurrence of moraine-dammed lakes ; and should the exit immediately over-lie the 

 old river bed, such a lake or tarn will necessarily have a comparatively brief existence. 



