812 THE POPULAR SCIENCE MONTHLY. 



and any force in an ocean current is clearly out of the question over 

 these scattered sx)Ots. Streams and rivers work along lines, form 

 ravines and gorges, but never a more or less circular basin of great size 

 in comparison with the stream, or river ; hence they can not be the 

 agents. The atmospheric powers — rain, snow, wind, and chemical 

 action — weather the rocks indeed, form tiny basins on almost every 

 stone ; but this is but nature's fretwork, the delicate carving around 

 the sculptured craggy tower or spire and smooth-scooped rocky front. 

 Yet there is one surface agent remaining, the moving glacier. Most 

 people are familiar with the proofs of former glacial action in Cumber- 

 land and Wales — proofs as clear as are those of the former greater ex- 

 tension of the Swiss glaciers. Now by far the greater number of our 

 tarns lie in true rock basins — hollows completely inclosed by rocky 

 sides, which are, moreover, smoothed and grooved in a manner peculiar 

 to ice-action. At the sides of many a tarn and lake you may see the 

 ice grooves and scratches passing beneath the water, so as to leave no 

 doubt whatever that ice has once occupied the rocky hollow. The 

 question is. Did the ice-movement form the hollow? I believe that 

 in most cases it did, and for these reasons : 1. The tarn lies almost in- 

 variably in the path of old ice streams or glaciers, as is proved by the 

 direction of the scratches in the surrounding rocks. 2. l^hey frequently 

 occur at the foot of slopes more or less steep, or where the ice-pressure 

 can be shown to have been great. 3. The position of the deepest 

 points in the larger tarns and lakes occurs almost invariably where, 

 from the confluence of two or more glaciers or the narrowing of the 

 valley, the ice-pressure must have been someAvhat increased. 4. The 

 depth of these tarns is very slight as compared with the thickness of 

 the ice which can be proved to have passed over them. 5. There is 

 every gradation from a tiny, rock-bound pool, glaciated on all sides, 

 and which all will admit must have been scooped out by the ice, to 

 the tarn or lake showing precisely similar phenomena on a larger 

 scale. 



Since the ice-plow passed over our land the atmospheric powers 

 have been at work for a long period ; and while many rock basins are 

 now completely filled up by stream-borne matter, all are being so filled, 

 and each age must witness a decrease in the number and size of those 

 sheets of water which form so marked a character of our Cumbrian 

 scenery. 



Before quitting this subject, however, I must remark that there 

 are a few tarns which seem to me to owe the whole or a part of their 

 depth to a moraine dam. That is to say, the rock basin is imperfect 

 on one side, and there an old glacial moraine may have helped to dam 

 the waters back ever since the retreat of the glacier which threw off 

 the moraine. It frequently happens that a little moraine material has 

 been left upon ice-rounded rocks at the foot of a tarn, and in such 

 cases a hasty observation might lead one to believe that the whole 



