452 Hugh Miller — Formation of Rock-Basins. 



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under tliat designation. A lakelet of these 

 dimensions, if admitted as the product of 

 glacial erosion, would at least serve to re- 

 move glaciers from the category of ''purely 

 hypothetical causes"^ in relation to larger 

 lakes. But I venture to contend that it does- 

 more. It is not necessary at this time of 

 day to urge that the admission commits one 

 to the beginning of a perfectly graduating 

 series of larger basins. Yet it seems to me 

 that if glaciers be accredited with scooping 

 powers at all, the initial stages of the process 

 are just those most difficult for them to pass, 

 and that, once surmounted to the extent of a 

 good-sized tarn, they begin more fully to feel 

 their power, and to work more easily and 

 rapidly. 



Supposing a basin half a mile long and 20 

 feet deep under a glacier or glacial sheet, 

 and assuming the continuance of the favour- 

 able conditions which were able to originate 

 it, there are now additional inducements to 

 vigorous scooping; and these are probably 

 twice as great as they were when the basin 

 was half the size. A diagram will illustrate 

 this. Entering the basin, it is clear that the 

 first decrease of obstruction at a is followed 

 by an increase of difficulty at h, from which 

 point onward, gravity and friction have to 

 be overcome by the accumulating pressure 

 behind. The force necessarily exerted to 

 impel a mass of ice equal to the area of 

 this acclivity, and of the great thickness 

 which glaciers have been proved to attain, 

 against gravity and friction, when considered 

 as additional to the first scooping incentive, 

 cannot but be great, and is a progressively 

 increasing quantity as the basin and the area 

 of inert ice enlarge. Whether the basin shall 

 deepen or lengtlien more depends on varying 

 circumstances ; but, obviously, increase in the 

 angle of slope tends in the same direction as 

 its lengthening. When the mechanics of 

 glaciers are better known, we may perhaps 

 know in terms tolerably precise what the 

 added force in given cases may be ; for the 

 mean time I think I am justified in holding 

 that, even for a slope of one degree and a 

 distance of 500 yards, it must be such as no 

 theorist can afford to disregard. 

 1 Judd, Geol. Mag. for January, 1876. 





