BOUNDARY OF THE GLACIATED AREA. 143 



moraine, are tilled to the rim with water ; others are bat par- 

 tially tilled; while others are for the greater portion of the 

 year completely dry. The angle at which the earth forming 

 their sides stands is usually as sharp as the nature of the 

 material will allow, and bowlders are as frequently found 

 upon the inside of the rim as upon the outside. 



The origin of kettle-holes has already been explained ; 

 but it is in place here to remark upon some of the general 

 considerations supporting the theory already advanced.* It 

 is evident, from a^ inspection of the depressions themselves, 

 that they can not be the result of erosion, since the depres- 

 sions are too irregular and too deep to have been formed by 

 the plunging movement of water, and the material is too 

 coarse for a water deposit. In some respects kettle-holes 

 resemble what are called sink-holes, frequent in limestone 

 regions, where a great amount of material below the surface 

 is removed in solution, leaving numerous caverns whose roofs 

 eventually sink in, and form the depressions characteristic of 

 such regions. But kettle-holes abound in regions where no 

 such caverns could have been formed, and are distributed 

 over the country according to a method which could not have 

 originated by the action of underground currents of water. 



While the iceberg theory was in favor to account for the 

 drift, it was not uncommon to hear these kettle-holes spoken 

 of as places where icebergs had stranded, and in turning 

 round and round had bored holes in the bottom of the ocean- 

 bed over which they were floating ; but, now that the ice- 

 berg theory is abandoned, and observations are more extended, 

 the origin of kettle-holes is readily understood as an inevita- 

 ble part of the glacial theory itself. Any one who will in 

 the early spring-time take pains to observe the melting of 

 masses of ice which have been covered by ashes and other 

 refuse, or which have been partially buried beneath the 

 debris of earth which some spring torrent has brought down 

 from a neighboring hill, will find before him a very perfect 



* See above, p. 66 et seq. 



