STUDIES FOR STUDENTS. 5 2 3 



their nature is such as to have suggested that they are the im- 

 mature nuclei of drumlins. Further investigation is proposed, 

 and they are here introduced tentatively. 



b. There are several classes of aggregations of till that are 

 not strictly drumloidal in form but which are thought to deserve 

 recognition, for the present, as varieties, for their possible sug- 

 gestiveness respecting the physical processes of subglacial accu- 

 mulation. 



{a) Crag and tail. — These embrace the well known accumula- 

 tions of till in the lee of rocky crags or embossments. 



(d) Pre-crag. — These embrace the less well recognized accu- 

 mulations of till in front of crags or embossments of rocks. 

 These two forms may co-exist in connection with the same pro- 

 tuberance of rock and may coalesce. From this has arisen the 

 suggestion that their coalescence might initiate a drumlin. In 

 support of this, it is cited that many drumlins have a core or 

 pedestal of rock. Against this is cited the fact that many drum- 

 lins have no such nucleus of rock so far as observation can dis- 

 cover. 



{c) Veneered hills. — -These are hills of rock, coated somewhat 

 uniformly with till, the surface conforming approximately to that 

 of the underlying rock. These differ from the crag-and-tail and 

 pre-crag accumulations in the genetically significant fact of a 

 much more uniform distribution of the till over the rock emboss- 

 ment and in the subordination of veneering to the pre-existent 

 contour rather than the formation of a new contour. 



(d) Till billows. — There is a class of drift accumulations 

 which take on a billowy surface. They differ from the drumlins 

 in their want of conformity to axes lying in the direction of the 

 drift movement. The drumlins are also usually separated from 

 each other by low flat ground. The till billows, on the other 

 hand, are arranged more closely together, are disposed more 

 irregularly, and are connected with each other by saddles or cols. 

 Between these billows are frequent undrained basins. The type, 

 it will be observed, graduates into, if it does not strictly belong 

 to, the class designated below as submarginal moraines. Tracts 



