174 Transactions of the Canadian Institlte. [Vol. VII. 



The gorges of the first type seem to be free from drift debris, and 

 their immature form would indicate that they are largely of postglacial 

 origin. The second and third types are usually more or less drift filled, 

 especially in the upper reaches of the third t\'pe. The valleys of the 

 second type are relatively narrow and steep walled. The level to which 

 their mouths are graded is not known. The valle^-s of the third type 

 are broadly open at the point of exit from the cuesta, and some of 

 them penetrate ten to eighteen miles back from its front. Occasionally 

 they are indicated by topographic depressions bej'ond the point where 

 the bounding rock scarps can actually be followed, though the amount 

 of drift material upon the cuesta has usually obliterated all rock-surface 

 features beyond the limits already mentioned. The rock-walls of each 

 valle\- (except the Dundas valley as far as can be traced at present) 

 tend to converge, but convergence to a point of union has only been 

 demonstrated for the walls of some of them. Some have also tributary 

 lateral gorges. Spencer has described several entering the Dundas 

 valley. In these tributaries the walls usually unite and the present 

 stream falls over a cliff. The tributary gorges may belong to any one 

 of the three types. 



Owen Sound, sometimes wrongly designated a fiord, Colpoy's ba\', 

 and other bays upon the Georgian Bay coast, may serve as illustrations 

 of the type (Map III). There are, however, between Owen Sound and 

 Burlington, a number of valleys, not submerged, and equally typical. 

 The north shore of Manitoulin Island seems also to possess many com- 

 parable with these, but developed on Trenton and older strata. 



As in the case of the rock-sided valleys at the eastern end of the 

 area, we lack an accurate knowledge of the precise form of valley which 

 a sheet-glacier, acting on homogeneous rocks in a region of very low 

 relief, might possibly be capable of eroding, and of the form of escarp- 

 ment-front, which it might, acting alone, produce. It is necessary then 

 to make the partial assumption, that if the sheet-glacier were capable of 

 producing such topographic features, the products would bear a definite 

 relation to the direction of ice advance, and would, in homogeneous 

 rock, assume forms less tortuous than those carved by the more mobile 

 erosive agent, running water charged with sediment. 



The direction of the valleys as a whole is entirely independent of the 

 general direction of the ice movement, whether it be determined from 

 the evidences out upon the lowland or from those upon the crest of the 

 cuesta at the edges of the valleys. They lie in all positions through an 

 angle of about i8o° ; all but one (that at Dundas) in such a position 



