iQoo-i.] Physical Geology of Central Ontario. 173 



cesses which opened the fissures must have operated during preglacial 

 time. In any event ice which was capable of scouring passed over the 

 area after the fissures were opened, removed some of the blocks from 

 small areas, but left still larger areas with the blocks still in position, 

 even on narrow ridges. 



The topography of these uplands is in many places similar to that 

 peculiar to a limestone region undergoing the process of subaerial 

 degradation. A comparison with ridges in like situations in the 

 unglaciated area of Wisconsin shows that the similarity is very striking. 

 In central Ontario, however, there are no pinnacles nor small promin- 

 ences in front of the escarpments. Many of the larger outliers still remain 

 as such, generally each with a steep cliff and talus slope in the direction 

 from which the thrust of the ice-sheet came. On the lee side there is a 

 long trail of rhomboidal blocks from the rear slopes of the outlier. 

 (Plate IV, fig. I.) 



Along the escarpments where the old cliff faced the ice thrust there 

 is always a well defined talus slope, sometimes right up to the crest. 

 (Plate IV., fig. 2.) When the direction of the cliff approaches parallelism 

 with the direction of ice motion, the talus is frequently much smaller 

 and occasionally nearly wanting. Where the valley sides are graded to 

 the edge of the upland, loose blocks usually seem to be altogether 

 wanting in the valle}'s. Whether any of the original soil cover is still in 

 situ it is at present impossible tosa}'. Certainly much of the present soil 

 is imported. 



Goj'ges and Valleys of the Niagara Cuesta. — Along the Niagara 

 cuesta from east of the Dundas valley, described by Spencer ('81), to 

 Cabot's Head on Georgian Bay, are a number of incisions transverse to 

 the escarpment, varying from deep and narrow gorges to deep but 

 broadly open valleys, sometimes as much as ten miles across the mouth, 

 whose bottoms are occupied by obsequent streams flowing to the inner 

 lowland. There seem to be three types of these valleys ; first, narrow 

 short and deep gorges, which in some cases might almost be described 

 as hanging gorges, since they are not yet cut down to grade with respect 

 to the rock floor in front of the escarpment. Second, narrow steep- 

 walled gorges, which so far as known appear to be graded with refer- 

 ence to the frontal rock floor. Third, deep broadly open valleys, 

 whose upper reaches may become gorges. They are graded with 

 respect to the rock floor of the inner lowland some distance, sometimes 

 a number of miles, away from the immediate vicinity of the escarpment. 



