igoo-i.] Physical Geology of Central Ontario. 179 



from the main escarpment, having a surface area of about four square 

 miles (Map IV). Were the land under water this would form a large 

 island, comparable to some of the islands already noted along the Bruce 

 peninsula. Similar outliers may occur elsewhere along the line of the 

 escarpment. In some other localities in front of the cuesta, outliers, 

 capped with Medina sediments and surrounded by drift-filled valleys, 

 were noted, suggesting a similarity to the Milton outlier, but being 

 further out on the lowland, they had perhaps undergone greater degra- 

 dation. 



In front of the Black River escarpment, notably in Hastings county, 

 are a number of outliers of limestone, with much jointed and fissured 

 upper beds. Some few of the outliers are of sandstone. If the region 

 were partly submerged these also would form islands in front of the 

 escarpment. As already noted, many of these outliers present a steep 

 face, with a talus slope at the base, towards the direction of the ice 

 advance, and a long trail of loose blocks on the lee side. 



Some of the islands and outliers were certainly in areas protected 

 from ice erosion. The case of the Manitoulin Islands cannot be con- 

 sidered, as the writer has not sufficient personal knowledge of the facts. 

 In the great majority of cases, however, they do not occupy such 

 protected places, and there is direct evidence that the ice transgressed 

 them. Their relation to the escarpments, and the effects which have 

 been produced by the ice, seem clearly to indicate that they had an 

 existence prior to the advance of the ice sheet. The more salient 

 features were smoothed off, but the essential features are still pre- 

 served. 



Depth of Excavation. — Another interesting fact is the remarkable 

 uniformity in the depth of excavation of the lowland below the crest of 

 the Niagara cuesta. At Cabot's Head the depth is about 800 feet, at 

 Collingwood 1,100, near Dundas 1,000.* The unsubmerged portion of 

 the Ontario lowland is located on rocks ranging from the Trenton to 

 the Medina ; the submerged portion is on Trenton In both cases. The 

 lowland has thus been excavated on rocks of four different horizons, 

 and of very diverse texture. 



Lozvla7id Rock- Surface. — An almost continuous transverse section of 

 the rock surface of the lowland is shown along the north shore of Lake 

 Ontario, parallel to the Dundas valley, from Hamilton to Lome Park 

 (twenty-five miles). Between here and the river Rouge (thirty miles) 



*743 measured, 1,000 calculated, Spencer, '81, 323. 



