THE BEDROCK GEOLOGY 213 



islands, crosses the Frontenac axis, a narrow neck of Archean rocks which 

 connects the Adirondack region with the greater Archean areas of Canada. 

 The Archean rocks, of which the Frontenac axis forms a part, underlie 

 the eastern, northeastern, and northern part of the area under discussion. 

 West and south of the Archean areas the region is underlain by rocks of 

 Ordovician age, chiefly Black River or Trenton limestones. Outcropping 

 from beneath the Black river, and also occurring in several cases as out- 

 liers on the Archean areas, are a few feet of sandstones, some of which 

 are classed as Potsdam. It is not impossible, in Ontario at least, where 

 they are very much less well developed than in New York state, and are 

 usually of a somewhat different texture, that they may be contempora- 

 neous with the lower part of the Black River formation. In the present 

 discussion these sandstones are only of relatively minor importance. 

 The dominant topographic features of the region are controlled either 

 by the limestone or by the Archean rocks. 



PLEISTOCENE AND RECENT GEOLOGY 



In this locality, from the close of the Trenton until the Pleistocene, 

 time is not represented by any deposits. Any that ever existed have 

 long since been eroded away. The Pleistocene deposits are represented 

 by typical boulder clay and by deposits of stratified sands and gravels. 

 The distribution of these materials differs greatly in the different parts. 

 Throughout the Archean portions of the region the deposits are confined 

 largely to the depressions, with but very few scattered boulders or patches 

 of till or sand and gravel on the sides or summits of the Archean ridges. 

 In very few places does the amount of material deposited obscure the bed- 

 rock topography of the Archean terranes, the thickness rarely exceeding 

 a few feet. The deposits are an important factor in the modification of 

 the topography of the Archean regions as to relatively unimportant 

 detail but not as to general features. 



On the other hand, in the parts of the region underlain by Ordovician 

 rocks the conditions are in places somewhat different. In the central 

 parts of the district, on either side of the Saint Lawrence, the Pleistocene 

 deposits form only a thin veneer, except very locally in a few cases. 

 There are many large areas where almost bare rock is exposed for sev- 

 eral square miles at a time. In very few cases does the soil cover exceed 

 2 feet, except in the bottoms of some of the valleys, and very frequently 

 not even there. As the measure of relief in the district under discussion 

 often exceeds 150 feet, and thus in comparison with the thickness of the 

 Pleistocene deposits is very great, it is possible to determine without the 

 slightest doubt the nature and character of the bed-rock topography of 



