18 THE ROYAL SOCIETY OF CANADA 



is merely the western continuation of the subdivision previously 

 described, but is distinguished by certain features which are either 

 absent or cannot be observed in the region to the eastward. It 

 consists of a broad depression, the irregularities in the surface of which 

 are occupied in places by outliers of Palzeozoic strata ranging in age 

 from the Potsdam to the Utica. The older Palaeozoic formations 

 represented in the Ottawa valley — the Potsdam, the Beekmantown, 

 and the Chazy — however, progressively disappear in these outliers in 

 an east-west direction so that in the western part of the depression as 

 well as in the Timiskaming depression farther to the north the Black 

 River formation rests directly on the surface of the Pre-Cambrian. 

 Similar overlap relationships can also be observed, here and there, 

 within the depression, thus in the vicinity of Clear lake in Sebastopol 

 township, Renfrew County, Ontario, the Utica formation rests on 

 the surface of the Pre-Cambrian at an elevation of approximately 800 

 feet above sea level, whereas in the valley of the Bonnechere a few 

 miles to the northeastward the Beekmantown occurs in the same 

 relationship at an elevation of less than 400 feet above sea level. It 

 would seem probable, therefore, that between [these two points there 

 was an original difference in elevation of the Pre-Cambrian surface, 

 upon which the Palaeozoic sediments were laid down, equivalent to the 

 thickness of sediments intervening between the Beekmantown and the 

 Utica or approximately 600 feet, an amount greater than the pre- 

 sent difference in elevation of the two localities. 



The fourth subdivision — the region adjoining the southern border 

 of the Laurentian plateau to the west of the Kingston district — differs 

 from the region to the eastward in that only the Black River and 

 Trenton formations are represented. These however, are distributed 

 in the usual successive parallel belts from north to south and as in 

 most other border localities dip to the southward away from the 

 Laurentian upland. Likewise where the Palaeozoic adjoins the Pre- 

 Cambrian in this locality the Ordovician limestone occurs as scattered 

 outcrops over the surface of the Pre-Cambrian throughout a wide zone 

 so that the border line between the Pre-Cambrian highlands and the 

 Palaeozoic lowland is most indefinite. 



Local Details of the Plateau Border 



The Eardley Escarpment: The southern border of the Laurentian 

 plateau in the region northwest of Ottawa is sharply delimited by an 

 escarpment several hundred feet in height which extends for a distance 



^ Ells, R. W., Report on the Geology of a Portion of Eastern Ontario, Ann. Rep. 

 Geol. Surv. Can., Vol. XIV, Pt. J, 1901, p. 7. 



