[WILSON] PALEOZOIC TO THE PRE-CAMBRIAN 19 



of 30 miles in a northwesterly direction parallel to the north shore of 

 Lake Deschene, from Kings mountain, northwest of the city of Hull, 

 to a point in Onslow township approximately three miles north of the 

 village of Quyon. Since this escarpment has its greatest development 

 in Eardley township and is known locally as Eardley mountain it can 

 be most appropriately designated the Eardley escarpment. At its 

 eastern end the escarpment is cut off abruptly by the Gatineau river 

 valley but at its western end it gradually diminishes in height in the 

 direction of its termination. From the Eardley escarpment westward 

 the Pre-Cambrian border everywhere presents the poorly defined 

 gently sloping relationships which characterize the southern border of 

 the Laurentian highlands in most other localities. 



From the Gatineau Valley to Montebello: In the region to the east 

 of the Eardley escarpment and as far as the village of Montebello a 

 distance of 40 miles the highland border like that to the west of the 

 Eardley escarpment is poorly defined, remnants of Palaeozoic sedi- 

 ments lying here and thei'e over the Pre-Cambrian surface throughout 

 a zone several miles in width. These relationships are typically 

 exemplified in the region included in the Buckingham map recently 

 published by the Geological Survey. In the southern part of this 

 area scattered outcrops of Potsdam sandstone overlain in places by 

 Beekmantown dolomite occur for a distance of 7 miles north of the 

 Ottawa river and 6 miles north of the point where the Pre-Cambrian 

 predominates as the bedrock formation. In a few of these outcrops 

 the strata appear to conform to the surface of the underlying Pre- 

 Cambrian but more generally they dip to the southward at low angles 

 and thus conform approximately to the regional slope of the Pre- 

 Cambrian floor upon which they were deposited. The maximum 

 thickness of Potsdam sandstone exposed in these outcrops is generally 

 not more than 30 feet, whereas the thickness in the vicinity of Ottawa 

 a few miles farther to the south is said to be 200 feet.^ This increase 

 in thickness of the Potsdam from north to south suggests that in 

 Pre-Palaeozoic time as at present the Pre-Cambrian surface in this 

 region had a gradient to the southward. 



The Grenville escarpment. The southern border of the Laurentian 

 highlands from the Montebello district eastward to St. Jerome, a 

 distance of approximately 35 miles, is marked by a second striking 

 escarpment which rises abruptly to a maximum elevation of 700 feet 



^Ami, H. M., Proc. and Trans., Royal Soc. Can., new series. Vol. VI, 1900, 

 p. 129. 



The thickness of Potsdam encountered in a well drilled in Dundonald Park, 

 Ottawa, according to Mr. E. D. Ingall was 265 feet. 



