33 



From our present knowledge of the distribution of the 

 faunas it would seem that at Montmorency we are near 

 the southern margin of a great area of Trenton rocks which 

 once extended far over the Laurentian highlands to the 

 north. Infaulted remnants of such an expanse of lime- 

 stone occur at Lake St. John, 200 miles (320 km.) northeast 

 of Quebec, and at various other places north of the St. 

 Lawrence. This limestone does not seem to have extended 

 to any great distance south of the St. Lawrence, and during 

 Trenton time there was probably a barrier here, to the 

 south of which was a sea containing the Altantic facies of 

 the Trenton fauna (Quebec City formation). South of 

 this barrier Lower and Upper Cambrian, Sillery and Levis 

 strata had also been deposited but after Trenton time the 

 barrier may have been submerged, so that Lorraine and 

 Richmond shales may have been deposited over both the 

 Trenton and the Quebec City formations. 



It is generally conceded that the pressure which accom- 

 panied the Appalachian mountain building was exerted 

 largely from the ocean side, and hence all overturning and 

 thrusting at Quebec would be expected to be, as it is, from 

 the southeast. It would seem that when the force was 

 exerted against the great mass of Cambrian and Lower 

 Ordovician strata which had accumulated south of the 

 St. Lawrence, the Lower and Upper Cambrian limestones 

 remained anchored, while the soft Sillery shales allowed 

 the development of a thrust plane within their mass, so 

 that a great thickness of Sillery, Levis, and Quebec City 

 rocks was pushed toward the northeast from their original 

 position. The drag at the bottom of such a thrust-block 

 would tend to delay the anterior end, thus swelling the 

 strata into an anticline. The front of this block, on reach- 

 ing the scarps of the normal faults which had developed in 

 this region would be stopped, and the anticline completely 

 overturned and secondary thrust planes developed, so 

 that the lower strata in the block would be thrust over the 

 higher ones. If the greater part of such an overturned 

 and fractured anticline were eroded away, the resulting 

 arrangement of the formations would be such as is shown 

 on the accompanying geological map of the Quebec area. 

 This would account for the fact that the oldest strata, 

 the Sillery, really appear highest in the section, and also 

 for the non appearance of the Cambrian strata from which 

 the boulders in the Sillery and Levis were derived. It 

 35063- 3 



