222 A. Y\\ G. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN 



side it. but immediately adjacent on the neighboring upland, would place 

 it at a minimum thickness of more than 1,000 feet. 



Summary of the post-Cretaceous geologic History 9 



Following the period of elevation which succeeded the Cretaceous 

 peneplanation epoch, a belted coastal plain topography had reached a 

 mature stage of development in the lake Xipigon basin. There were 

 still many remnants of the lower members of the coastal plain sediments, 

 scattered over the main lowland between the Archean oldland and the 

 (probably) paleozoic sediments. At a number of different points in 

 this and in the adjacent areas, huge masses of molten diabase were forced 

 upward from below through fissures. In those parts of the district 

 where the cover of Paleozoic sediments was not removed the diabase was 

 forced between many of the beds in laccolitic masses or broke across the 

 beds as dikes. Xear the edge of the cuesta some of the laccolitic masses 

 probably forced their way (horizontally) to the edge of the escarpment 

 and the molten diabase poured down upon the lowland. Through other 

 fissures passing only through the Archean rocks of the lowland, fluid 

 diabase was also forced upward. The flows followed each other with suffi- 

 cient regularity and so closely that a large amount of liquid rock col- 

 lected in the lowland basin in front of the inner cuesta, between it and 

 the oldland, eventually reaching such a height that it overflowed upon 

 the adjacent uplands. Owing to its depth, the rock cooled very slowly 

 except in the lower basal portions and in the upper part. Probably the 

 middle portions congealed nearly simultaneously throughout the whole 

 mass, after a long period of slow cooling. 



Subsequent erosion has removed over 90 per cent of the diabase (pos- 

 sibly 95 per cent) and a considerable portion of the underlying sedi- 

 ments. Here and there, however, portions of the fossil post-Cretaceous 

 coastal plain have been preserved by its diabase cover. The post-diabase 

 degradation has developed a type of topography very similar to that of 

 the original coastal plain. Subsequent to the outpouring of the diabase 

 a system of block faulting has slightly affected the region, disconnecting 

 contiguous portions of the diabase sheets and sills, modifying locally the 

 type of topography developed, and differentially affecting the rates of 

 erosion in different parts. The presence of laccolitic sills and capping 

 sheets of diabase, much harder and more compact than the underlying 

 sediments, has resulted in the development of the numerous mesas and 

 buttes so characteristic of the region. 



9 Post-Cretaceous on the assumption that the Great Lake basins (except Superior) are 

 of this date. 



