[whittaker] bottom DEPOSITS OF McKAY LAKE 143 



south end where the shoreline for a short distance is composed of 

 sand only. 



A glance at the map of McKay lake shows the areas well repre- 

 sented. The area between the shore and the dotted line surrounding 

 it represents mucky deposits. 



As may be seen from the map, the outlet of the lake is at the 

 extreme north end of the lake. It is a small stream two to four feet 

 wide and about one-half a mile in length, which flows into the Ottawa 

 River. This stream has cut a valley from twenty-five to forty feet 

 deep and from eighty to one hundred feet wide at the top through the 

 Champlain clays. The former level of the lake must have been nearly 

 at the top of these deposits on the emergence of the land from the sea. 

 Erosion by the outlet stream through the soft unconsolidated clays 

 must have been rapid. As the level of the lake fell and the grade 

 decreased, the erosive power of the stream diminished, and now is 

 almost negligible. Subsequent to the withdrawal of the sea the lake 

 has been depositing its sediment products in its own freshwater basin 

 up to the present time. 



Ancient Bottom Deposits of McKay Lake 



As noted above the lake is surrounded on the north and east sides 

 by marine clays and sands while an escarpment of rock of Chazy age 

 borders the west side. On the south side there is a bed of marl con- 

 taining freshwater shells, two to four feet in thickness, and about 15 

 feet above lake level. This marl represents one of the deposits of the 

 earliest stage of McKay Lake. It rests upon sand which frequently 

 shows crossbedding and folding with occasional boulder and gravel 

 contents, suggesting that these underlying beds are of fluvioglacial 

 origin. Elsewhere near the lake these fluvioglacial deposits are 

 covered by marine clays. 



These marl beds date back probably to the closing stage of marine 

 submergence. The former McKay Lake which was much larger than 

 at present must have remained at nearly the same high level for a 

 period during which these marl beds were laid down. When the 

 stream finally cut back from the Ottawa River to the outlet of the 

 present lake, the water dropped very rapidly to its present level, eigh- 

 teen feet below the top of the marl beds. As the lake was thus reduced 

 in size it was removed from direct contact with the marine clays, and 

 marl deposition practically ceased. Although at present occupying 

 only a restricted area at the southern end, eighteen to twenty feet 

 above the present lake level, they were probably of much greater extent 



