60 
RELATIONSHIPS OF THE MARL DEPOSITS 
McKay lake is 500 yards long and 200 yards wide, with its 
longer axis approximately north and south. On the west side 
there is an escarpment of Chazy sandstone, but on all other 
sides Pleistocene and Recent deposits form the shore. Cross- 
bedded gravel and sand border the south side, but on the south- 
east only sand occurs. To the north are typical Leda clays, 
rising, like the sand and gravel, 18 to 25 feet above the present 
level of the lake, or about 40 feet above Ottawa river. Above 
the sands, on the south and east sides of the lake, extending 
to its northern end, is the marl deposit which at present is 
disconnected owing to the large amount of material removed 
for commercial purposes. At the time of the retreat of the 
Champlain sea from Ottawa valley the water of McKay lake 
stood at an elevation at least 20 feet higher than at present. 
The waters from the lake commenced eroding a channel to 
Ottawa river through the soft, unconsolidated, marine clays 
now found at its north end. The marl was deposited while 
the water remained at this high level. Its source was doubtless 
lime leached from the marine clay and subsequently precipit- 
ated either from solution chemically, or by the Chara which 
is still found in the lake, or in both ways. The marl consists 
in part of shells and shell fragments, but for the most part of 
a fine, impalpable powder. Owing to the high grade of the 
outflowing stream through this material, erosion must at first 
have been very rapid. When the stream had cut back to the 
outlet, the waters of the lake probably fell rapidly to about 
their present level, and being no longer so continuously in con- 
tact with the marine clay, marl deposition practically ceased. 
The valley of the stream is from 80 to 100 feet wide at the top 
and from 25 to 40 feet deep. Owing to its low grade the stream 
is now eroding its channel much more slowly. 
The marl as noted above is from 18 to 25 feet above the 
present level of the lake, and overlies crossbedded gravels and 
sands. It is from 3 to 5 feet thick and is overlain thinly by 
soil and peaty matter on which grows a luxuriant forest of 
large trees and underbrush. The marl is yellowish white to 
