14 



The Ottawa Naturalist 



[April 



THE RELATIONSHIP OF THE FOSSIL MARL FAUNA OF MACKAY LAKE, 

 OTTAWA. TO THE PRESENT MOLLUSCAN FAUNA OF THE LAKE * 



By E. J. Whittaker. 



Physical Features of the Lake 



MacKay or Hemlock Lake is a small body of 

 water in Rockcliffe, just east of the city of Ottawa. 

 It is irregular in shape, about 500 yards in 

 length and 215 yards in greatest breadth. The long 

 axis of the lake runs approximately north and 

 south. One eighth of the total water area is occu- 

 pied by a deep bay indenting the eastern shore to a 

 depth of 150 yards. The surface of the lake is 

 1 5 feet above the Ottawa River and its greatest 

 depth is only thirty feet. The history of this basin 

 dates back to the end of the Pleistocene, when the 

 land was emerging from the Champlain sea. The 

 shore on the west side consists in part of bedrock of 

 Chazy age, while on all other sides are marine 

 sands and clays and some small areas of recent 

 deposits. The topography about the lake reflects 

 these two contrasting types. In the part of the lake 

 enclosed by bedrock the shores are high, small ram- 

 parts of sandstone outcrop, and there is a complete 

 absence of peaty or mucky deposits. Elsewhere the 

 shores are low and owing to their boggy nature the 

 water cannot be approached on foot. A small 

 area at the extreme southern end where the muck 

 deposits are absent, is the only exception. 



At the south end there are two small rills which 

 form the only visible inlets to the lake. A consider- 

 able volume of water is brought down by them in 

 time of spring flood, but in summer they are nearly 

 dry, and the only supply comes from seepage and 

 springs from the surrounding land areas. As this is 

 inconsiderable in amount, the water becomes quite 

 stagnant. The waters of MacKay Lake find an 

 outlet through a small stream, half a mile in length, 

 which flows into the Ottawa. This creek has cut a 

 valley from 25 to 40 feet deep and from 80 to 100 

 feet wide at the top through the Pleistocene clays. 

 Originally, on the emergence of the land from the 

 Champlain Sea, the erosion must have been very 

 rapid through this soft unconsolidated material. At 

 the present time, however, owing to the insignificant 

 volume of water carried the bed of the stream is 

 being lowered very slowly. Its erosive power is 

 further diminished by the abundance of water plants 

 over much of its course. 



Though of small extent, the marl deposits of 



MacKay lake have been known for a long period.** 

 They are very accessible, and new parts of the beds 

 have been constantly brought to view as further 

 advances were made into the sand beds which they 

 overlie. The elevation of the marl beds above the 

 present level of the lake is 18-20 feet, and is without 

 doubt due to the lowering of the lake since their 

 deposition by the cutting down of the outlet. This 

 erosion must have occupied a considerable period of 

 time. Formerly the lake must have been somewhat 

 larger than now, although, at present, the deposits 

 of marl are found only at the south end of the lake, 

 at a distance of about 100 yards from the water. 

 Elsewhere, presumably, the beds have been removed 

 by erosion. The marl is from three to five feet in 

 thickness and is overlain by a small amount of super- 

 ficial soil and peaty matter on which grows a luxuri- 

 ant forest of both large trees and undergrowth, 

 whose roots have filled the beds with a network of 

 interlacing fibres. The underlying material is for 

 the most part sand, which frequently shows cross- 

 bedding, and is occasionally replaced by heavy 

 gravel or boulder suggesting that these lower beds 

 are of fluviatile origin. 



The marl was formerly used in making brick and 

 cement but is not being worked at the present time. 

 In appearance it is yellowish-white to pure white, 

 but is occasionally rust-stained from overlying de- 

 posits. It is very slightly coherent, crumbling 

 readily between the fingers, and a block placed in 

 water will break down like loaf sugar. It consists 

 of a large proportion of fresh water shells well pre- 

 served in a matrix of almost impalpable powder 

 which is not made up of shell fragments as in the 

 case of many marls. Its origin will be discussed 

 later. 



A chemical analysis made in 1894* shows over 

 93% of calcium carbonate, and nearly 5% of 

 organic matter such as root fibres and humus, indi- 

 cating a very pure marl. 



The Faunas. 



The fresh water shell remains in the marl of 

 MacKay Lake are the most modern fossils in the 

 area and belong to a late Pleistocene or early 



*P'ublished by permission of the Director of the 

 Geological Survey of Canada. 



**Geol. Survey of Canada Report 1845-46, p. 96. 

 Report of Progress, 1863, p. 765. 

 Annual Report 1893, vol. VI, p. 70AA. 

 Annual Report 1894, vol. Vir, pp. 23, 24R, 



(Chemical analysis). 

 Annual Report 1899, vol. XII, p. 47. 

 *Annual Report, Geol. Survey of Canada, 1894, 



vol. VII, pp. 23-24R. 



