A Study of the Ontario Basin. 356 



join and make a continuous subaqueous shelf alon^ the coast 

 where the process is at work. The under-water forms of the 

 two portions of the shelf may be studied by means of sound- 

 ings, and it is found that the part carved from the projecting 

 shore has a gentle slope to the edge where the attack began, 

 and a steeper slope beyond towards the depths of the lake ; 

 while the built terrace terminates outwards in a much more 

 rapid descent into deep water. The slope just mentioned is 

 usually about at the angle of stability for the materials of 

 which it has been built. 



Such a shelf consisting of two distinct parts formed in differ- 

 ent ways and having different contours may be recognized 

 under water by soundings, and the edge where the more rapid 

 descent into deep water occurs represents the end of the 

 promontory before the carving of the waves began. 



Fortunately ancient Lake Iroquois did the same kind of 

 work as Lake Ontario and carried its work to about the same 

 degree of completion, so that one may study these subaqueous 

 earth forms conveniently on dry land. An old shore cliff of 

 Lake Iroquois is well seen at Davenport ridge in Toronto, ris- 

 ing from 50 to 75 feet above a terrace carved from rolling 

 bowlder-clay country, and sloping down for nearly 170 feet to 

 the shore of Lake Ontario. This descent is pretty uniform, 

 and the contours are fairly evenly distributed over the three 

 miles of distance between the old shore cliff and the present 

 water front. The carved portion of the terrace, sometinjes 

 veneered with more or less stratified sand, joins on the east the 

 terrace built up of cross-bedded sand of great thickness south 

 of the old Humber bar. These relationships were described 

 by Sir Sandford Fleming in 1864 and a comparison was made 

 between them and the relations of Toronto island to Scar- 

 boro' Heights.* 



We are now ready to consider the extent of the recession of 

 Scarboro' Heights by the wave action of Lake Ontario. It will 

 be seen from Dr. Spencer's map, in spite of the fact that it is 

 taken from an old and very imperfect survey, that the shelf 

 carved from the cliffs extends with a gentle slope for 2 or 3 

 miles to about 100 feet in depth and then falls off rapidly to 

 greater depths. The southward projection of the contours 

 shown on his map near the middle of the water front is due to 

 an error. The edge of the shelf is in reality very uniform as 

 may be seen on the map accompanying my former paper, or 

 the one given here, which has been prepared from the last 

 Hydrographic Chart published in 1916. The point where the 

 carved terrace joins the built terrace, with its steeper slope, is 

 well marked and the extent of the promontory removed by the 



* Journ. Can. Inst., 2nd Series, vol. vi, pp. 247-253. 



Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 263.— November, 1917. 

 25 



