354 A. P. Coleman — Wave Wo7'k as a Measure of Time : 



out of consideration also. The thirteen measurements that 

 appear to be normal give an average of 81 feet in the fifty 

 years, which v^orks out to 1*62 feet per annum. 



The rate of recession having been settled, the next problem 

 to determine is the distance through which the cliffs have been 

 cut back. At first sight this might appear to be almost insol- 

 uble, but a study of the history of the former lakes of the 

 Ontario basin and of the mode of action of waves in such lakes 

 gives quite definite information as to the point where the work 

 began. 



The earliest lake of which there is positive evidence occurred 

 in interglacial times, when the water stood 150 feet or 

 more above the level of the present lake and a great river 

 flowing from the northwest entered at Scarboro'. This rivei', 

 an interglacial successor of Dr. Spencer's preglacial Lauren tian 

 river, built a delta of clay and sand out into the lake causing 

 an obstruction to the next advance of the ice. This delay in 

 the motion of the glacier was repeated at each successive 

 advance, and the result was the building of a ridge of bowlder 

 clay and interglacial materials, which begins at the interlobate 

 moraine between the Ontario and Greorgian Bay basins and 

 extends with a fairly uniform elevation to Scarboro' Heights, 

 making the highest point on the whole shore of Lake Ontario. 

 The transformation from a river valley into the highest ridge 

 in the region is an impressive one. 



That such a long and uniform ridge did not end suddenly 

 but once extended farther into the Ontario basin is in itself 

 probable ; and to determine just how far it reached beyond 

 the present shore will give a measure of the length of time dur- 

 ing which wave work has gone on under present conditions. 



It is a well-known fact that a cape projecting into a body of 

 water is more strongly attacked than the rest of the shore, and 

 that the materials removed by the waves are shifted in the 

 direction of the greatest reach of the prevalent storm winds 

 and built out across the next bay as a spit. If the bay is shal- 

 low the spit may grow across its whole width and form a bar. 

 If the bay is deep the spit advances only so far across as the 

 lower limit of wave action permits and then bends shorewards 

 forming a hook. Successive hooks are built out into the deeper 

 water forming a platform which rises a few feet above the 

 water and encloses shallow depressions or lagoons. As the 

 work continues and the cape is cut back farther and farther, a 

 gently sloping terrace is carved from it, and at the same time 

 the spit or hook is shifted in a direction inland forming a ter- 

 race also, but in this case one that was first built up from the 

 bottom and afterwards cut down a little by wave action. 

 These two varieties of terrace, though formed in diilerent ways, 



