Birth of Lake Ontario. 445 



In the survey of the Iroquois Beach, the shore-line has been 

 followed by one or another of its characteristics, even across 

 areas of broken physical features. The altitude of the highest 

 ridge, where the beach is broken up into a series of ridges, is 

 that which has been everywhere taken, for it is the one giving 

 most accurate results. No elevations have been adopted except 

 those of the summit of the crests (as in fig. 1), or of the spits 

 (at h or b, fig. 2). The measurements consequently represent 

 the maximum height of wave-action, in place of the mean sur- 

 face of the water, which was a few feet below. The writer's 

 leveling has everywhere been done instru mentally. 



The coast materials, out of which the Iroquois shores have 

 been carved, are mostly bowlder clay, or stratified clays or 

 sands, deposited upon the floor of the lake when the waters 

 were at higher levels. At a few places the shores rest against 

 Paleozoic rocks, in which case the materials of the gravel beach 

 are more scanty, as the pebbles were mostly derived from the 

 stony drift, or there may be an absence of the beach. 



Except in spits across old valleys, the thickness of the sand 

 and gravel of the beach does not usually exceed 20 feet, but in 

 front of valleys it may reach a thickness of 100 feet (h, fig. 2). 

 The internal structure always shows stratification, with such 

 sloping and false-bedding as are characteristic of beaches. 



There are frequent exposures which show that the Iroquois 

 Beach rests upon stratified stoneless clay — the silt washed into 

 the waters when the waves were encroaching upon older and 

 higher shore-lines, and assorting the bowlder clay, which, at 

 the higher elevations, formed the coast. Eastward of Water- 

 town, the beach rests upon stratified sand in place of clay, as 

 there was but little stony clay in the drift to furnish silt for the 

 older lake floor. 



From near Trenton to the head of the lake, and thence 

 around the southern and eastern borders to near Watertown, 

 the Iroquois Beach is not hard to follow ; but eastward of that 

 point the features are more complex. The old coast of stony 

 clay is there replaced by stony drift sand, and hence there is 

 but little lithological distinction between the frontal plain and 

 the older sandy drift shores. Moreover, such a coast is apt to 

 be defaced by the sand being heaped into dunes. Again, in 

 the region beyond Watertown, the Iroquois Beach is inter- 

 rupted by promontories of Paleozoic limestones and shales, 

 rising out of deep water, upon which at most only benches 

 were cut. Farther, northeastward, the beaches trend among 

 bold headlands and islands of crystalline rocks. Wave action, 

 which carves broad terraces out of drift materials, can cut only 

 moderately well-marked benches out of limestones. But when 

 the same intensity of wave force is applied to hard crystalline 



