THE lEOQUOIS BEACH. 



127 



always reached again by following the margin of the old shores, which are generally 

 well defined. In front of the ridges, there is often a very heavy pavement of boulders 

 (fig. 6 r). The blocks seldom reach a length of more than from three to five feet (the 

 largest seen liud a volume of 150 cubic feet), and they are generally smaller. The smaller 

 stones may have been arranged by the waves, but the larger were stranded in the shallow 

 waters by the coast-ice — no heavier than that seen to-day— or were left just below water- 

 level, as the finer materials were washed out of the shores composed of boulder clay 

 owing to wave action. The pavement is most widely developed, when the subaqueous 

 plains descend very gradually, where it may be hundreds of yards wide, although generally 

 it is only as many feet. The boulders upon the hill-sides or within the beach, are more 

 scattered, and do not form a pavement.' 



Fig. 6. — Vertical section showing a beach in the form 

 of a terrace of construction (a), in front of whicli 

 there is a pavement of boulders (, P) ; former water- 

 level (1(0. 



Fig. 7.— Vertical section showing a cut or erosion ter- 

 race floor, with the true beach wanting, but re- 

 placed by a pavement of boulders (P). 



(6.) Difficulties in following the Be.\ch. — There are several large river valleys, 

 especially in the country composed of drift, so modifying the topography as to render 

 the following of the beach often uncertain, or locally impossible. In front of exposed 

 clay bluffs, and indeed of rocky shores, along which, in water that was deep enough 

 to allow the free action of coastal currents, there was no beach formed, as all the material 

 was carried out into the lake with very little assortment. The gravel-beach often ends 

 abruptly in clay banks. When the conditions are favorable there may be considerable 

 gaps in the beach of even several miles (fig. 4.), but still the cut-terrace is there, and 

 along its foot the shore may be followed. The occurrence of islands in front of the coast 

 complicates observations. "Where the shores have been steep and the gravel deposits have 

 been left upon their flanks, the highest part of the beach cannot always be recognized, as 

 the older gravels may be mistaken for those of the beach. Again, when the beach is a 

 constructed terrace against the shore, it is apt to be obscured by overwashes from the 

 hillsides. One last cause may be added to these difficulties : there is often a covering of 

 clay and cobble stones, unlike the rest of the beach, to a depth of two or four feet. This 

 deposit has a significance as pointing to a temporary submergence, yet it is no deeper 

 than frost action may have been. 



(*7.) Sources of the Beach Materials. — The boulders, gravels, and sands are alike 

 almost entirely derived from the drift hills of stony clay, or these capped with stratified 

 sand and gravel with boulders, forming the shores of the lake. The gravel is mostly 



' For the fuller study of ancient beach-structure, see Ancient Beaches, Boulder-Pavements, etc., by the 

 author, in Bull. G. S. A., i. 71—86, 1889. 



