BOULDER PAVEMENTS AND ANCIENT BEACHES. 75 



this line by the waves forcing up the coast-ice to which these boulders have 

 been frozen. When these deposits occur adjacent to the modern beach, they 

 may be seen rising out of the water, but they are also found outward in the 

 lake to the depth of several feet (Plate 1, fig. 1). 



In front of an elevated shore, the boulders may be arranged iu the form of 

 a zone, even a few hundred yards in width, throughout a vertical range of a 

 few feet, which may be increased to thirty or forty feet where there is a suc- 

 cession of beachlets close together, marking the gradual recession of the 

 waters. But the upper level of these zones never quite reaches that of the 

 beaches. In travelling along a flat country these pavements of boulders are 

 as certain indications of shore-lines as are any other forms of the beaches 

 (Plate 1, fig. 2). Boulders left on the hillsides by the action of rains, washing 

 out the finer materials of the drift clay, are not arrauged in belts of symet- 

 rical level. The boulder pavements do not usually occur where the adjacent 

 coast is not composed of boulder clay, nor where the beaches are separated 

 from the land by what is now or has been a bay or lagoon. Pavements of 

 boulders are not as commonly seen in front of modern shores as iu front of 

 some of those more elevated and ancient. 



Turning to the more typical form of the beach structure, as shown in the 

 raised shores, there may be seen sand or gravel ridges, most frequently from 

 one hundred to sometimes five hundred feet across, rising to fifteen or 

 twenty-five feet above a flat or very gently descending plain, whose surface 

 is most commonly composed of fine clay. Sometimes this descent is so very 

 gradual as to be inconspicuous ; at other places the descent is quite sudden. 

 The depression behind the ridge is generally less than that in front of it, and 

 here also the floor may be composed of clay. When the beach is broad, it is 

 apt to be broken up into a number of ridgelets (e, fig. 1). Indeed, some of 

 the larger and more important beaches mark the recession of the waters by 

 separating into several ridges, often at considerable distances apart, each a 

 few feet below the preceding, where the lake floor is sloping very gently ; 

 but where the slope is more rapid, all unite into one large ridge. The beach 

 has rarely a thickness of more than fifteen or twenty feet, and rests upon the 

 clay or drift deposits, which constituted the floor of the former lake. As 

 the plain recedes from the shore, the materials become finer and finer clay 

 and freer from sand ; but at varying distances, of sometimes a mile or more 

 in front of the beaches, there may be found thin belts of sand resting upon 

 the lake deposits. Again, the beaches may take the form of terraces of con- 

 struction, resting against clay banks; or against these banks the ridges may 

 abruptly (but only temporarily) end like the modern beaches (b, fig. 6). 



In measuring the comparative altitudes of a beach at different points, the 

 summit of a well marked ridge should be chosen, rather than that of the 

 beach in the form of a terrace (a, fig. 2) against the shore, or the junction 



