22 ME. T. F. JAMIESON STJPPLEMEIfTARY 



ceases, and then settles down again. The retreating wave has a 

 tendency to draw the sand and stones down the incline to its onter 

 edge ; if the slope below is steep they will then roll down until their 

 momentum is extinguished by friction, the largest and heaviest 

 stones going farthest. In this way the slope beneath the beach is 

 clothed with a sheet of sediment which accumulates according to the 

 supply of material from above. 



On the other hand, the advancing wave has a tendency to carry 

 sand and stones up the beach, and more especially everything light 

 which floats upon the water. Some of these will be stranded where 

 the retreating wave is unable to carry them off again, especially 

 when a storm is subsiding. Eut where the slope is steep both above 

 and below the beach, as it often is in Glen Roy, this result would 

 seldom happen with the heavy material. The prevailing tendency 

 of the stones in such a case, I think, would be downwards, as 

 Lubbock insists. Accumulation would take place chiefly at the 

 outer edge of the beach and below it, not upon the beach itself. 



Such appears to be the effect on the bottom. The action on 

 the bank facing the water is different ; in calm weather it is almost 

 nil, but during a gale the advancing wave strikes the bank hori- 

 zontally or obliquely upwards, like a liquid battering-ram, while the 

 surf washes up and down against it, licking hollows in its surface. 

 If the materials are porous and friable the water penetrates them 

 and loosens their cohesion, whilst the shock of the wave dislodges 

 them and sets them in motion. The bank is in this manner under- 

 mined and a portion of it brought down. The debris thus tumbled 

 down is spread out by the waves and acted upon as part of the 

 bottom. The result is to cut back the bank until the upward slope 

 of the beach extends as far as the highest limit reached by the waves 

 during storms. The fretting of the water against the hillside in 

 this way would therefore produce an escarpment, or steeper slope, 

 just above the edge of the lake. Although the gradual crumbling 

 down of the materials from above has now almost obliterated this 

 little scarp, yet traces of it may still be perceived in many places, 

 and the x^rotuberance of the front of the terrace beyond the general 

 slope of the hill may also generally be noticed. Both are observable 

 in MacCulloch's ' real profile ' of one of the ' roads,' and they have 

 been remarked by other observers. For example. Prof. H. D. 

 Rogers says : — " Seen in profile, as when looked at horizontally, 

 they [the ' roads "] resemble so many artificial hill-side cuttings, the 

 back of each terrace lying within the general profile of the mountain 

 slope, while the front or outer edge is protuberant beyond it." ^ 



The action of the lake-margin, then, would produce a flatter sur- 

 face at its line of contact with the hill and a steeper slope or scarp 

 just above this line ; it would transfer part of the matter from the 

 slope above the beach to the slope beneath it, and cover the beach 

 itself with some washed sand and gravel, which are now hidden 

 by turf and fallen debris. The general absence of an ordinary 



1 Proc. Eoy. Inst. vol. iii. (1861) p. 342. 



