ACCUMULATIONS OF SURFACE BOULDERS. *79 



stone to be seen, and others where only an occasional block occurs, as if 

 dropped down from some meteoric source, there are other localities literally 

 so covered with large boulders as to prevent agricultural pursuits. These 

 boulder accumulations are superficial and do not peuetrate the subjacent 

 earths. They occur along certain zones, outside of which they are not 

 found. 



The presence of these surface boulder accumulations has been most com- 

 monly explained alike by those who believe in the glacial origin of the drift 

 and those who do not, as having been dropped by melting icebergs at the 

 close of the drift epoch. A few glacialists regard these boulders as having 

 been deposited from glaciers where they now rest. It has also been hinted 

 that they have been left upon the hills, as the finer materials of the boulder 

 drift have been washed away by atmospheric agencies ; but it was only since 

 the recent systematic studies of the high-level beaches, compared with modern 

 lake shores, have been made that the natural explanation of boulder pave- 

 ments and distribution of erratics become possible. 



There are three conditions under which boulder accumulations are found. 

 The most important is where the boulders form pavements stretching as belts 

 across a level country, usually in front of ridges which once constituted old 

 shore-lines, or forming zones of stones resting upon hillsides or capping the 

 summits of ridges. Of lesser importance is the occurrence of blocks scattered 

 sparsely and irregularly on the sides of hills. Lastly, occasionally erratics 

 are found alike over the hilly and over the flat country. That the boulders 

 were brought from their original sources in the later Pleistocene days and 

 dropped by either icebergs or glaciers where we now find them is an unten- 

 able hypothesis, for their birth places are now often covered with the older 

 drift or are hundreds of feet below the elevations where they are now found. 

 The relation of the boulders to the older drift are such that the erratics can 

 commonly be recognized as of secondary origin, being derived from the 

 earlier accumulations of boulder clay or sand. The manner in which the 

 blocks have been brought to the surface has been by the removal of 

 the finer earths from the drift, principally by the action of the waves or 

 currents enci'oaching upon the hills or ridges of such materials, charged 

 with occasional boulders. Thus the coast-line has been moulded into steep 

 shores, in front of which there is the gently descending plain, once sub- 

 merged — the floor of a terrace since the recession of the waters (figs. 

 2 and 3). 



Thus the boulders throughout the whole thickness of the drift, which were 

 too large for transportation by the waves, were reduced to water level and 

 were accumulated upon the floor in the form of pavements or fringes 

 along the former water-margins. The removal of the earth beneath the 

 boulders continued until they had settled to the maximum depth of wave 



