DISTRIBUTION OP SURFACE BOULDERS. 81 



Upon the steep hillsides, as along the Mahoning valley, near the crossing 

 of the Ohio-Pennsylvania line, there are zones thickly covered with boulders. 

 There we find the records of old water-margins, as well as in the pavements 

 associated with the well marked beaches and shore-cliffs facing the lake 

 basins. The finer materials have been washed out of the associated drift to 

 form bars, in the valley, which was once filled with water. On some 

 of the higher hills between the southern part of Georgian bay and Lake 

 Huron, to the west, the tops of ridges are covered with boulder pavements. 

 These ridges were islands in a former expanded lake or sea, whose surfaces 

 were encroached upon by the waves, until they were reduced to partially 

 submerged reefs covered with great erratic blocks, as the finer mud was borne 

 into the deep water. That these were island shores may be seen from the 

 boulder covered ridges, although miles apart, being reduced to a common 

 altitude. 



On the hillsides, behind the fringes, there are only here and there irregu- 

 larly deposited blocks, exposed by the action of rains. Besides, the meteoric 

 effects upon any of the hills are small, compared with the encroachments 

 of the waves, in exposing enough stones to make boulder pavements. 



The occasional erratic blocks often reposing upon fine lacustrine deposits 

 are of little importance, and indicate only an occasional stone entangled in 

 old coast-ice from an adjacent shore, when the region was covered with 

 water, just as the boulders resting upon the sunken ships in the mouth of 

 the Baltic have been deposited from the coast-ice moving out of that sea. 



The study of the relation of the pavements of boulders to beaches sets at 

 rest the speculation upon the origin of these fringes, and obviates the necessity 

 for appealing to either icebergs or glaciers in later Pleistocene days to account 

 for the erratics, popularly called "hard heads," which are scattered over 

 the country in the form of pavements or fringes ; for these are usually seen 

 only where they can now be referred to some old coast line, or a succession 

 of shore lines, acted upon, in former days, by frost and coast-ice. 



Chapter III. 

 High-Level Gravel Deposits in the Region op the Great Lakes. 



Rather than rummage through the talus heaps of geological literature for 

 the different kinds of gravel deposits which may represent beach structure, 

 it is easier to go into the field of observation and investigate those forms 

 which may be modified beaches, or be related to, or be mistaken for them. 

 This method is the more satisfactory, as the geological literature often con- 

 founds different forms, and leaves others unnoticed, or not considered in the 

 light of the present investigation. The object of this chapter is to describe 



XI— Bull. Geol. Soc. Am., Vol. 1, 1889. 



