144 SEA AND LAND 



crowded by the great pressure from above and below until 

 they form extensive ramparts which are frozen solidly together. 

 When these masses float away they often carry with them 

 bowlders weighing one to two tons, and drop them at a dis- 

 tance from the land. In their collision w^ith the shore these 

 fields of rock-covered ice exercise a very great disruptive force. 

 At many points, even in Southern New England, it is difficult 

 to build piers, even of firm masonry, strong enough to with- 

 stand their assaults. The fact tliat these artificial construc- 

 tions fail to withstand the action of this floating ice, shows 

 how vigorous is tlie attack which it makes on the shore. All 

 persons who are familiar with the coast of New England, 

 especially those who have an intimate acquaintance with the 

 districts of any particular portion of it, must have remarked 

 the often conspicuous changes which a ver\' frigid winter 

 brings about in the appearance of the bowldery shores. We 

 have then only to conceive that in the ages these actions are 

 to be multiplied by the million-fold, in order to make it clear 

 that this agent is capable of accomplishing a great deal of 

 geologic work. In general this work is most effectively done 

 in the embayments of the shore, for on the headlands the 

 waves prevent the ice from forming, or keep it broken into 

 small blocks, so that it cannot seize upon the large stones ; 

 in this way the floe-ice serves to wear away the shores of re- 

 entrants more than those of the headlands. Sand-beaches are 

 but little affected by the ice which may form upon them, for 

 the reason that the continual movement of w^ater from the 

 land side j)revents their materials from freezing, so that they 

 do not become entanijled in the floe. 



When geologists first sought to interpret the ice marks so 

 common on the surface of the lands in high latitudes, they 



