MUSEUM OF COMPAKATIVE ZOOLOGY. 187 



necessary that there should be a sufficient distance between the selected 

 locality and the sea to afford a chance for the display of a bowlder 

 trail of such length as to afford indications of value. After much pre- 

 liminary search of various localities in New England, I found that the 

 boulder train from the so called Iron Hill in Cumberland, R. I., pre- 

 sented by far the most satisfixctory basis for the proposed study. 



General Description of the Train. 



The accompanying map will afford the reader a sufficient idea as to 

 the geographical conditions of the district in which the trail from the 

 Iron Hill lies. It will be observed that the iron deposit is situated in the 

 town of Cumberland, which lies on the eastern margin of the valley of 

 the Blackstone River, at a point about fifteen miles north of Providence, 

 R. I. Although the contour map which accompanies this report, and 

 -which is reproduced from the plates prepared by the U. S. Gee -gical 

 Survey, will afford an excellent idea of the topography, the scaie on 

 which it is printed is somewhat smaller than is desirable for the com- 

 plete display of the matter with which we have to deal. It would have 

 been possible, from the original maps of the Survey, to have made this 

 illustration on a more considerable scale, but the chart would have then 

 been too large for convenient use. 



It will be observed that, except for the occurrence of a few somewhat 

 isolated hills, the reliefs of this district have no great height. They 

 rarely, indeed, exceed one hundred feet of elevation above the neigh- 

 boring valleys. For about thirteen miles from its point of origin, this 

 glacial trail lies upon a surface of bed rock of varied hardness, which 

 still retains in good part the topography given to it by erosive agents 

 which operated before the advent of the last Ice Age. The boulder 

 clay or ground moraine of which the train forms a part is, except where 

 it crosses these ancient valleys, usually not more than ten feet thick- 

 It is likely that in this district there may be many deep valleys in the 

 bed rock which have been entirely effaced by the thick deposits of drift 

 which mantle this part of the coast between Valley Falls and Aquidneck 

 Island. On that island the bed rock is again near the surface, the drift 

 being rather more than fifteen feet in average depth, and except next the 

 shores, where some waslied drift occurs, consisting altogether of boulder 

 clay. In this part of its course the movement of the glacial stream 

 which bore the material deposited in the train may have been somewhat 

 guided by the long ridge which forms Aquidneck Island, and by the 



