GLACIAL ORIGIN OF CHANNELS ON DRUMLINS. 9 



an area as the whole state has perinitted only a general reconnaissance, rather than 

 a detailed survey, and much more time is re(iuired to produce a well linished result. 



Symmktry of Dku.mlins a rROTErnoN AGAINST Erosion 



The typical druiiilin or lenticular hill of Hitchcock, with its smooth, flowing- out- 

 lines, long, gentle slopes on the ends, and somewhat steeper slopes on the sides, 

 and with no break in its curves, is the most symmetrical and graceful hill that na- 

 ture i)roduces. This very perfection of symmetry in form has enabled it to with- 

 stand the attacks of the rains and snows by causing the water to flow off with an 

 even distribution, and thus preventing it from collecting in sufficient quantities to 

 produce an incipient crease, which in time might be enlarged to a small gully and 

 stream channel. The small amount of erosion that has taken i)lace has been so 

 evenly distri])uted over the whole surface that it is now j)ractically impossible to 

 even make an approximate estimate of its amount. 



Erosion of Drumlins 



CHARACTERISTICS OF POSTGLACIAL EROSION 



But the single typical drumlin is not the only form. There are groui)S of two, 

 three or more, each member of which is nearly or (laite perfect in itself and only 

 touching its neighbor at its lowest periphery. In such cases the valley between has 

 its bottom nearly or quite on a level with that of the surrounding country. In 

 other cases the members of a group may be so closely joined together as to rise 

 from the same base and have a quarter or a third of their mass in common, and 

 then the bottom of the valley between is at an elevation far above the surrounding 

 country level— an elevation one-third, one-half or two-thirds that of the summits on 

 each side, but in all these cases the valley is one of construction, and its origin is 

 in common with that of the drumlins themselves. Rain-water flowing down the 

 adjacent sloi)es accunuilates in these valleys in little streams, which erode small 

 channels from either end of the valley to the base of the group. These features of 

 postglacial erosion are common. 



CHARACTERISTICS OF GLACIAL EROSIOS 



Of a very distinct type, however, are the channels of erosion which form the 

 subject of this paper and which are glacial instead of i)()stglacial in their origin. 



The longer axes of the drumlins are closely i)arallel in direction with the glacial 

 grooves upon the rock surfaces in their own neigh])orhood, as in the central i)art 

 of the state they are both nearly north and south, while in the neighborhood of 

 Boston they are about southeast. The eskers, which are now generally believed 

 to be formed by the accumulation of material in the bed of streams flowing on, in 

 or under the ice-sheet, also agree fairly in direction with the above. This latter 

 fcict tends to show that the drainage of the glacial sheet was practically in the 

 direction of its motion. So likewise these glacial stream channels with very few 

 exce[)ti()ns obey the same law and run southward. They cut the typical, synnnet- 

 rical drundin at all altitudes from the base to the exact summit ridge, and while 

 winding as is usual with the course of a stream their general direction is parallel to 

 tlie axis, though often they break through their lower bank and seek the shortest 

 way down the steeper side to the bottom of the hill. Often they occur, passing 

 1 1 -Bum.. Gkol. 8oc. Am., Vol. 6, 1894. 



