970 ■ HISTORICAL GEOLOGY. 



along the front. But in some large submerged valleys, like that of the 

 Connecticut, in which the bottom ice had a movement of flow in the direc- 

 tion of the valley, there were sometimes obstructing conditions which pro- 

 duced a forced deposition of bowlders and till, and thus made an accumulation 

 somewhat moraine-like, which might be called an obstruction moraine. 



A good example exists along the loest side of the south end of the Connecticut valley, 

 in the vicinity of Xew Haven. This declivity is rather abrupt, and has a nearly north- 

 and-south direction, while the course of the valley ice-stream, as described on page 956, 

 was S. 15° W. The ice-stream, in meeting the obstructing ridge or declivity, dropped along 

 it a large amount of till and many great bowlders of trap and sandstone. The top of the 

 ridge, five miles from the Sound, is about 300' above the lower land to the east, and 400' 

 above the sea level. One great bowlder of 1200 tons, and several others of large size 

 near by, were a little too low in the ice to pass the top of the ridge, and consequently 

 became stranded against its slopes, or combed out by its summit ledges. Half a mile 

 north is another trap bowlder of 500 tons, and several exceeding 100 tons lie to the south. 

 A mile and a half to the east, but separated by an open valley 300' deep, stands the West 

 Rock trap ridge, of equal height ; and on this ridge, and almost in an east and west line 

 with the 1200-ton bowlder just mentioned, at a like height, there is a 1000-ton bowlder, 

 which was similarly stranded. For a distance of 10 miles from Long Island Sound 

 the great bowlders are common, and the till against the slopes has unusual thickness. 

 The upper part of the glacier above the level of the ridges kept on its southeastward course 

 (S. 30°-40° E.), carrying bowlders of gneiss from the northwest. But some, if not all, 

 of these gneiss bowlders, while on their way over the valley, dropped down so as to come 

 within the lower or valley ice-movement ; and they are now, as a consequence, part of 

 the obstruction moraine along the eastern base of the West Rock Ridge, and other north- 

 and-south trap ridges of the valley. 



Among the formations produced by the melting, besides moraines and 

 deposits of till, clay, and other ordinary materials, there were glacial accumu- 

 lations of loose materials called drmnlins, and eskers or kames, — formations 

 that were much less common in connection with the early partial retreat 

 than with the final. Kettle-holes, also, were a feature of many moraines, 

 from the Coteau des Prairies to Cape Cod. 



Kettle-Jwles are bowl-shaped depressions, usually 30 to 50 feet deep and 

 100 to 500 feet in larger diameter. Each depression, according to the accepted 

 explanation, was the resting-place, and often the burial-place, of a huge mass 

 of ice that became detached during the melting ; and the final melting away 

 of the ice left a hole where the ice lay. The great Wisconsin moraine about 

 Green Bay is called by Chamberlin the "Kettle Range," from the great 

 numbers of its kettle-holes. Near Wood's Hole, in southeastern Massachu- 

 setts, opposite Martha's Vineyard, 1000 kettle-holes occur, according to B. F. 

 Koons, in a distance of about 12 miles. Kettle-holes occur sparingly over 

 Long Island ; but it is possible, since there is clay beneath the drift, that 

 the weight of the overlying drift, with the addition of the resting glacier in 

 some cases, forced aside the clay, flexing its layers in the process, and thus 

 made the bowl-like depressions. 



Drumlins are hills or ridges of till, 30 to 200 feet high, made ordinarily 

 by deposition from the glacier, or in the course of its dissolution ; and 



