PRE-GLACIAL DRAINAGE AND EROSION. 513 



and in somewhat larg-er proportions in the area just Ijehjvv the mouth of the 

 tributary as a resultant of the transjjorting- power of the main stream and 

 tlie tributary, so that upon the lowering of the waters of the main stream 

 and their retreat from the western lateral valley each tributary found its 

 way southward dammed np by its own delta dej)osits, and, ponding back 

 behind them, flowed again through its old notcli to join the diminished 

 waters of the Connecticut. The heavy sands which fill up the latei-al valley 

 below each of these tributaries, from the western border across to the divide 

 range, do in fact show, both by the derivation of their material and by then- 

 structure, that they are the ancient deltas of these streams. 



The tlu'ead of the current of the main stream, driven clear across toward 

 the eastern foot of the divide range by the great delta of the Millers and 

 Chicopee rivers, had continued to pass through (a) the narrow passage 

 between Deei-field Mountain and Mount Toby and (b) the Holyoke notch, 

 two portions of its old channel, partly, perhaps, because these lay in the 

 main artery of the pre-Glacial drainage, but more because they were out 

 in the center of the lake, far from all lateral streams and their deposits, and 

 on the recession of the waters the western or lateral valley was filled up 

 to such a height by the Westfield River that the Connecticut was compelled 

 to shrink down to this line and reoccupy its old notch in the Holyoke range. 



THE PRE-GLACIAL COURSE OF THE CONNECTICUT AND ITS TRIBUTARIES. 



The pre-Glacial bed of the Connecticut across Massachusetts lay 

 below the present sea level. (See map, PI. XI, p. 510.) Hence, where 

 the river passes over rocky bottoms with rapids and waterfalls it has been 

 expelled from its pre-Glacial bed by Glacial and Champlain deposits. In 

 each case the old bed of the stream is marked by a broad band of depres- 

 sions in the high terrace sands — kettleholes — partly empty and partly 

 water-filled. The ice seems to have persisted in the deep channel until it 

 was covered by the flood sands and then to have melted to form the 

 depressions. This is most mai-ked south of Millers Falls across the Mon- 

 tague plain, in the great loop of Millers River and the succession of ponds 

 extending southwai'd, of which Lake Pleasant (its bottom about 67 feet 

 below the plain) is the largest. From the State line to Northfield fai*ms 

 the river has regained its old bed. South of this point the great delta of 

 Millers River crowded it 6 miles west to the foot of the trap ridge and 



MON XXIX 33 



