THE REPULSION OF TRIBUTAKIES. 747 



river, do not show the pecuhiirlty, since the erosion terraces are tliere nar- 

 row or wanting, but the five brooks south show it most clearly. Cushmaus 

 Brook (called Mill River on the new map) runs down west of Mount 

 Warner. The next two brooks south do the same, and then Fort Ri\er, the 

 . last of the series, illustrates the rule in the most striking manner, and indeed 

 formerly ran much farther south than now, parallel with the Connecticut, 

 and entered the latter above Hockanum at the boat landing of the Mount 

 Holyoke House. This has here plainly the following explanation: The 

 water sank very suddenly in the lake, and the oldest position of the present 

 river of which any trace remains was the eastern edge of the terrace S3'stem. 

 On this sinking of the lake water the streams followed it by the shortest 

 course, cutting gorges in their old deltas, and at one time each one joined 

 the main stream at the point where it at present cuts the boundary between 

 the lake bottom (1 h t) and the terrace system (t'-t^). As the Connecticut 

 swung west and built up its terrace behind it the tributary elongated and 

 kept its com-se across this newly formed terrace, and since this terrace flat 

 or flood plain was built up as a series of bars Avhich grew to be islands, 

 behind each of which there is for a long time a long groove opening south 

 (see p. 726), the brook occupied this and entered the main stream round its 

 south end, and at last this operation, many times repeated, gave the streams 

 their present course. 



It was the observation and study of this law several years ago which 

 caused me to doubt the then prevalent idea held by those most competent 

 to judge, that the Connecticut Valley had been filled up to the height of its 

 high teiTace — the lake bench — and then excavated, and led me to map the 

 teri'aces, as I have done, into («) a high bench or string of deltas bordering 

 the valley; (h) a succession of lake bottoms sloping from the above center- 

 ward and broadening in each of the wider stretches of the valley, and (r) a 

 comparatively small area occupied by the "oscillation terraces" of the river 

 proper — the "meadows." 



