388 THE JOURNAL OF GEOLOGY. 



top of the gorge at Tariffville is about 190 feet above the sea-level. 

 It does not seem probable that these highest terraces were ever 

 continuous over all the Farmington valley. But if they repre- 

 sent the level reached by the maximum flood accompanying the 

 melting of the glacier, the great difference in their height 

 on the two sides of the trap ridge, in connection with 

 the other evidence already noted, gives strong reason for 

 believing that the gorge as it exists to-day had not then been 

 cut. A mile and a half east of Tariffville there is a lower ter- 

 race which is wide-spread. Its general height is about 190 feet, 

 in places a little more. In this terrace the lower part of the 

 Farmington has cut a trench 90 to 100 feet deep. The shape 

 of the valley makes clear the fact that before this trench was cut 

 the river flowed at about the 190 foot level, which is the height 

 of the bottom of the sag at Tariffville. On the west side of the 

 trap ridge there is also a more or less wide -spread terrace at 

 about the same height. It seems very probable therefore that 

 the river was raised to the level of the old sag in the trap ridge 

 by the building of these terraces. 



The present average southward slope of the highest terraces 

 west of the trap ridge from Northampton, Mass., to Farmington, 

 Conn., forty-four miles, is seven inches per mile,' and Professor 

 Dana is inclined to believe that this is approximately the slope 

 at the time the terraces were built. The character of the 

 deposits shows that the current which formed these deposits 

 flowed south. The present river, flowing north, falls twenty feet 

 between Farmington and Tariffville, or i ^ feet per mile. The 

 reversal of the river was probably determined by two factors. 

 Near the village of Farmington, the waters of 200 square miles 

 of territory are poured into the valley by the upper Farmington 

 and its tributary, the Pequabuck. During the terrace building 

 stage the great mass of debris contributed by these streams was 

 deposited where the steep gradient of the highlands was 

 exchanged for the gentle slope of the lowland. The main north 

 and south valley was thus choked by the debris of its tributaries 



'J. D. Dana, Amer. Jour, of Sci. 3d ser. vol. xxv, p 446. 



