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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 represent 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 terrace 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,^[1] 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 1⅔ 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 débris 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 débris of its tributaries