52 SURFACE GEOLOGY. 



amount as the current descends, and sometimes more. It is on the smaller and 

 more rapid streams that we see this slope most conspicuously ; indeed, on these it 

 is so obvious that I deemed measurements unnecessary. I have made only a single 

 one, and that shows the slope in a delta terrace in the west part of Deerfield, which 

 terrace was produced by a small stream called Mill river, which, as it entered the 

 former estuary, thrust forward a quantity of sand marked as a terrace on Plate IV. 

 This deposit would of course be thickest nearest the shore and diminish outwardly. 

 The amount as I measured it by the aneroid barometer, is thirty-nine feet, in less 

 than half a mile, a slope which of course had no reference to that of the current. 



I have said that the slope in some cases is greater than that of the stream. To 

 illustrate this, let us refer to the wide and long basin from Mt. Holyoke to Middle- 

 town, in which the current of the Connecticut must have been gentle, nor could the 

 tributaries have brought in materials sufficient to fill up the broad valley as high as 

 where it is much narrower. Hence we should expect, that as we pass south from 

 Holyoke, the upper terrace would become thinner and thinner. Such I suppose 

 to be the fact, as stated in my description of the sections in the part of Connecticut 

 valley above alluded to. In a distance of forty or fifty miles, I have thought we 

 have evidence of a descent of more than 140 feet, besides the descent of the river. 

 The only doubt I have in the case, arises from the difficulty of determining 

 whether the upper terrace, to which my sections extend, is continuous throughout 

 this whole distance. 



10. Terraces are usually the highest about gorges in river courses. Such is 

 the fact at Bellows Palls, at Brattleborough, at Montague, at South Hadley, and 

 a little above Middletown, where Rocky Hill on the west side of the river pro- 

 duces a narrow gulf for the river. Also between Tekoa and Middle Tekoa, on 

 Agawam river (Section No. 19.) The materials are not accumulated around these 

 narrow passes because they were then closed, for we have shown that since the 

 drift period most of them have not been closed. But the narrowness of the valley 

 at these spots would, to some extent, retard the streams when swollen, and cause 

 it to deposit more of its suspended matter than in the middle part of the basin. 

 In general it is on the lower side of the gorge that the accumulation is the greatest, 

 because there the waters would spread out laterally and produce eddies or ponds. 

 But sometimes it is above the gorge where the terrace is highest, as on Tekoa. 



11. The chief agent in the formation of terraces and beaches appears to have 

 been water. The following facts establish this conclusion beyond' all reasonable 

 doubt. 1. The materials have been so comminuted and rounded as no other agent 

 but water can do. Glaciers and stranded icebergs may, indeed, crush and some- 

 times partially round abraded fragments of rock, but they do not produce deposits 

 of rounded and smoothed pebbles, such as form most of the terraces and beaches. 

 2. The materials are sorted, so that those of different sizes occupy distinct layers. 

 This effect water alone, of all natural agencies, in the form of waves or currents, 

 can produce. The size of the fragments indicates the strength of the breaker, or 

 the current. 3. The deposition of the layers in horizontal or nearly horizontal 

 position, can be effected only by water. In order to produce the level tops of the 

 terraces water must have once stood above them, while currents strewed the mate- 



