568 W. M. DAVIS — DATES OF TOPOGRAPHIC FORMS. 



excavated for the greatest part in the weaker members of the series, the 

 Triassic belts, the Cambrian limestones of the Great valley, and the Siluro- 

 Devonian members of the folded Appalachians ; but the hard rocks are also 

 trenched by cross-valleys. 



Tertiary Work in New England: the Connecticut River. — Beginning in 

 southern New England, the valleys may be classified under three heads, viz : 

 the broad lowland of the Triassic belt ; the upland valley of the Berkshire 

 limestones ; and the narrow valleys of the crystallines. The Triassic rocks 

 are nearly all so weak that they have been reduced, as a whole, close to the 

 Tertiary baselevel. There are a few exceptions to this rule : First, the lava 

 sheets, of which the thicker ones in a general way retain the altitude of the 

 plateau : these are roughly indicated by curved black lines in the Triassic 

 area of figure 6. Second, Mount Carmel, a group of dikes which probably 

 supplied the lava sheets, and which now rises a little above the plateau, as if 

 representing a low hill of the Cretaceous cycle.* Third, the heavy conglom- 

 erates, which in Mount Toby, north of Amherst, Massachusetts, rise as high 

 as the plateau next toward the east. 



The upland valley of Berkshire has its floor at an elevation of 800 or 

 1,000 feet ; this is probably to be explained first by the absence of any large 

 master river to drain it; second, by the belts of hard rocks that its streams, 

 the Housatonic and the Hoosick, must cross on their way to the sea ; and 

 possibly, third, by a shift of drainage that I suspect has occurred here. The 

 plateau between the Connecticut and the Berkshire valleys is drained almost 

 entirely by branches of the Connecticut, the divide being close along the 

 western margin of the plateau ; it is quite possible that in the Cretaceous 

 cycle these branch streams drained the Berkshire limestone area as well, 

 and that the present outlets of the valley are the result of headward cutting 

 of formerly external streams ; if this be true, it is quite natural that the 

 valley should not yet be excavated to the same depth as the Connecticut 

 lowland. 



The narrow transverse valleys of the crystallines are excellently illustrated 

 by the two pairs of streams that the east-and-west railroads follow; the 

 Quaboag and the Westfield, followed by the Boston and Albany railroad ; 

 and Millers and the Deerfield, followed by the Fitchburg railroad. All of 

 these are deep cut, but still retain steep side-slopes ; their depth manifestly 

 depends as much on the altitude of the uplands in which they are sunk as 

 on the approach that they have thus far made toward baselevel. The 

 Belchertown, Massachusetts, sheet exhibits the narrow and deep longitudinal 

 or structural valley of Swift river. The finest illustration of these features 

 is in the Connecticut river itself. On the northern side of Massachusetts, 

 where the Connecticut is only 200 feet above sea-level, the plateau on either 



* See figure 2, p. 420 of this volume. 



