84 THE BUTTERFLIES OF NEW ENGLAND. 



below this point, it wanders about over tlie Ossipee region and the adjacent 

 parts of Maine, upon broad sand-plains by which the old surface of the 

 country is completely concealed : it seldom touches rock until it crosses a 

 broad ledge over which it falls in picturesque cascades near Hiram, Me., 

 about twenty miles above its mouth. As it happens that the river settled 

 upon this ridge at a high level, the terracing of all the sand-plains further 

 up stream is delayed by the slow cutting of the rock barrier ; hence the 

 beautiful intervale at North Conway, lying smooth between the steep, rocky 

 slopes that enclose it. The Ammonoosuc, on the other side of the moun- 

 tains, must also once have flowed over an open plain, like that still followed 

 by the Saco in its middle course ; but the northern river did not happen to 

 encounter a rocky reef until it had cut deep into the plain and carved it 

 into wide open terraces, as at Littleton. These contracted examples may 

 be taken as the extremes of a series, whose many intermediate members 

 are represented by other rivers in New England. 



The general conditions of terrace-making, as stated above, included an 

 increase of river slope, as a result of the greater value of postglacial ele- 

 vation in the north than in the south. This applies to most of the New 

 England rivers, but manifestly not to northward flowing streams, like the 

 Concord and Nashua rivers : here the eflfect of unequal elevation must have 

 been to decrease the rate of descent, and it has been suggested that the 

 flat, marshy, unterraced character of their valleys is in good part owing 

 to their weakened flow, thus determined. 



One of the most constant results of recent glaciation is the occurrence 

 of lakes, and New England aflfords good illustration of this rule ; for there 

 is small probability that any of the lakes, now so plentifully distributed 

 over its surface, existed before the last glacial period. Some of the lacus- 

 trine basins may be due wholly or in part to glacial erosion, but by far the 

 greater number result from obstruction to drainage by the irregular depo- 

 sition of drift, and thus further characterize the immaturity of drainage 

 already indicated by the rocky rapids and diluvial terraces of the rivers. 

 Some little advance from this immaturity has already been made : the 

 lakes held by drift barriers in the steeper valleys have generally been 

 drained by cutting down at the outlet, and thus the rarity of lakes in the 

 White and Green Mountains and in the plateau valleys is best explained ; 

 lakes have lately existed there, as many of the flat meadows attest, but 

 their life was short by reason of the strong slope of their outlets over their 

 weak barriers. The largest and most numerous lakes occur in the lower 

 country from eastern Massachusetts to northeastern Maine, where the plen- 

 tiful drift is most eflTective in barring off" the old valleys, and where the 

 gentle slope of the streams allows the longest life to the lakes ; l:)ut here 

 also the water surface has in many cases been somewhat lowered, revealing 

 the former shores as sandy terraces and benches above the present margin. 



