GEOLOGY OF FRANKLIN COUNTY 79 



ciently long to permit the complete wearing down of the region, 

 not only once but more likely a half-dozen times from an elevation 

 such as the present one to the condition of a plain only moderately 

 elevated above sea level. But the opposite to this process has 

 also been in operation, and periods of quiet, during which the 

 surface has suffered steady decrease in elevation through erosion, 

 have alternated with intervals during which slow movements of 

 the earth's crust have served to reelevate it. Where such inter- 

 vals of quiet are sufficiently protracted the surface may be wholly 

 worn down to a comparatively level plain known as a peneplain. 

 Above its fairly level surface low, rounded hills composed of the 

 more durable rocks may rise and are known as monadnocks. 1 If, 

 after erosion has produced this result, the region undergoes uplift, 

 the streams will commence to carve valleys in the elevated plain, 

 and, by the deepening and widening of these valleys with the 

 lapse of time, the old plain is carved into a hilly region, whose 

 valley bottoms are at a new base level. The former low relief 

 of the surface is indicated by the nearly uniform altitude to which 

 the hill tops rise, so that the observer has merely to imagine the 

 valleys refilled with the material which the streams have removed 

 to obtain a clear mental picture of the surface as it was before the 

 uplifting took place. 



Two old base levels and two periods of uplift are clearly indi- 

 cated in the northern Adirondacks, even without the aid of good 

 maps and in spite of the difficulty of getting good views from most 

 of the hills, due to the heavily-forested character of the country. 

 One of the base levels is represented by the uneven line of the 

 hilltops, excluding however many of the higher summits; and the 

 other by the valley levels. The first period of uplift lowered the 

 base level from the hill summits to the valley bottoms, and the 

 second has produced a new base down toward which the streams 

 are actively cutting but which they have not yet reached. 



In the northern Adirondacks the hills rise to such diverse alti- 

 tudes that the precise horizon of the old base which they repre- 

 sent has not been made out. The same difficulty is met when the 



iNamed from Mt Monadnock in New Hampshire, the type of the class. 



