GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



417 



level. The forces of erosion work incessantly and according to 

 stereotyped methods, but cease their activity at sea level, hence 

 tend to wear down the lands to a gently sloping surface, rising 

 inland with recession from the shore line. The longer these 

 agents are permitted to work at this task, without interruption 

 from the other set of forces, the larger the proportion of the 

 whole task which will be accomplished. The streams will pro- 

 gressively cut their valleys down to this slope, or grade, after 

 which the work consists mainly in valley widening, bringing a 

 steadily greater amount of the region down to the new level, 

 with a constantly diminishing portion remaining at the old. 

 During the progress of the work a varied topography will be 

 produced, depending on a host of minor factors, rock arrange- 

 ment and rock resistance being the two most important. The 

 weak rocks will yield most quickly, and many of the streams will 

 adjust themselves to these weak rock belts. The more resistant 

 rocks will persist longer at the old levels, hence tending to be- 

 come stream divides. The weak belts may be owing to weak 

 rocks or to structural lines of weakness. The rock dip is a most 

 important matter in determining the character of the valleys 

 and uplands. Where it is gentle, flat topped divides and a tend- 

 ency to radial valleys result. Where it is steep, parallel val- 

 leys and sharp backed ridges are produced. 



Griven sufficient time, the resistant rocks slowly reach the 

 lower level, and the surface becomes comparatively smooth, the 

 interstream areas having low, gentle slopes, with perhaps here 

 and there a low hill or ridge of extraresistant material. Be- 

 ginning as a plain, the district reappears as a plain, though 

 less smooth than before. Such an erosion surface is known as 

 a peneplain. 



If now this process of wear is interrupted at any stage by 

 an oscillation which changes the relative level of land and sea, 

 the grade of the streams is altered, and the whole erosion pro- 

 cess must recommence its work with reference to this new grade. 

 If the movement be an upward one, the streams at once com- 

 mence the task of cutting down the region to this new level, 

 leaving their old task in the condition in which the beginning 

 upward movement found it. Such portion of the region as had 

 been worn down to grade, will carry this evidence of graded 



