34 NEW YORK STATE MUSEUM 



ill a way that renders the interpretation of the caverns a simple 

 matter and affords some basis for a systematic prosecution of 

 the work. 



The influences controlHng this drainage may be summed up 

 under these heads : ( i ) the character of the successive terranes and 

 the topographic forms resulting therefrom, (2) the strong south- 

 west dip of the strata, and (3.) the insequent stream valleys which 

 have been cut in the peneplain to the level of the cavernous horizons. 



The normal retreat of the Helderberg escarpment along its north- 

 ern and eastern face results in a series of terraces caused by the re- 

 moval of the soft strata from between the harder, and the differen- 

 tial retreat of the terraces has left in places wide areas of permeable 

 limestone exposed to the action of percolating water. Plateaus of 

 variable extent are formed by the Onondaga, the Coeymans and, 

 west of the Cobleskill creek (where the Oriskany quartzite appears 

 to be absent or very thin), by the Becraft limestones. 



When carbonated and acidulated meteoric water has once found 

 its way into the joints and between the beds of the limestones thus 

 rendered accessible, the excavation of caverns by solution will begin, 

 provided the water can find an outlet at a level below that of its 

 point of entrance into the rock. But the inclination of the strata 

 southward precludes the possibility of exit above the line of strike 

 passing through that point and, excepting along that part of the 

 Helderberg escarpment which trends southeastward from near Alta- 

 mont, it is only where a surface stream has cut its valley to the 

 requisite depth and below the line of strike that an outlet is afforded. 

 It is evident, therefore, that the age of these underground channels 

 can be approximately fixed by estimate of the time at which their 

 mouths were uncovered ; and caverns of the same zone may be 

 compared in this regard by reference to their distance from the 

 head of the valley into which they open. 



The diameter attained by a cave is primarily dependent upon the 

 amount of water passing through it and the relative solubility of 

 the rock. Disregarding meteoric conditions the volume of water 

 is largely governed by the topography of the area drained by under- 

 ground passages, the changes caused by surface erosion and the 

 deposit of glacial material having in this way affected the Helder- 

 berg caverns considerably. 



The life of a cave ends when the roof gives way and the channel 

 is opened to day, a fate which may overtake it early in its develop- 

 ment or be postponed for a long time, but is inevitable. As the 



