468 NEW YORK STATE MUSEUM 



Peneplains^ 



The even sky line of some of the mountains [pi. 4] suggests 

 an uplifted peneplain. It is best displayed on Treadway mountain, 

 but may also be seen on Springhill, Trumbull and several small 

 unnamed mountains. Wherever found this flat rises slightly toward 

 the northwest. Apparently this peneplain antedates the last period 

 of faulting, since the eastward slopes of the fault blocks are invari- 

 ably flat [pi. 4 and 5]. Since the rocks are igneous or meta- 

 morphic it is impossible to correlate beds or to restore the prefaulted 

 surface with exactness. The fact that when developed on sedimen- 

 tary gneiss these flat surfaces often cut across the bedding [pi. 4, 

 fig. 2] is snflicient evidence of their origin from some process of 

 down cutting. But whether one peneplain or more is represented 

 is impossible to say until some estimate of the amount of faulting 

 can be made. These flats are developed alike on undoubted igneous 

 rock (Owl Pate, Bald Pate, Moose mountain) and on gneiss, pos- 

 sibly of sedimentary origin (Bear mountain. Skiff mountain. Knob 

 mountain). 



A second temporary base level is represented by the present valley 

 floors. Like the older level this one is higher towards the northwest, 

 but this second level follows the formation of the great faults. 



Summary of the preglacial erosion history 



In Prepotsdam time eastward flowing rivers were established on 

 the limestone; these rivers were drowned by the Potsdam sea, and 

 the whole country covered by the Lower Siluric. From the Siluric 



^A peneplain, i. e., almost a plain, can best be defined in terms of a " base- 

 level " or " graded slope." The last two terms are both used by different 

 authorities to describe that condition of a land surface which is reached when, 

 after erosion by streams, the slope is just sufficient to carry off the water 

 without permitting either additional erosion or transportation. It is a plain 

 as near sea level as river erosion can bring it, and it is a limiting condition 

 which is approximated even if never reached. At a stage shortly preceding 

 base-level, the surface would be a peneplain. A peneplain then is a nearly 

 plane surface at sea level, produced by the erosion of streams. Should such 

 a peneplain then be elevated and subjected to erosion again, evidence of it 

 would remain in that the tops of the hills would be flat. These flat hilltops 

 'stand at about the same elevation, and the rock forming such flat topped hills 

 may be of any kind or of any structure. 



