GEOLOGY OF THE BROADALBIN QUADRANGLE 53 



POSTrALEOZOIC PHYSIOGRAPHY 



In the northern Appalachians and in southern New England the 

 great areas upraised at the close of the Paleozoic underwent vast 

 erosion during the Mesozoic era so that by its close the well-known 

 Cretaceous peneplain had been developed. There is considerable 

 reason to think that this Cretaceous peneplain was developed more 

 or less perfectly over the Adirondack region although there is no 

 evidence in favor of this view within the Broadalbin quadrangle 

 itself. Granting the presence of this peneplain, the topography of 

 the Broadalbin district must have been rather smooth and feature- 

 less with the fault scarps' practically removed by erosion. 



This great peneplain was elevated about the close of the Alesozoic 

 era and thus the region of the Broadalbin quadrangle was rejuve- 

 nated and the revived streams vigorously renewed their work of 

 erosion which has continued to the present time. The present 

 topography of the quadrangle, except for the local glacial deposits, 

 is the result of this long period of erosion and most of the faults 

 have again been made prominent as topographic features by the 

 unequal erosion of the harder and softer rocks on opposite sides of 

 the faults. 



The detailed topography of the district has, of course, been quite 

 appreciably affected by the distribution of glacial drift. 



PRESENT SLOPE OF THE PRECAMBRIC SURFACE 



The extensive faulting has precluded the possibility of studying 

 the slope of the Precambric surface except in a limited way in the 

 southeastern portion of the quadrangle. At Barkersville the Pre- 

 cambric surface clearly slopes southwestward at the rate of over one 

 hundred feet per mile. From just north of Barkersville to the 

 north end of the Amsterdam reservoir is about four miles and the 

 difference in elevation of the Precambric is fully four hundred feet 

 so that the general slope southwestward along this line is at least 

 one hundred feet per mile. Similar results have been obtained by 

 Professor Cushing and the writer in the Little Falls and Trenton 

 Falls districts and also two or three miles northwest of Saratoga 

 Springs. Thus we see that a southwestward slope of a little over 

 one hundred feet per mile of the Precambric surface under the 

 Paleozoics appears to be general along the southern Adirondacks. 



One-half of a mile south of Round lake the Precambric lies at 

 about 1550 feet while just north of Barkersville, and three miles 



