GEOLOGY OF THE PORT LEYDEN QUADRANGLE 4/ 



Erosion of the sedimentaries 



Turning our attention to the sedimentaries, we find that ice erosion 

 was much more effective upon them. In fact the writer believes 

 that in the Black river valley we have one of the best examples of 

 ice erosion in northern New York. One factor favoring the ice 

 work here was the comparative softness and highly jointed char- 

 acter of the rocks, while another factor was their position with ref- 

 erence to the ice current. 



The figure on page 48 shows the profile and geologic structure 

 across the Black river valley 2^ miles north of Lyons Falls. One 

 of the striking features is the terraced character of the sedimentar- 

 ies, particularly from Port Leyden northward [see topographic 

 map]. Along the river course there is a slight notch in the Pre- 

 cambrics and just west Oif this, on the northern part of the Port 

 Leyden sheet, there is a steep slope rising 300 feet above the Pre- 

 cambrics. The formations outcropping on this slope are shown in 

 the section. Resting upon the Precambrics are several feet of weak 

 sandstones which are followed by the sandy limestones of the Pa- 

 melia ; then come the hard Lowville and Black River limestones ; 

 while the summit is capped by hard, crystalline limestones. The 

 streams passing over this slope are characterized by gorges with 

 waterfalls and rapids. From the summit of this slope and extend- 

 ing for several miles westward there is a well defined terrace 

 developed upon the limestone. 



Rising from the western side of the above named terrace there is 

 a second slope higher and much steeper than the first. The rise is 

 commonly about 450 feet within ys mile. The soft Utica shales 

 outcrop at the base of this slope and they are followed by the 

 Lorraine shales with an upward increasing sandstone content. The 

 summit of this terrace, known as Tug hill, is more irregular and 

 stream dissected than the limestone terrace below. All streams 

 flowing across the steep slope of this terrace have high gradients 

 and have cut deep, narrow gorges locally called " gulfs." 



At first these terraces, in their present form, were thought to have 

 been due entirely to water action, but an examination of the region 

 shows that some other explanation must be sought. The steep 

 fronts of the terraces are certainly young topographic features, 

 which precludes the possibility of their having been formed during 

 the long preglacial period of erosion in this ancient region. On the 

 other hand, Black river has done very little work of erosion, between 

 Lyons Falls and Lowville, in postglacial times as proved by the fact 



