REPORT OF THE DIRECTOR I915 93 



The rock under the valley is almost wholly shale, from Trenton 

 to Lorraine in age, the so-called Hudson river formation, but out- 

 liers of Siluric limestones occur here and there, remnants of a 

 once continuous mantle which is now all but destroyed by erosion. 

 The shales are upturned at a high angle and have a northerly to 

 northeasterly strike to which the course of the river approximately 

 conforms. 



The valley, it is apparent from several considerations that need 

 not be set forth here, was marked out and largely formed as early 

 as Tertiary time. The gorge represents a stage of erosion during 

 a period of renewed uplift which accentuated the cutting power 

 of the streams. 



The rim of the gorge lies at about 200 feet above tide for the 

 most part, but varies somewhat from place to place. It is defined 

 rather sharply by a bench or terrace at the line of intersection with 

 the outer valley which has a very gradual rise toward the bounding 

 hills. The gorge is breached by old tributary channels now in part 

 masked by morainal and water-laid beds. A large channel that 

 may have carried the preglacial Mohawk lies under the site of 

 Albany as indicated by the rock contours revealed in test holes that 

 have been put down through the unconsolidated sediments. The 

 soundings have shown that the clays here rest upon morainal 

 gravels and sands, the bottom of which for some distance back 

 from the river is below sea level. The Mohawk channel, if it 

 does mark the ancient course of that stream, enters the gorge nearly 

 at grade, and the same is probably true of the Stockport channel 

 above Hudson, as well as of other streams ; the indications there- 

 fore point to a long period for the gorge cutting during which the 

 tributaries were able to adjust themselves to the lowering of the 

 main channel. Glacial erosion was of minor importance in the 

 formation of this part of the valley. 



One feature, however, that may be ascribed to the ice or to the 

 work of subglacial streams is the deep furrowing of the shales 

 whereby their surface in many places shows a succession of longi- 

 tudinal troughs betAveen parallel ridges or hummocks. The bed 

 of the gorge, especially, appears to be of this uneven character. 

 A single ridge does not continue for any great distance but as it 

 dies out through gradual decrease of height and contraction 

 laterally, it is succeeded by another on either side of its axis. 

 Soundings in the section about Albany indicate there is a great 

 variation in the size and distribution of the ridges and hollows. 



