LOWER SILURIC SHALES OF THE MOHAWK VALLEY 39 



attributed to the Canajoharie shale, since Cumings (op. cit. page 466) 

 measured 950 to 1260 feet of black shales in the lower Mohawk 

 region and 300 feet at least at the top belong to the Indian Ladder 

 beds, a formation that follows the Schenectady beds in this region 

 and will be discussed later (see page 50). There remain then 1800- 

 2000 feet of Schenectady beds, since we properly assume that 

 neither the Normanskill nor the Snake Hill beds reach as far west 

 as Altamont. The Normanskill and Snake Hill beds extend con- 

 siderably farther west than their original basin of deposition (see 

 diagram, page 69) through extensive overthrusting which, however, 

 has not reached the meridian of Altamont where the beds are 

 undisturbed. 



There are other lines of evidence which indicate a similar great 

 thickness for the Schenectady shale and sandstone. One of these 

 is the width of the belt of shales from Amsterdam to the Helder- 

 bergs which at the rate of 140 feet dip in a mile, observed for the 

 Glens Falls limestone, corresponds to a thickness of 1925 feet of 

 shale (13.75 miles by 140), to which must be added 900 feet repre- 

 senting the difference in height of the base of the shales at the north 

 and their top in the south, subtracting from this 2825 feet again the 

 1 140 feet found in that meridian for the Canajoharie shale, we 

 obtain approximately 1700 feet for the Schenectady beds, the 

 Indian Ladder beds having disappeared along the Schoharie creek 

 section. The dips along the line Aqueduct to Schoharie, which are 

 mostly southwest to west and amount to i°-2°, frequently as much 

 as 5°, as at Aqueduct, would also indicate a thickness of more than 

 2000 feet (2681 feet at 1° dip). 



The cause of the astonishing thickness of the Schenectady shales 

 is to be sought in their deposition in a basin, namely, the sinking 

 foreland in front of the rising Green Mountain folds to the east; 

 which basin was rapidly being filled with sediments. The shallow 

 water origin of most of the shales and sandstones of the Schenec- 

 tady beds is proven by the sun-cracks found in the thinner sand- 

 stones (as at the Bozen kill), the frequent layers of mud pebble 

 beds,^ cross striation with plunge structure, very rapid change of 

 thickness of beds and other features. 



^ These mud pebble layers occur mostly on top of the sandstone beds, 

 when the latter are followed by argillaceous shales, indicating that the first 

 clay deposits were rolled about by the waves upon the sand and formed into 

 mud balls. 



