CHAXGE OF COURSE OF IROQUOIS-MOHAWK RIVER 417 



At the same time a portion of the flooded waters escaped over the rock 

 strata bounding the Ballston Channel at Aqueduct, 3 miles below Sche- 

 nectady. Eventually these currents undercut the northward channel, 

 thus establishing the present postglacial course of the Mohawk from 

 Aqueduct to its place of discharge into the Hudson Eiver below Cohoes. 



For a long time, however, the main volume of the Mohawk waters 

 coursed northward through the Ballston Channel. ^Vith the further re- 

 cession of the Lake Albany waters the Mohawk currents developed a valley 

 extending southeasterly from the northern end of the Ballston depression 

 to the Eound Lake inlet. At a later stage, with the further subsidence 

 of the lake waters the Mohawk currents eroded out the broad and deep 

 valley, now followed by Anthony Kill, which opens into the Hudson Val- 

 ley at Mechanicsville. 



The above statements are deductions from the present topographic fea- 

 tures of the localities referred to, especially the following : 



(1) The basin of the Mohawk at Schenectady bounded to the south by 

 a crescentic bluff of the Lake Albany clays marking the course of the 

 Mohawk currents as they cut into the delta deposits and swerved north- 

 eastward. 



(2) The continuity' of the basin with the preglacial rock valley (Balls- 

 ton Channel) extending northeasterly to the Saratoga-Eound Lake region. 

 The bottom of this valley stands at 300-foot elevation, which is 60 feet 

 below the summit of the clay bluff. 



(3) The broad and deep rock gorge extending from the Ballston Chan- 

 nel at East Line southeastwardly to the Eound Lake depression. There 

 is a fall of 97 feet from Ballston Lake, which lies in the northern portion 

 of the Ballston Channel, to Eound Lake, 4 miles to the east. 



(4) The broad and deep valley (now followed by a small stream, An- 

 thony Kill) cut in the Lake Albany deposits from Eound Lake to the 

 Hudson Valley at Mechanicsville. 



The writ-er may add that this interpretation of the course of the Iro- 

 quois-Mohawk during the period of the subsidence of the Lake Alban;^ 

 waters was first described by him in Xew York State Museum Bulletin 

 Xumber 154, pages 31-33, and has since been recognized by Professor 

 Fairchild in Xew York State Museum Bulletin Xumber 195, page 12. 

 (See also the accompamdng sketch map, figure 1.) 



The Terraces at Mechaxicsville 



The main purpose of the present paper is to call attention to the topo- 

 graphic features of the Hudson Valley at Mechanicsville, where the Iro- 



