TERRACES OF MECHAXICSVILLE 419 



both sides of the Hudson Valley. The terraces of this formation are ter- 

 races of deposition, representing the surface of sediments deposited in the 

 middle portion of Lake Albany and converted into terrace form through 

 cutting down by southward flowing currents of the Hudson Valley waters. 

 The 240-foot terrace at Mechanicsville is a segment of the general terrace 

 on the west side of the valley, cut off from its northern extension by the 

 Iroquois-Mohawk Valley above described and from its southern extension 

 by a gully formed by a small stream tributary to the Hudson south of 

 Mechanicsville. 



The next lower (or middle) terrace is at the 200-foot level. It is sepa- 

 rated from the upper terrace by a bluff 40 feet high, as shown by the 

 contour lines of the sheet. The trend of this bluff shows that it was 

 formed by the cutting down of the inner portion of the area of the upper 

 terrace by strong currents issuing from the western tributary valley — 

 that is, Iroquois-Mohawk currents. This terrace is therefore a terrace of 

 erosion in contradistinction to the upper terrace, which is a terrace of 

 deposition. 



•On the east side of the valley there is a terrace symmetrical to the one 

 just described in location, elevation, and form relations. This was formed 

 by Iroquois-Mohawk currents sweeping across the waters of the narrowed 

 Lake Albany and impinging against a frontal lobe of the Hoosic Eiver 

 delta, which had been built out into the lake in this quarter at the time 

 when the lake w^aters were at their height. These currents cut laterally 

 into the materials of the delta, carving out a shelf -like terrace and reduc- 

 ing it by downward cutting to the 200-foot plane. 



The third (lowermost) terrace at Mechanicsville stands at the 100-foot 

 level. It is separated from the middle terrace by a bluff 80 feet in height. 

 The trend of this bluff northwesterly shows that it was formed by the 

 cutting action of currents flowing in from the western tributary valley. 

 This terrace also has its companion on the opposite side of the Hudson 

 with quite definite outlines. The striking bluff extending northwesterly 

 from EcA^nolds and forming the outer margin of the 100-foot terrace was 

 quite evidently formed by currents issuing from the western tributary 

 river and, sweeping across the lake waters, eroding away the inner por- 

 tion of the 200-foot terrace and cutting downward to the 100-foot level. 

 All the features of the topography of this complex of terraces and bluffs 

 become harmonized on the interpretation that they represent the erosive 

 work of Iroquois-Mohawk currents discharging into the Hudson Valley 

 waters. The trends of direction of the bluffs bounding the erosion ter- 

 races could not have resulted from southward flowing currents of the 

 Hudson Valley waters. 



XXIX — Bull. Geol. Soc. Am., A^ol. 30, 1918 



