56 NEW YORK STATE MUSEUM 



The Hoffmans fault enters the Saratoga quadrangle on its west' 

 margin i mile northwest of East Galway and, pursuing a general 

 north-northeast course passes on to the Luzerne sheet a mile west 

 of Corinth. At Hoffmans Ferry on the Mohawk (Amsterdam quad- 

 rangle) the displacement is estimated by Cumings at 1300 feet and 

 by Prosser at 1600 feet.^ Near the north edge of the Amsterdam 

 quadrangle the fault sends off two branches to the east, each of 

 which takes part of the throw of the main fault. These two 

 branches, called by Miller the West Galway and East Galway faults, 

 continue across the Broadalbin quadrangle on to Saratoga. Across 

 Broadalbin, Miller estimates the throw of the Hoffman fault as 

 but 250 feet, but states that this rapidly increases in its course across 

 the Saratoga quadrangle.^ 



The scarp of the Hoffmans fault is the most prominent topo- 

 graphic feature of the Saratoga region, though nearly equalled by 

 the McGregor scarp. The summit knobs of the range reach eleva- 

 tions of over 2000 feet, towering as the west wall of the Kaya- 

 derosseras valley, the valley floor not greatly exceeding 600 feet 

 altitude. None but Precambric rocks occur west of the fault within 

 the quadrangle ; but east of it such heavy drift is banked up against 

 the face of the fault scarp all the way from East Galway to near 

 South Corinth that no rock exposures are seen. About South 

 Corinth Precambric rocks are at the surface east of the fault; 

 about East Galway rocks of the Theresa formation are at the sur- 

 face. What lies between the two is chiefly conjectural. But the 

 rapid increase in prominence of the fault scarp in passing north 

 from East Galway can, in this instance, be due to nothing except 

 increasing throw. The surface of the western block is tipped to 

 the south more than that of the eastern block. Back from Corinth 

 therefore the throw is equal to the height of the fault scarp plus 

 an unknown amount; hence Miller's estimate of 1000 feet is modest, 

 and the throw is likely 500 feet in excess of that. 



To recapitulate: the great throw of the Hoffmans fault at Hoff- 

 mans Ferry is split into three parts by the branching of the fault so 

 that, across the Broadalbin quadrangle, the main fault retains only 



1 In vertical faults in nearly horizontal rocks, such as these, the displace- 

 ment is practically all throw, so far as can be told. At Hoffmans Ferry the 

 surface beds on the western, or upthrow, side of the fault are buried under 

 a thickness of from 1300 to 1600 feet of younger rocks on the opposite side, 

 according to these estimates. Relatively to the beds on the western side the 

 corresponding ones on the east have been vertically dropped by that amount. 



2 N. Y. State Mus. Bui. 153, p. 46. 



