20 



NEW YORK STATE MUSEUM 



at 1200 feet, has a little swash channel across it, but was never cut 

 by a river. The failure of flow across this col seems to be due to 

 the Hudson ice flow blockading the Catskill valley. 



When the ice front receded on the Helderberg scarp the Mohawk 

 lake ( including the Schoharie waters) found escape to the Hudson 

 valley. The face of the Helderberg is strongly terraced by cen- 

 turies of river work. This channeling continues southward along 

 the west side of the Hudson valley where the rivers flowed along- 

 side the ice tongue. 



The lowest pass of this episode lies between Esperance and 

 Delanson at 840 feet, now utilized by the Delaware and Hudson 

 Railroad. This outlet correlates with numerous delta plains in the 

 Mohawk valley, striking examples of which may be seen about 

 Prospect and Trenton Falls, at 1100 to 900 feet. 



The Delanson outlet was abandoned when the ice front weakened 

 on the rock scarp west of Schenectady and at Rotterdam Junction. 

 The blockade at this stage correlates with many plains in the upper 

 Mohawk valley and with broad plains in the Sacandaga valley, 

 already noted by Brigham. These have altitudes of 900 down to 

 700 feet. 



The low altitude of the glacial waters in the upper Aloha wk 

 region forces us to the conclusion that the rock barrier at Little 

 Falls must have been removed early in the history of the Postwis- 

 consin drainage through the Mohawk valley, and long before it 

 received the flood from the area of the Great Lakes. It appears to 

 be more likely that the pass was chiefly cut by Prewisconsin 

 drainage. 



By the study of the several points of outflow of the glacial waters 

 as indicated above, in comparison with their correlating sand 

 plains reaching west to beyond Rome, it becomes possible to deter- 

 mine with some degree of exactness the limits of the two ice lobes 

 (Ontario and Hudson) at certain critical stages of their waning. 

 A series of large maps has been prepared to show in a general 

 way the recession of the ice sheet over New York. 



Industrial geology 

 Iron ores. The investigation of the Slate's iron ore fields — a 

 work that has been under way for some time — was brought to 

 partial completion during the past year by the publication of final 

 reports upon the Adirondack magnetite district and the Clinton 

 belt of hematite ores. These arc the two largest fields and have 

 the greatest commercial importance at present as well as for the 



