GEOLOGY OF OGDENSBURG 59 



plain; or it may represent one of the later ones. Much work 



remains to be done before more definite statements regarding the 



history of the region during this long lapse of time will be 

 warranted. 



QUATERNARY HISTORY 



Another upward movement of the region occurred toward the 

 close of the Tertiary and the present low grounds of the region 

 have been worn down below the level of the previous peneplain, 

 since that occurred. Glaciation of the region followed, a series of 

 ice sheets invading it, advancing, reaching a maximum, waning and 

 disappearing, and separated from one another by long, interglacial 

 intervals. There were certainly three such ice sheets, and very 

 probably five or even six, which followed one another over the 

 district. The later ones largely obliterated the traces of the earlier. 

 The glacial deposits of the region consist chiefly of those left by 

 the last retreating ice sheet, and the polish and striation of the 

 underlying rock are chiefly also of that date, though in some places 

 the rocks bear two sets of striae, the one superimposed on the other. 1 



The last ice sheet retreated from the district toward the north- 

 east. It vanished from the Ogdensburg vicinity while it still 

 remained blocking the St Lawrence valley lower down. The dis- 

 trict also had sagged in level during the Glacial Period and was at 

 a level some 400 feet or more below its present-day altitude. The 

 lake waters which collected behind the retreating ice front could 

 not pass down the St Lawrence valley because of the ice blockade 

 lower down, and they rose until they found an outlet down the 

 Mohawk valley to the Hudson. This lake, known as Lake Iroquois, 

 occupied the Ontario basin, extended east to Rome, N. Y., where 

 its outlet began, and extended itself down the St Lawrence valley 

 behind the retreating ice wall. It has left a well-defined shoro 

 line all around Lake Ontario, distinctly recognizable as a continuous 

 beach to a point some 5 miles east of Watertown, Cape Rutland, 

 where its altitude, a terrace and cliff on the limestone nose of the 

 promontory, is at 733 feet above sea level. This old shore line has 



1 The glacial striae which have been observed in the region are indicated 

 upon the geologic maps. The usual direction shown is south, from S. 10 W. 

 to S. 10 E. showing that to be the general direction of the latest ice motion. 

 The older striae, however, trend from S. 40 W. to S. 30 W. Whether 

 these were produced by an earlier ice sheet, or during an earlier stage of the 

 last ice sheet, is not yet definitely established, though quite probably it was 

 the former. There is considerable evidence to the effect that the action of 

 the last ice advance in the region was very feeble, erosively. 



