POSTGLACIAL FEATURES OF THE UPPER HUDSON VALLEY 7 



was the predecessor of the St Lawrence and rivaled it in volume, for 

 it bore not only the drainage of the Great Lakes but the copious 

 waters from a great stretch of the melting front of the Labradorian 

 glacier. This river had a complex relation to the Hudson valley 

 which will be considered later in this writing. 



DELTA SAND PLAINS 



• Fvlly one-half of the area described in this paper is buried under 

 the deluge of detrital deposits that were laid down in the sea-level 

 waters that followed and continuously laved the receding front 

 of the latest ice sheet. The more conspicuous, level stretches 

 which would be evidence to even the casual observer of deposition 

 in standing water are indicated by purple shading on the map. 

 The shore lines of the summit level of the marine waters are marked 

 by the heavy^ broken lines. It must be understood that all the 

 breadth of the great valley was submerged by the waters, up to the 

 plane as shown by the isobasal lines, in solid brown. Large tracts 

 not marked in color received much deposit of sand, silt or clay. 



The uplifted marine plane is now 355 feet above tide at Sche- 

 nectady, 400 feet at Saratoga Springs, and 440 feet at Glens Palls. 

 As indicated by the isobases, the direction of greatest differential 

 uplift or tilting is 20 degrees east of north. The average deforma- 

 tion of the tilted plane in the distance of 50 miles between the 350 

 and the 475 feet isobases is 2.5 feet a mile, the rate increasing slowly 

 northward. On the meridian the gradient is 2.2 feet. 



The extensive sand plains, making the profusion of purple color on 

 the map, were mostly subaqueous, only occasionally marking the 

 highest water level, or rising slightly above. They were aU swept 

 by the relatively falling waters. This is an important distinction, 

 because many students make the mistake of regarding the highest, 

 broad and conspicuous plain of some particular locality as the summit 

 water level, while in fact it may be far inferior. The sand plain 

 southwest of Schenectady is full height, lying near the eroded head 

 of the Iromohawk delta. The plain at Malta, north of Round lake, 

 was strongly swept by storm waters. The desertlike plain north of 

 Ballston Spa is about 10 feet over the marine plain, and was the 

 delta of a powerftil stream, the glacial Hudson, with later contribu- 

 tion by the Glaciomohawk and the Iromohawk. Some of the plains 

 west and northwest of Glens Falls are at or above the summit of 

 the sea-level waters. In the latter district the deposits made in 



