GLACIAL AND POST-GLACIAL HISTORY 451 



that made shallows at the highest stages of the water body must have 

 produced islands and peninsulas at lower stages. 



Depressions below old lake- or old sea-floor. — Below the level of 

 the sea-floor or lake-floor plain there are two classes of depressions: 

 (i) lake basins and similar depressions not occupied by lakes; (2) 

 valleys produced by erosion. The depressions occupied by lakes are 

 those like the Saratoga Lake basin and Round Lake basin, the origin 

 of which has been referred to above and will be discussed below. 

 The basins may be small — a few yards across and shallow; or they 

 may be like the basin of Saratoga Lake, 5-8 miles in length and I-2 

 miles in width, and with a depth below the plain surface of 60-100 feet, 

 plus the depth of the water in the lake. 



The second class of depressions below the old lake- or old sea-floor 

 are the valleys of the present streams, of which the Hudson is the 

 chief. (Fig. 15.) Below Troy the Hudson is now an estuary, but above 

 that place the tide does not reach. The course of the Hudson is inter- 

 preted as marking the trough of the depression down to which the 

 old sea- or lake-floor sloped from each side, and which was followed 

 by the main stream when the floor emerged from the waters in which 

 it had been built. Some of the details in the course of the Hud- 

 son may be explained as due to the greater building out along one 

 side of the lake or sea, as for example the westward bend of the 

 Hudson opposite the mouth of the Hoosick River, where the building 

 out of the delta from the eastern side of the valley crowded the depres- 

 sion farther out into the midst of the plain than to the north or to the 

 south beyond the influence of the delta deposits. This westward 

 bend amounts to about two miles. There is no doubt that similar 

 explanations will account for the fact that the Hudson is nearer one 

 side of the valley than the other in different portions of its course, and 

 for other details ; but it is probable that other slopes of the floor were 

 the resultant of the interaction of several factors — the original topog- 

 raphy of the valley, the rate of .deposition by the agencies building 

 up the floor, and the length of time the building continued. At any 

 rate, the course of the Hudson may be considered a consequent course 

 determined by the slopes of the old lake- or old sea-floor across which 

 it flowed when the floor emerged from the lake or sea that had occu- 

 pied it. Since that time it has trenched its course below the level of 



