l8o NEW YORK STATE MUSEUM 



As the Sihelf approaches low water it reaches a level where the action 

 just described is inoperative, and the surface sheet of rock has not 

 been removed. This surface sheet is swept clean of sediment and 

 is much as the ice sheet left it. The bed here had its constituent 

 sediments assorted by vibration of the bottom waters of the Chazy 

 sea, due probably in part to wave motion on the surface and in part 

 to tidal flow of waters over the surface of the sediments. A series 

 of interlocking ridges containing much silicious material were thus 

 segregated and the bottoms of the miniature valleys between filled 

 with the finer and purer calcareous material of the bottom. Solu- 

 tion aided by mechanical attrition has carried away the exposed 

 softer portions of this bed to a depth frequently measuring 24 milli- 

 meters and sometimes slightly exceeding this depth. A view of this 

 shelf at a time of low water is shown in plate 17. The ridges 

 remain as flat surfaces of irregular outline. Abrasion has removed 

 the glacial polish and finer striae from these surfaces and there is 

 a slight relief of a fraction of a millimeter in some places which 

 reveals figures like branching algae with stems from 2 to 6 milli- 

 meters in diameter. The amount removed from the higher surface 

 can not well be more than i or possibly 2 millimeters, the aver- 

 age loss of surface from the softer and harder portions being in 

 the vicinity of 15 millimeters. The bay opens to the east and 

 there are no strong winds from this direction in this region. North 

 or south storms disturb the waters here but very little. The bay 

 however has a muddy bottom and storm action enables its waters 

 to take up enough sediment to become discolored. At such times 

 small and gently moving waves with an abundance of fine abrasive 

 material must lap this area. Careful examination of the lowered 

 surfaces of the rock plainly show the presence of water vortexes 

 carrying silt. The difference between the 55 millimeters of the 

 block last studied and the 24 millimeters of surface removed here 

 must be largely due to the great difference of wave energy in the 

 two localities. Whatever part has been played by solution in the 

 lowering of the Sloop bay surface has been aided in no little 

 measure by plant action for blocks taken from this surface show a 

 dried organic film still attached to the very irregularly pitted sur- 

 faces of the depressions. The depth of 24 millimeters is not to be 

 fotmd next the harder ridges but near the middle of the softer 

 area. Solution alone would have worked with greater uniformity. 

 As in even the lesser depths, erosion by vortex action is still a 

 factor, it hardly seems possible that pure solution could have 



