GLACIAL GEOLOGY OF THE COHOES QUADRANGLE 33 



from layers of less coarse composition. Some of the layers are 

 of dark color, due mainly to water-worn fragments of the local 

 rock. At the base of the bed there are cobbles and boulders that 

 have been separated from the sands in the work of excavation. 



It is believed that these deposits are morainic in character and 

 were laid down under conditions of standing water. They evidently 

 occupy a preglacial depression and the retreating ice front may 

 have halted or entered upon a phase of slow recession at this place. 

 At the same time waters gathered in the depression and the 

 materials derived from the melting ice were partially sorted as they 

 were laid down. 



This formation of sands and gravels, and others of similar com- 

 position and structure as they occur in the Hudson valley where 

 lacustrine conditions immediately supervened upon the withdrawal 

 of the ice sheet, may be designated as subaqueous recessio7ial 

 moraines. 



Residual clays of postglacial age. The Mohawk waters wihich 

 coursed through the Anthony kill channel swept from their path 

 the Lake Albany deposits and laid bare the underlying rock. Much 

 of this rock surface thus exposed to weathering is now covered 

 with residual clay soils. In general, the floor of the valley from 

 Willow Glen eastward to the river (except where bare rock is now 

 exposed) is mantled with a thin layer of rock detritus soils of post- 

 glacial age. They are well shown in cuttings made in grading the 

 macadamized road, and the transition from the fine-grained surface 

 soil to the slightly altered rock below is readily noticeable. The 

 depth of the soil, as observed, varies from a few inches to several 

 feet. The park area of the city of Mechanicville is composed of 

 this residual clay soil with some additions of materials from the 

 lacustrine clays and sands that have been washed down from the 

 slope bounding the erosion terrace. 



When the Mohawk river became established in its present course 

 (the Aqueduct^Cohoes channel) its flooded waters likewise swept 

 away a broad path through the Lake Albany deposits. In this way 

 was formed the broad depression extending southeastward from 

 Crescent to the islands at the mouth of the Mohawk. The greater 

 portion of the floor of this depression lies east of the present course 

 of the river and is marked by striking irregularities of surface. 

 The larger features are clearly due to the effects of weathering and 

 stream erosion, mainly in preglacial times, of steeply tilted rocks 

 composed of unequally resistant strata. The topography of this 



