34 NEW YORK STATE MUSEUM 



ing water. Or again (i) those accumulated rapidly with very irreg- 

 ular supply of material at the margin of the ice-forming, hummocky 

 or hill and kettle surface (kames, eskersj, (2) those carried along 

 valleys or general lines of drainage to a considerable distance beyond 

 the ice margin aggrading the valley with the overload of gravels 

 and sands (valley trains J, (3) those washed out from the ice margin 

 in more even distribution forming a gently sloping and thinning 

 extramarginal fringe (outwash or apron plains), (4) those fine 

 matters that are carried by glacial streams into the margins of more 

 quiet waters, either a temporary or a permanent lake or a larger 

 and slower stream or other body forming more perfectly assorted 

 and more evenly stratified deposits (delta deposits), (5) those 

 finer rock flours and clays that remain suspended longer and carry 

 out much farther settling only in the very quiet waters of lakes 01 

 estuaries or temporary water bodies of this character forming the 

 perfectly banded clays (glacial lacustrine clays). 



It is evident then that modified drift has in the process of its 

 accumulation suffered chiefly a separation of fine from the coarse 

 particles and that in most cases the fine clay filling that makes the 

 till dense and impervious to water, has been washed out and de- 

 posited by itself in the more inaccessible deeper waters. As a re- 

 sult most modified drift deposits are pervious and easy water 

 conductors, but poor or questionable ground for dikes or dams or 

 basins [see discussion of Ashokan dam, pt 2]. 



Some of them, the medium sands and gravels, furnish an excel- 

 lent and already cleaned structural material for concrete or mortar, 

 such as the Horton sand deposit, or coarser kinds may be crushed 

 and sized before using as is done at Jones Point on the Hudson. 



The finer silts and clays, usually overlain by assorted sands, are 

 abundant along the Hudson, having been deposited there at a time 

 when the water of this estuary stood 50 to 150 feet higher than 

 now. Recent erosive activity of the ri\'er has cut the greater pro- 

 portion of the original deposits away but at many places large quan- 

 tities still rem.ain above water level in the banks and still greater 

 quantities extend beneath the river. These deposits are the support 

 of the brick industry of southeastern New York. The till deposits 

 are very difficult to penetrate in making borings because of the 

 boulders, the wash rig being almost useless. Modified drift of the 

 medium and finer sorts is easily and cheaply penetrated, and, if it 

 lies on bed rock, such exploration gives reliable results. 



Structure. But this is stating the actual conditions too simply. 

 The glacial epoch was a complex one The continental ice sheet may 



