GEOLOGY OF THE NEW YORK CITY AQUEDUCT 33 



with the ledges that may be regarded as their source, that the gen- 

 eral ice movement was from north to south swerving along the 

 southerly extension to east of south. Therefore it is not unusual 

 to find abundant boulders of Palisade trap stranded in New York 

 city or on Long Island, or boulders of the Cortlandt series, or of 

 the gneisses of the Highlands, or, in occasional instances, of sand 

 stones from the Catskills, or the limestones from the Helderbergs 

 or perhaps in rarer cases even rocks from greater distance, as the 

 Adirondack mountains. 



Kinds of drift. There are in the region two fundamentally differ- 

 ent types of drift as to method of deposition. They are (a) unas- 

 sorted drift (till or hardpan), and (b) modified drift (stratified or 

 partially assorted gravels, sands, clay, etc.). The former (o) repre- 

 sents deposition directly from the ice sheet at its margin (terminal 

 or marginal moraines) or beneath ("ground moraine") without 

 enough water action to rework and assort the material. It there- 

 fore contains boulders, pebbles, sand and clay of a heterogeneous 

 mixture of the most complex sort both as to size and character. In 

 such deposits there is almost always sufficient intermixture of clay 

 and rock flour of the finest sort to make a very compact and dense 

 mass that is usually quite impervious to water. Such deposits are 

 distributed rather unevenly over the surface and where this uneven- 

 ness leaves hollows or basins, or obstructs the outlets of other de- 

 pressions, they may hold water and form small lakes or ponds or 

 swamps. This is almost universally the origin of the many thou- 

 sands of lakes of the northern lake region. It is evident that ma- 

 terial of this character, a type that commonly serves the purpose of 

 a natural dam or reservoir, would be especially important and useful 

 at certain places on the Catskill system. As a matter of fact, so far 

 as geologic features are concerned, it is the chief factor in choice of 

 location for the Ashokan dam [see discussion pt 2] and is a con- 

 trolling factor in the plans for the erection of the miles of dikes 

 at less critical margins of the reservoirs. Till is an extensively 

 developed type but frequently passes abruptly either laterally or 

 vertically into assorted materials of very different physical char- 

 acter. 



(b) All materials associated in origin with the glacial occupation 

 that have been materially modified especially in the direction of an 

 assorting of material are referred to as " modified drift " deposits. 

 They include (i) deposits made by both water and ice together, 

 (2) those formed by running water, (3) those laid down in stand- 



