GEOLOGY OF THE NEW YORK CITY AQUEDUCT III 



in part the ice itself, which came in from the northeast, helping to 

 complete the barrier. Into this lake the streams from the melting 

 ice margins deposited their load of silt. This is well shown in the 

 trenches cut across the terrace ^ of a mile above Olive Bridge. 



A similar occurrence is seen at the cemetery near West Shokan. 



Laminated sand and clay. In all cases where silts were carried 

 into the lake basin the finest materials were carried in suspension 

 to greater distances from the margins, and slowly settled out in the 

 form of alternate laminae of clay and fine sand. Each sandy layer 

 represents a fresh supply of material and rapid precipitation of the 

 comparatively heavy grains ; while each clay layer represents a 

 period of greater quiet or decreased supply during which the finest 

 particles settled to the bottom. A predominance of fine sand indi- 

 cates either abnormal supply or proximity to the supply margin, 

 while a predominance of clay represents either uniform and mod- 

 erate supply or greater distance from the supply margin. 



These deposits are nearly impervious to water moving vertically, 

 but much more pervious laterally and especially so in the most 

 sandy portions forming the marginal facies. 



This type of deposit is to be seen at the surface at about the 

 700 foot contour 2 miles north of Shokan, above the " big dugway," 

 also in the trenches cut into the terrace at about the 500 foot con- 

 tours ^ of a mile north of Olive Bridge, and it is probable that 

 this same type underlies the northern half of the " Tongore " site. 

 The material marked " fine sand " at and below the 400 foot line 

 on the accompanying ''geologic section" G-H is judged to be of 

 this type. 



Pebbly clays. These are developed to only limited extent and 

 indicate probably floating ice in addition to the other methods of 

 distribution. 



Gravel streaks and assorted pebble beds. Wherever water flowed 

 with considerable current across the material either before or after 

 deposition the finer particles were removed and only gravel and 

 pebbles, too heavy to transport were left behind. Some of these 

 gravel beds were developed in the intervals of successive advances 

 and retreat of the ice when for a time the lower valleys were unoc- 

 cupied. In many places the succeeding advance of the ice would 

 plow all these surface materials up again and mix them into the 

 usual till ; but occasionally the oncoming glacier simply covered 

 these deposits with its own till mantle, and they are preserved as 

 records of these minor interglacial stages. Such behavior would be 



