PLEISTOCENE GEOLOGY OF NASSAU CO. AND BOROUGH OF QUEENS 639 



on the Malaspina glacier ; or 3) deposits made in water-eaten cavities 

 in tlie ice front. As a rule, the gravels are seldom so well exposed 

 as to reveal the structures on which a decision as to their precise 

 character can be arrived at ; and their origin in the presence of the 

 ice in all cases being so intimate as to permit the falling of erratics 

 on their surfaces makes it difficult to discriminate them from the 

 gravelly till. This is particularly true where the growth of trees 

 and the overturning of the superficial deposits have broken up the 

 original stratification in the surficial portion so that the materials 

 have the structural appearance of till or at least ice-deposited 

 gravels. The deposition of the surface gravels by direct ice action 

 is sometimes shown by the scratches on hard silicious pebbles. 

 These scratches are usually miscroscopic and would have been 

 quickly effaced by water action. Such pebbles occur in the churned 

 up gravelly drift on the surface of the Columbia north of the 

 moraine. The structure of the principal knob in this moraine 

 chanced to be revealed in the summer of 1900, and the following 

 notes on Harbor hill show the surprising development of these 

 water-w^orn gravels in the deposit. 



Harbor hill. The precise mode of accumulation of the materials 

 in the terminal moraine still demands explanation in numerous 

 details, particularly in regard to those j^ortions which are mainly 

 composed of stratified gravels and sands. Nowhere in the moraines 

 on the islands ofl: the southern shore of New England does this 

 problem become more urgent for a satisfactory answer than in 

 Harbor hill, a towering mass of stratified gravels, forming the cul- 

 minating point of the moraine on this quadrangle at the eastern 

 side of the pass through which the glacial drainage escaped from 

 Roslyn bay to the great south plain. This hill rises with steep 

 slopes into four knobs, the highest of which has an elevation of 391 

 feet, its base on the outwash plain being roughly circumscribed by 

 the 200 foot contour line. 



At its eastern base, the hill is separated from the extension of 

 the morainal wall in that direction by a distinct depression, or 

 trough, one of those numerous channels wliich gave exit to the 

 intraglacial waters on to the outwash plain. On the west, its slopes 

 fall off to sealevel at the head of Hempstead harbor. The high 



