WARREN fl'ITAM — DISTRIBUTION OF ENGLACIAL DRIFT. 145 



may be somewhat brief, the chief point to be brought into view being the inequality 

 of its distribution. 



Englacial Till. — The chief characters of the englacial upper portion of the till, as 

 compared with the subglacial lower portion, are its looser texture ; its more plenti- 

 ful and larger bowlders ; the prevailingly angular or subangular shapes of its bowl- 

 ders ami smaller rock fragments, whereas they are mostly worn smooth and planed 

 by glaciation in the lower till ; and the usually more gravelly and sandy and less 

 clayey composition of the englacial till, owing to the washing away of much of its 

 finer material by the superglacial drainage. To these originally inherent characters 

 we must add the very noticeable postglacial peroxidation and hydration of the 

 -mall ingredient of iron, giving to the upper part of the till a yellowish-gray color, 

 while the lower part, holding the iron in protoxide combinations and as pyrite, 

 has a darker and bluish color. This change has generally extended through the 

 englacial till, stopping at the more impervious subglacial deposit. Between the 

 two there is also frequently a layer of subglacial stratified gravel and sand from a 

 few inches to several feet thick. 



The proportion of the englacial drift dropped as till and that borne away by 

 streams in New Hampshire appear to he approximately equal ; and probably it is 

 also true for most other parts of our drift-bearing area that about half of the en- 

 glacial material became till and half modified drift. 



The extremes of thickness of the englacial till in Xew Hampshire, so far as ob- 

 served, are one foot and seventeen feet. Its inequality of distribution in other 

 states appears to range, as before described, from almost nothing or only a few feet 

 for its minima to about forty feet for its maxima near massive terminal moraines 

 and where great currents of the ice-sheet converged. 



Perched Blocks. — Bowlders and all rock fragments and other drift enclosed in the 

 Lee at a considerable height above the ground were borne forward without attrition. 

 This higher part of the englacial drift supplied most of the material forming the 

 terminal moraines, which therefore have a remarkable profusion of bowlders and 

 angular gravel. When the ice-sheet was finally melted its enclosed bowlders were 

 dropped, and they now lie frequently as conspicuous objects on both the tower and 

 higher parts of the land. Perched on the sides and tops of hills and mountains, 

 they at first suggest transportation and stranding by icebergs or floe-ice. Some 

 of these blocks are very huge and have 1 raveled far, as the "Three Maidens" at 

 the lied Pipestone quarry in Minnesota, where a single immense bowlder has 

 fallen into three pieces thai measure each about 20 feet in length and 12 feet in 

 height, besides several smaller masses* Two perched blocks, measuring respect- 

 ively 42 by 40 by 20 feet and 40 by 30 by 22 feet, found by Dr. G. M. Dawson on 

 the eastern font-slope of the Rocky mountains about 3,300 feet above the sea, ami 

 others in the same region up to 5,280 feet, were derived from the Axchean area 

 some 700 miles distant/! But the longest distance of transportation of drift within 

 the ice-sheet know n on this continent is fully 1,000 miles, from the eastern side of 

 Hudson bay, where il narrows into .lames bay. to the southwest and south into 

 southern Minnesota. 



Karnes. — McGee \ ami Chamberlin^ have judiciously proposed the restriction of 



*( reologj Of M il Ol. i- I v " I, 1'. 540. 



FQeol. Survey of Canada, Report of Progress for L882-'83-'84, pp.147 1 19C. 



| Reporl of the International Geological gress, 9 ud session, B 1881, p. 621. 



-. 1 1 Survi y, Third annnal report, for l881-'82, p. 299; Am. Jour. Sei., :;.i series, vol. \\\ ii 



X I \ I'.i 1 1 S01 V.i . \ 01 



