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SCIENCE 



[N. S. Vol. XLII. No. 1093 



normally rapid rate during tlie seasons imme- 

 diately following the retreat of the ice sheet, 

 and that a mass of angular blocks might be left 

 which, because of its uniformly weathered 

 aspect, would appear to have gained little or 

 nothing from annual frost action within more 

 recent time. In either case, an earthquake 

 would be unnecessary to the theory, (c) I 

 agree with Mr. Sayles that the extent of frac- 

 turing and dislocation in the gorge itself is 

 too great to be attributed to frost action under 

 ordinary circumstances, both as to the depth 

 reached and the amount of movement regis- 

 tered by the blocks; yet I can not see why 

 plucking or quarrying of blocks by the ice 

 sheet, supplemented by abnormally severe 

 freezing and thawing, as the last vestiges of 

 the ice sheet melted away from the pass, is not 

 a perfectly valid alternative hypothesis. In 

 plucking a large joint block from its place in 

 the ledge, the ice sheet might rotate it so that 

 one end of the block would be moved a few 

 inches in a direction opposite to glacial move- 

 ment, while the other end was moved forward. 

 Even a case where a block had been moved 

 bodily in a direction opposite to glacial move- 

 ment, but through a space of only a few inches, 

 as reported by Mr. Sayles, could be accounted 

 for by the action of ice in crevices and angular 

 " cavities " between the blocks at the close of 

 the period of actual glaciation, when the ice 

 surrounding the blocks had lost its ability to 

 move en masse, but expanded and contracted in 

 response to temperature changes, somewhat as 

 capillary water and frost behave, in crystalline 

 rocks, but on a much large scale. " Lateral 

 movement " among the blocks, and " pell-meU 

 arrangement " would be natural results of this 

 type of ice action.^ For these reasons it seems 

 to me that the facts thus far reported do not 

 demand the occurrence of an earthquake at 

 Lost Eiver, but are adequately met by the 

 hypothesis of glacial plucking, followed by 

 rock falls and frost work on a scale larger than 

 has been possible since the last remnants of 

 glacial ice vanished from the Kinsman Notch. 



2 See paper by J. B. Tyrrell, on "Rock Glaciers 

 or Chrystoerenes, " Journal of Geology, Vol 

 XVm., 1910, pp. 549-553. 



The size of some of the " giant potholes " is 

 extraordinary. Although they may have been 

 produced by a glacial torrent passing through 

 the notch and on through Agassiz basin, where 

 tortuous channels and large torrent-worn 

 cauldrons are well developed, there are two fea- 

 tures which lead me to question the reality of 

 such an origin, (a) In more than one place, 

 where a concave niche or alcove in the wall of 

 the gorge suggests the side of a pothole, from 

 which the other sides have been removed, I 

 saw a curved joint crack in the ledge, one or 

 two feet back of the concavity, and approxi- 

 mately concentric with it. The detachment 

 of the intervening concavo-convex slab, by 

 frost or glaciation, would have left an alcove 

 equally cylindrical in form to, but of larger 

 radius than the " pothole." (&) I was shown 

 by Mr. E. E. Grinnell, superintendent of the 

 reservation, examples of blocks with convex 

 sides, which seem to match the concave niches 

 or incomplete " potholes." One of these abuts 

 against one of the giant potholes, near "the 

 guillotine," and from its shape and form a peg- 

 matite vein which traverses it appears to have 

 dropped from the side of the ledge so as to 

 produce what at first sight would be accepted 

 as part of a huge pothole. Nowhere else have 

 I seen examples of joint cracks with such 

 sharp curvature, yielding partial cylinders of 

 15 to 25 feet diameter; but their existence 

 here is certain. The question whether the 

 giant potholes are surviving portions of real, 

 torrent-carved potholes, or are imitative forms 

 left by the extraction of joint blocks with 

 curved sides can only be settled by careful 

 measurements of the concave and convex sur- 

 faces and a geometrical study of the relations 

 between the ledges and the blocks which still 

 rest against them. As yet no large water-worn 

 boulders seem to have been found in the 

 largest " potholes." 



As regards both the true character of the 

 giant potholes, and the earthquake theory, it 

 appears, therefore, that the geological history 

 of Lost Eiver deserves further study. 



j. w. goldthwatt 



Dartmouth College, 

 October 2, 1915 



