492 PROF. T. G. BONNET ON THE [Aug. 1906,'' 



Boulder-Clay at these pits, as is usual in the eastern parts of 

 England south of Flamborough Head, is full of more or less well- 

 rounded fragments of chalk which are often striated. This Mr. 

 Woodward cites (like several other authors) as if it proved the 

 action of land-ice. Striations are no doubt frequent on stones 

 which have been transported between such ice and the underlying 

 rock, but they could not often be produced either in ground-moraine 

 (that is, a sub -glacial stony clay) or in englacial material. Even if 

 the latter were sufficiently abundant to make contact between two 

 stones a common occurrence, that contact would usually affect only 

 a very limited surface-area on either of them, and the difference in 

 their rate of movement would hardly be sufficient to produce a 

 scratch of any length. Eor similar reasons, it is difficult to 

 understand how striations could be made in a ground-moraine, when 

 that is at all thick 1 ; but, as every one knows, they are easily 

 produced when a stone held in the grip of ice is rubbed against a 

 rock-surface. Again, a glacier is a much less efficient pebble-maker 

 than moving water. The former can smooth the broader faces of 

 fragments ; but can produce only slight effects on the narrower, 

 because of the obvious difficulties in holding them against the 

 grinding surface of rock. I doubt Avhether the form of a stone 

 shaped only by ice-action will ever be more than subangular. 

 But the larger chalk-fragments which are so abundant as to give 

 an epithet to the Boulder-Clay in the above-mentioned parts of 

 England 2 are true pebbles, most of which are at least subrotund, 

 like those on a sea-beach at the foot of chalk-cliffs. They might also 

 be formed by streams, but fluviatile chalk-gravels are not, so far as 

 I know, common in this country. Thus it is more probable that 

 the chalk-fragments in the Boulder-Clay are seaworn pebbles from 

 a beach ; but if so, we are confronted with some difficulties in the 

 laud-ice hypothesis, which, as I have referred to them elsewhere, 3 1 

 am contented with mentioning, and that only lest they should be 

 ignored. Enough to say that, as Col. Eeilden proved more than a 

 quarter of a century ago, 4 the tidal movements of an ice-foot can 

 striate beach-pebbles which afterwards may be transported to very 

 considerable distances. Any such explanation of the English 

 Boulder-Clay is of course an hypothesis. My point is that the 

 attribution of it to land-ice is at least equally hypothetical, and thus 

 cannot be used as an axiom, on which a logical superstructure can 

 be safely erected. 



Mr. Woodward also assumes that an ice-sheet, when advancing, is 

 capable of shearing off and thrusting before it large masses of such 



1 The clay in these East-Anglian deposits is always, so far as my experience 

 goes, sufficiently abundant to keep the stones from coming frequently into 

 contact. Doubtless this would sometimes happen, but even then the scratches 

 would, as a rule, be short. 



2 It covers (intermittently) a large area. I have myself examined it in places 

 from Yorkshire to Essex, and inland to the neighbourhood of Narborough 

 in Leicestershire. 



3 ' Ice-Work ' 1896, pp. 164-88. 



4 Quart. Journ. Geol. Soc. vol. xxxiv (1878) p. 566. 



