354 S. V. Wood, Jun. — On Glacial Deposits. 



Mr. Geikie mentions the occurrence on the tops of the Ochils, 

 npon the watershed of the Renfrew uplands, and on the crest of the 

 Pentland Hills, of what he regards as true Till, containing, along 

 with rock debris furnished by these hills themselves, material 

 brought from the Highlands ; and as this material, he considers, 

 must have been brought by land-ice, filling the low ground between, 

 he urges this as fatal to my views. 



He has evidently misunderstood the meaning I attached to the 

 word "deposit" in the expression I used that "where the ice rested 

 deposit occurred." Of course, although no permanent deposit of it 

 occurred, there was always a certain quantity of the material under 

 the ice-sheet, or else it could not be incessantly ti-avelling outwards 

 to the sea : and in protected hollows the temporary accumulation 

 of this might be considerable. Necessarily, therefore, as the sheet 

 wasted back, there was a certain quantity of the material left on the 

 land surface as the ice deserljed it, and this would represent the Till 

 which Mr. Geikie thus instances against me ; but that material was 

 the product of the ice-sheet just preceding its desertion of these 

 particular spots ; and was formed ages after that which occupied the 

 same spot during the earlier part of the period, and which had all 

 travelled out to sea long before these hills were relieved of the ice- 

 sheet. The very circumstance instanced by Mi-. Geikie of material 

 from the Highlands finding its way to the Ochils and other isolated 

 hills, proves that the material was thus in motion. 



In a footnote to the Introduction to the Supplement of the Crag 

 Mollusca (p. 26), this occurrence of Mollusca at Dimlington near the 

 top of the great Chalky Clay (No. 9 of that Introduction) is noticed ; 

 but I had not seen the specimens. Since then Sir Charles Lyell has 

 been kind enough to send them to me, and it is highly satisfactory 

 to me to find this colony of Nucula Cohholdice occurring at an horizon 

 in the glacial sequence so near to that at which I and my coadjutors 

 had placed the Bridlington bed,^ with the Mollusca of which, inclusive 

 of this Nucula, these Dimlington shells agree identically, as far as 

 they go, both in species and in mineral condition. When it is thus 

 seen that a nest of Nucula Cohholdice lived in the British sea, after 

 100 feet of Glacial-clay, teeming with chalk debris similar to that 

 forming the lower part of Dimlington Cliif, had been deposited (for 

 to that depth is it shown by adjacent borings that this clay descends 

 before the uniformly even chalk floor is reached), the prolonged 

 glaciation of the Chalk wold which must have preceded the dying out 

 of that shell will be, I think, better recognized. The existence of 

 another 100 feet of Glacial-clay overlying this Nucula seam in actual 

 section shows that a prolonged period of glaciation also succeeded it; 

 and when it is observed that in this overlying 100 feet, the Chalk 

 debris begins to diminish immediately above the shell seam, and 



1 Besides the observations as to the position of the Bridlington bed in the Crag 

 Supplement Introtluctiun, and in various papers of mine in this Magazine, the re- 

 specrive horizons of the Uridlington bed and of Dimlington Cliff base will be found 

 marked in my vertical section of the Glacial sequence of the East siile of England, 

 at page 90 of vol. xxvi. of the Quarterly Journal of the Geological Society. 



