55G TRANSACTIONS OF SECTION C. 



time, tliei'e is supposed to have been a regrowth of valley-glaciers that came down 

 to sea-level during the 'Lower Turbarian ' stage. Then another melting away 

 marked the 'Upper Forestian/ followed by a fresh appearance of glaciers in the 

 glens of the higher mountain groups during the ' Upper Turbarian ' glacial epoch. 



But all the phenomena on which this scheme is built seem explicable on the 

 hypothesis of a gradually waning glaciation, during which there were occasional 

 local advances of the mountain-glaciers in their glens, due to temporary increase 

 of snowfall. We have already discussed the probability that the growth of the 

 individual ice-sheets was largely influenced by the local impact of snowfall under 

 changing meteorological conditions, and it seems equally probable that similar 

 changes, in reverse order, accompanied the waning of the same sheets. 



Indeed, from the study of recent glaciers, it has been shown that the presence 

 of separate moraines need not indicate separate stages of advance in the ice. 

 In discussing the influence of englacial debris on ice-flow, the late Professor 

 Israel C. Russell has the following pertinent remark : ' The considerations . . . 

 lead to the suggestion that a series of terminal moraines in a formerly glaciated 

 valley, or a similar succession of ridges left by a continental glacier, are not 

 necessaril}' evidences of repeated climatic oscillations, but may have been formed 

 during a uniform and continuous meteorological change favourable to glacial 

 recession. That is, a d6bris-charged sheet may retreat for a time, then halt, and 

 again retreat, owing to its terminus becoming congested with foreign material, 

 in response to a climatic change which would cause a glacier composed of clear 

 ice to recede continuously and without halts.' ^ 



Professor Geikie states his case for the ' Mecklenburgian district ice-sheets' 

 with intrepid but unconvincing persuasiveness.^ He acknowledges that no inter- 

 glacial deposits of the preceding Neudeckian epoch have been recognised in 

 Britain, and bases his argument upon the relation of the hill-drift to that of the 

 lowlands. Into the intricacies of this argument it is impossible for me to enter, 

 but there is one point which requires particular notice. The shelly boulder-clay 

 around Loch Lomond is held to represent the Mecklenburgian glaciation, and its 

 marine detritus to have been derived from a sea-floor belonging to the ' 100-foot 

 raised beach,' which is supposed to mark an early stage of the same glacial epoch. 

 But, as Mr. T. F. Jamieson ' has shown, there is no valid reason for regarding this 

 boulder-clay as newer than the bulk of the shelly boulder-clays of Scotland; it 

 rests directly upon the solid rock, except at one place, where a wedge of blue clay 

 with shells was found beneath it ; and no older boulder-clay is known in the 

 district. Even from the original description of the deposit given by Dr. R. L. Jack,'* 

 quoted with approval by Professor Geikie, we can gather no other interpretation ; 

 for although Dr. Jack thought that the shells were more probably derived from an 

 iiiterglacial than from a preglacial bed, he still regarded the boulder-clay in which 

 they occur as older than the ' great submergence ' — i.e., than the Helvetian 

 interglacial epoch of the new classification. 



The evidence yielded by the freshwater deposits that overlie the drifts in 

 Scotland, so far as I can judge, runs parallel with that of the similar deposits in 

 Yorkshire and the Isle of Man. The researches of the late James Bennie brought 

 to light several instances in which arctic plants and other remains occur in such 

 deposits, but always at or near their base, and sometimes overlain by higher beds 

 containing a temperate flora. By Mr. C. Reid, who has determined most of the 

 material, these arctic plant beds are classed as ' Late-Glacial,' and the subsequent 

 deposits as ' Neolithic' ■" 



' ' The Influence of D6brison the Flow of Glaciers.' Journ Geol., vol. iii. (1895), 

 p. 8.S1. 



' Great Ice Age, 3rd ed., chap. xx. 



' « Some Changes of Level in the Glacial Period.' Geol. Maf]., dec. v., vol. ii. (1905), 

 pp. 487-488. 



* 'Notes on a Till or Boulder-clay with Broken Shells . . . near Loch Lomond, &c. 

 Trans. Geol. Soc. Glasgow, vol. v. (1874), pp. 5-26. 



* Origin of the British Flora, p. 53. 



