BANKS OF THE TWEED AND SOME OF ITS TRIBUTARIES. D597 
a series of long smoothly-rounded banks, or ‘Drums’ and ‘ Sowbacks,; which run parallel in the 
direction taken by the ice. This peculiar conformation of the till, although doubtless modified 
to some extent by rain and streams, yet was no doubt assumed wnder the ice-sheet ; the ‘sow- 
backs’ being the glacial counterparts of those broad banks of silt and sand, that form, here and 
there, upon the banks of rivers. Perhaps the most admirable example in Scotland of this 
peculiar arrangement or configuration of the till, recwrs in the valley of the Tweed, between the 
Cheviot hills and the Lammermuirs. In this wide district, all the ridges of till rwn parallel to 
one another, and in a direction approximately east and west.” 
The foregoing passages ascribe the formation of these parallel ridges some- 
how to the action of land ice. 
But in another part of his book (page 243) the author apparently ascribes 
the formation to river action. 
After again (pages 241-2) referring to the “ gravel beds,” “‘ the well-marked 
ridges,” and the “typical kaims of the Tweed and some of its tributaries,” he 
says, that— 
“Putting these various considerations together, the conclusion seems forced upon us, 
that all those accumulations of water-worn materials owe their origin to currents, that once 
flowed down the valleys. Not only so, but one must also admit, that those currents were 
proportionate in size, to the extent of each particular valley-system, in which such accumula- 
tions were found. In short, we can only, as I think, account for the appearances described, by 
attributing the deposition of the greater areas of gravel and sand to river uction. But if so, 
then the rivers must have greatly surpassed in volume and breadth their present puny repre- 
sentatives.” (Page 243.) ; 
“The explanation appears to be simply this :—The great ice-sheet, underneath which the 
till accumulated, had, after depositing the boulder clay, continued to retire, until it was reduced 
to a system of gigantic local glaciers. In summer time, such streams and rivers as flowed in 
glacier valleys, would be vastly swollen by the water derived from melting snow and ice. 
Great currents would sweep down the valleys, carrying with them the angular debris derived 
from terminal moraines, and from freshets running down the slopes of the hills. As this debris 
was hurried along, it would be gradually rounded by attrition, and eventually pass into good 
gravel. At the sametime, the till and ancient moraine debris over which the rivers rushed, 
would be denuded and washed away from exposed positions.” (Page 245.) 
In the foregoing passages, the author calls in, for the formation of the 
till, the agency of an ice-sheet; and for the formation of beds of gravel 
and sand, the agency of rivers swollen in summer by the melting of the snow 
and ice. 
In the following passage, however (page 246), he not only admits but adopts 
a very different explanation :— 
“But whilst thus admitting, that many mound-shaped hillocks of gravel and sand are only 
the denuded remains of what were once continuous flats of fluviatile origin, still there are 
appearances connected with the more typical assemblages of kaims, cones, and mounds, which 
can hardly be explained by what we know of rain and river action. To account for some of the 
phenomena, we are apparently compelled to call in the agency of the sea. The deep circular 
depressions, surrounded on all sides by smoothly-rounded cones and banks, and often occupied 
by lakelets or peat mosses, cannot possibly be due to the action of rivers.” 
VOL. XXVII. PART IV. 1 _ 
