Vol. 62. ,] GLACIAL PERIOD IN ABERDEEXSHIEE, ETC. 15 



debris of Crag-shells and fragments of Secondary limestone. These 

 mounds, I find, form part and parcel of this Red-Clay Series, for the 

 Eed Clay occurs both above and below them. 



From the foregoing account it will be seen that this Red-Clay 

 Series presents features somewhat resembling the Drift- deposits of 

 Lancashire and Cheshire, and like them it occasionally contains sea- 

 shells, generally more or less broken. In certain localities, as at 

 Aberdeen and Peterhead, clay of a different colour is interstratified 

 or mixed with the red, showing that there has been in such places 

 a mingling of sediment from different sources. 



The esker-like mounds of gravel occur along what seems to have 

 been the western border of the mass of ice which came northward 

 along the coast. These mounds may have been formed by streams 

 of water tumbling from the margin of the glacier, and washing off 

 the mineral debris embedded in or lying on the ice. The purer 

 masses of clay seem to have resulted from clouds of muddy sediment 

 subsiding in a sheet of water lying in front of the ice, between 

 it and the land. 



In connection with these Red-Clay beds one noteworthy circum- 

 stance is the evidence which they afford that, at the time when this 

 northward flow took place along our coast, the Aberdeenshire ice 

 was shrinking, or had already shrunk, much within its former limits. 

 During the preceding period, when the subjacent grey Boulder-Clay 

 was deposited, the Aberdeenshire ice came down to the coast, and 

 even advanced beyond it ; but, during the deposition of the lied 

 Clay, this local ice had made a decided retreat, and the coastal 

 district was under water. Specially was such the case in the Valley 

 of the Dee. This river takes its rise among the mountains of 

 Braemar, the highest group of hills in Britain, and flows along a 

 well-defined valley terminating in the sea at Aberdeen. It might 

 therefore be thought that here, if anywhere, we should have had a 

 very large and persistent glacier. Nevertheless it is quite clear, 

 that during the deposition of the Red Clay the Glacier of the Dee 

 had receded some distance up the valley ; for - the Red Clay lies right 

 across the mouth of the stream at Aberdeen, and wasted patches of it 

 have been noted for about a couple of miles up the bed of the river. 

 This shows that the Aberdeenshire ice must have been comparatively 

 thin, so that it melted aw T ay long before the heavy ice which 

 covered the Western Highlands and the valleys of Perthshire. 

 There is nothing unintelligible in this, for the Tay and its main 

 tributaries take their rise in the rainy district of the West, where 

 the fall of snow during the Glacial Period seems to have been very 

 heavy ; whereas Aberdeenshire and its rivers lie on the drier side 

 of the country, where the fall of snow was doubtless much less. 

 Therefore, from meteorological considerations alone, we might expect 

 the local ice there to have been much thinner. Consequently, the 

 glacier that filled the Valley of the Dee would shrink long before 

 the glacier which came down the Valley of the Tay, and along 

 Strathmore to Lunan Bay. 



