GLACIATION OF THE SHETLAND ISLES. 809 
plained by the passage of land-ice over Shetland. It is sufficient 
for our present purpose if we show that, during the general glaciation 
of Scotland, Boulder-clay was transported across important  hill- 
ranges by the ice which radiated from the Grampians. On the 
south of the Sidlaw range, as well as on the south side of the Ochils, 
the Boulder-clay contains fragments of schist, gneiss, and granite, 
which must have been transported from the Highlands. Further, on 
the top of Allermuir hill small patches of Boulder-clay were observed 
by Dr. Croll containing striated stones derived from the Highlands 
to the north-west. Itis evident, therefore, that the Scotch ice- 
sheet was powerful enough to override such important ranges as 
the Sidlaws, the Ochils, and portions of the Pentlands, and must 
likewise have rolled forward the bottom moraine, depositing it in 
the lee of the hills. And if such was the case in Scotland, then 
why may not the same thing have happened in Shetland? Indeed, 
had Shetland formed a part of the western sea-board of Scotland, 
there would have been no hesitation in ascribing the striated surfaces 
and the Boulder-clay to the action of land-ice. 
The land-ice which glaciated Scotland could only have come from 
Scandinavia, as the striated surfaces clearly point in that direction. 
And we must now briefly consider what grounds there are for be- 
levying that the Scandinavian mer de glace was powerful enough to 
invade the North Sea. The researches of Krdmann, Horbye, Ksmark, 
Helland, Tornebohm and Linnarsson haye revealed to us the extent 
of the ancient glaciation of Norway and Sweden. ‘They clearly 
show that Scandinavia was not glaciated by Polarice moving south- 
wards from the Arctic regions ; for the ice-markings generally radiate 
from the great tablelands as they do in Scotland. It must have 
been buried underneath an ice-sheet which moved off the land in all 
directions. It has been generally supposed that this mer de glace must 
haye broken up in the form of bergs when it reached the shallow 
North Sea; but fortunately we are now supplied with data which 
enable us to prove that this could not have been the case. If 
we take the estimate given by Helland for the minimum thick- 
ness of the ice in Sogne Fjord during the period of extreme cold, it 
follows that, instead of the ice breaking up in the form of bergs, it 
must have invaded the North Sea and moved in a westerly direc- 
tion towards the Shetland Isles. He gives 6000 feet as the estimate 
at this point; and when we remember that the average depth of 
the German Ocean is about 240 feet, we can readily understand 
how such a mass of ice could never haye floated between Norway 
and Shetland, much less between Norway and Scotland. 
When this mer de glace impinged on the Shetland frontier, it 
would necessarily be deflected to some extent by the opposing high 
ground. Hence, as we move southwards from Unst, where the 
average trend of the ice-markings is W. 10°-20° S. towards Bressay 
and Lerwick, the deflection increases to S8.W. and in some cases to 
S.8.W. But as soon as the ice reached the crest of the Mainland, 
it would naturally follow the path of least resistance, veering 
round to the N.W. and N.N.W. It is highly probable that this 
northing may be due in part to the resistance offered by the Scotch 
