THE GEOLOGY OF ORKNEY. lxxxili 
The ice sheets which followed wiped out all the flora 
of the islands, and when they melted left a surface 
which at first must have been destitute of vegetation. 
The history of the present flora dates from that time. 
Not much is known about it as yet, but there is 
sufficient evidence to prove that there have been 
considerable changes of climate and geographical 
conditions, all of which must have directly affected 
the flora of the islands. 
It is not altogether certain at what period the 
great submergence took place that drowned the 
valleys of the old land surface, and converted 
Orkney into an archipelago of which the individual 
islands represent the hill tops of the previous period. 
Before the oncoming of the Ice Age, the land stood 
three or four hundred feet higher than at present, 
and was of course very much more extensive. It is 
possible that a considerable subsidence took place 
immediately before the ice sheets over-rode the 
islands. After the ice melted, the land was probably 
very much at the same level as at present, or perhaps 
a few feet higher. Along the east coast of Scotland, 
all the way south of Helmsdale, there are numerous 
raised beaches, but these are either absent or very 
badly developed in Caithness and in Orkney. The 
most important of these beaches are the hundred-foot, 
fifty-foot, and twenty-five-foot beaches, so called be- 
cause they stand approximately at these heights above 
present sea-level. The hundred-foot is the oldest of 
these, and next to it the fifty-foot beach. They show 
that there have been periods since the Ice Age when 
the land was sunken to the extent marked by each 
