Ixxvill FLORA ORCADENSIS. 
there were small lochans on the irregular surface of 
the boulder-clay, and most of these are now occupied 
by alluvial silt often overlying thin layers of fresh- 
water marl. The streams are of insignificant size, but 
along their courses there are many flat alluvial 
meadows usually drained and cultivated. The small 
peaty lakes in the moors afford a nidus for a swamp 
flora. The larger fresh-water lochs are often held up 
by dams of clay, but some of them seem to be rock 
basins, as they discharge over rock, and there are no 
signs of an old outlet filled up with boulder-clay. 
They are all quite shallow, and the water in them is 
distinctly calcareous. 
The principal post-glacial changes in the surface 
configuration of Orkney are the silting up of many 
small lakes, the erosion of narrow gorges through 
rock by some of the streams, and the invasion of parts 
of the sea coast by blown sand. The sea cliffs also 
have receded before the attack of the waves, especially 
on the west coast, but few data are available to enable 
us to determine how far. 
In Sanday, as the name implies, there 1s a very 
large area covered by blown sand, and all the features 
of sandy links are there developed in great perfection. 
In Westray, Deerness, Burray, and South Ronaldshay, 
there are also fairly large sea-side links. 
PHYSICAL FEATURES OF ORKNEY. 
We may now consider the physical development 
of the Orkney Islands and the origin of the more 
important surface features of the county. This can 
only be done in a very imperfect, manner by means of 
