THE GEOLOGY OF ORKNEY. lxxvii 
numbers in the boulder-clay ; the shells are usually 
striated and broken ; though sometimes single valves 
may be found whole. 
By tar the larger number of the rock fragments 
that occur in the boulder-clay, however, are of local 
derivation, and belong to varieties of rock that are 
common in the islands, such as flagstone and sand- 
stone. They have all been transported from a greater 
or smaller distance, and consequently the boulder-clay 
is not strictly similar in composition to the rock on 
which it rests. In Westray, for example, there is 
often much sandstone, derived from Eday, in the 
boulder-clay ; and this is also the case near Kirkwall 
and in the Kast Mainland. The presence of shell 
fragments and of sand and silt from the sea-floor also 
makes the clay somewhat different from what it 
would be if it were entirely of local derivation. For 
these reasons it varies a good deal in different parts of 
the islands, but it is always a stiff, stony, unstratified 
clay, somewhat caleareous and of mixed origin, and 
the best soils on the islands are derived from this clay 
where 1é has been improved by artificial drainage and 
long cultivation. 
The later phases of the Ice Age have left few 
traces in the Orkney Islands except that at the 
mouths of the higher valleys small conical or crescent- 
shaped moraines may often be seen, but there are no 
extensive sheets of fluvio-glacial sand and gravel such 
as cover enormous tracts in Sutherland, Ross, and 
Inverness. There are also none of the raised beaches 
which are so characteristic features of the shores of the 
Cromarty and Moray Firths. After the ice melted 
