THE- GEOLOGY. OF ORKNEY. lxxv 
BOULDER CLAY. 
The “drifts” or loose deposits that overlie the rocks: 
are of the greatest importance as determining the 
soils and agriculture of Orkney. Of these by far the 
most wide-spread is the “boulder-clay,’ a glacial 
deposit that covers most of the lower grounds. No 
accurate maps have as yet been made to show the 
distribution of the drifts, but it is probably a safe 
estimate that boulder-clay occupies one-third of the 
area of the islands. It varies in thickness from a foot 
or less up to forty feet and more. Cliffs of boulder- 
clay are to be seen at the east side of Kirkwall, the 
north of Shapinshay, in Deerness, and in many other 
places. Some of them are thirty feet high, but the 
deposit may be much thicker than this, and, in default 
of borings, it is impossible to estimate definitely what 
may be its maximum thickness. In its distribution 
there is one main characteristic, viz., it is confined to 
the lower grounds and seldom occurs in continuous 
expanses at elevations exceeding 250 feet. Conse- 
quently, on most of the hilly ground the soil is thin 
and derived from the decomposition of the underlying 
rock. Below that altitude, however, the reverse holds 
good, and boulder-clay, usually only a few feet thick 
is almost universal. Its presence may be observed in 
the shore sections, on the banks of streams, and in 
quarries and artificial openings of all sorts. 
The boulder-clays of Orkney are seldom sandy or 
loamy, and yield stiffish soils, which tend to be wet in 
winter unless they are drained. On the other hand 
they are generally calcareous, because they are 
