Ixxxil FLORA ORCADENSIS. 
epoch of erosion belongs to a comparatively late 
period in the earth’s history, probably to the Pliocene. 
At any rate it was complete before the climate began 
to change, and with increasing rainfall and greater 
cold the conditions supervened that inaugurated the 
Ice Age. 
HILLS AND VALLEYS. 
The valleys and the hills, however, were carved 
out of a great plain, of low relief, that belonged to 
a still earlier set of geographical conditions. Any- 
one who ascends Wideford Hill or one of the 
Hoy Hills on a fine day can see around him many 
eminences that rise to nearly the same level. This 
varies from 800 to 900 feet on the Mainland to 1400 
or 1500 feet in Hoy, and these hill tops are the 
remains of a great plateau that extends southward 
into Caithness and Sutherland. In Harray and 
Birsay there are wide level summits to the principal 
hills, which are the remaining parts of this plateau; 
this is also true in Hoy, but at Wideford Hill and in 
some of the Firth Hills the ridges are narrower and 
the old plateau is less easily recognised. Geological 
evidence which is available in several parts of the 
West of Scotland goes to prove that this plateau out 
of which the scenery of Orkney has been carved (as 
the wood-carver cuts a pattern in a block of wood) 
belongs to the Miocene period, which preceded the 
Pliocene. 
These changes carry us back a long distance, and 
concern the botanist and plant ecologist only in- 
directly, as determining the relief of the land surface. 
