Earth Sciences / 25The British landscape was formed by a series of orogenies. The first, at a time when Scotland was
still joined to North America, began about 500 million years ago and produced the Caledonian-
Appalachian mountain chain (WINDLEY, 1984, pp. 181–208) as well as the mountains of northern
Norway. The Appalachians were later affected by the Acadian orogeny, about 360 million years ago,
and the Alleghanian orogeny, about 290 million years ago. Europe was affected by the Hercynian
and Uralian orogenies, both of which occurred at about the same time as the Alleghanian. Figure 2.3
shows the area of Europe affected by several orogenies.^5
Igneous intrusions can be exposed through the weathering away of softer rocks surrounding them. Such
an exposed intrusion, roughly circular in shape and with approximately vertical sides, is called a ‘boss’ if
its surface area is less than 25 km^2 and a ‘batholith’ if it is larger (and they are often much larger).
Dartmoor and Bodmin Moor, in Devon and Cornwall, Britain, lie on the surface of granite batholiths.
Mountains are not always formed from igneous rocks, however. There are fossil shells of marine
organisms at high altitudes in the Alps and Himalayas, showing that these mountains were formed
by the crumpling of rocks which had formed from sea-bed sediments.
Many sedimentary rocks are composed of mineral grains eroded from igneous or other rocks and
transported by wind or more commonly water to a place where they settle. Others, said to be of
‘biogenic’ origin, are derived from the insoluble remains of once-living organisms. Limestones, for
example, are widely distributed. Most sediments settle in layers on the sea bed, to which rivers have
carried them. Periodic changes in the environmental conditions in which they are deposited may
cause sedimentation to cease and then resume later, and chemical changes in the water or the sediment
itself will be recorded in the sediments themselves and in the rocks into which they may be converted.
Figure 2.3 The mountain-forming events in Europe
Note: The thick lines (- • - • -) mark the Alpine orogeny