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continent and so when the ice finally melted the area experienced
isostatic rebound. Today the northern part of Norway continues to rise
at a greater rate than southern Norway. This naturally imposes differ-
ences on the dynamics and flow regimes of rivers in the two ends of the
country. This change in Norwegian elevation is rather local. If two
continental masses collide, as happened 450 million years ago when
the Iapetus Ocean closed and a land mass broadly coincidental with
NorthAmerica of today crashed slowly into a landmass that makes up
modernwestern Europe, the result is uplift on a large scale. Mountains
with the altitude of the modern-day Mount Everest were thrown up in
Scotland and northwest Ireland and the area was dotted with volcanic
activity. The pimples that make up the Scottish peaks collectively
known as Munros are the eroded remnants of these great mountains.
Similar collisions have produced the Appalachian Mountains and
the Alps.
Uplift is sometimes associated with volcanic activity, and per-
haps the classic case of this is the Andean Belt that was produced as
the Pacific Plate was subducted beneath the South American Plate. As
the oceanic rocks of the Pacific Plate were pushed deeper, theymelted,
and the molten rock found its way towards the surface where it was
extruded in volcanoes such as andesite and rhyolite. Many of these
volcanic slopes are highly unstable and have yielded copious quanti-
ties of sediment in a relatively short period. Fifty million years ago, as
the Atlantic Oceanwas opening andNorth America was moving away
from Europe, hot plumes of volcanic material centred on the area
around Mull and Ardnamurchan on the western side of Scotland led
to emplacement of volcanic rocks and associated uplift. This uplift
affected the flow regimes of rivers in the area and the erosion of the
pre-Tertiary rocks by those rivers. As has been recently demonstrated
by various researchers interested in the denudation of northeast
Ireland, it is not sufficient to examine the flow patterns of extant rivers
in order to achieve an understanding of how the ancient rivers behaved.
Not all rocks are the same and it is rare to find a river catchment
area draining only one type of lithology, and equally unlikely that one
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