Upland Glaciation
Glen Rosa, Isle of Arran
A View of Glen Rosa, Looking North
Arête: A’ Chir (745m)
Striations: Rosa Slabs
Nunatak: North Goatfell (818m)
Roche Moutonnées
Pyramidal Peak: Cir Mhór (799m)
Corrie: Fionn Choire
Arête: Witch’s Step (758m)
Glacial Breach: The Saddle (432m)
Flat-bottomed, vertically sided glacial trough (‘U’ shaped valley),
occupied by Glenrosa Water, a misfit stream.
Cir Mhór: A Pyramidal Peak
Arête
Arête
Corrie
• Goatfell (874m) is another, excellent example from Arran.
• A steep-sided, isolated mountain summit produced by the
convergence of the backwalls of adjoining corries. The
precipitous cliffs of this horned peak, or horn, are separated by
steep arêtes on the intervening ridges.
• By virtue of their elevation, the pyramidal peaks on Arran
were also nunataks during the glacial maximum (18000 BP).
The Sleeping Warrior: An Arête
Witch’s Step
• The Witch’s Step (the deep ‘V’ shaped notch in the ridge) is part of the
arête better known as The Sleeping Warrior.
• Arête is the French term which has been widely accepted to describe a
narrow, rocky and often jagged ridge which divides the steep walls of two
adjacent corries. It may be on the actual crestline of a mountain or merely
on a subsidiary ridge.
• Lying above the ice they are subject to intense frost shattering.
The Saddle: A Glacial Breach
Glacial diffluence is the local
overriding of a pre-glacial
subsidiary watershed by glacier ice
so that the ice flows through a col
into a neighbouring valley. The
erosive effect of ice on the col is
such that it is lowered and the
watershed is said to be breached.
• The Saddle (432m) separates Glen Rosa from Glen Sannox.
• During the glacial maximum an ice sheet 600m deep overwhelmed Arran
from the north. It flowed up Glen Sannox before breaching and eroding a
low point in the intervening valley side. The ice then continued flowing
southwards, down Glen Rosa.
Fionn Choire: A Corrie
• Fionn Choire sits alongside Coire Buidhe on the southwestern
flank of Cir Mhor. It represents one of the source areas for the
valley glaciers that flowed into Glen Rosa during the Loch
Lomond Readvance (11ka – 10ka BP).
• Coirein Lochain and Coire na Ciche (The Devil’s Punchbowl)
are excellent examples from elsewhere in Northern Arran.
Roche Moutonnées in Glen Rosa
• Roche Moutonnées can be
found on the western valley
side in Glen Rosa.
• They are glacially moulded
rock mounds exhibiting an
asymmetrical form, with the
gently sloping and smoothly
abraded, up-valley face (stoss)
contrasting with the steeper,
broken, ice-plucked, downvalley face (lee).
Glacial Troughs on Arran
•
•
•
Glen Rosa (top), Glen Catacol (bottom) and
Glen Sannox are three of the most impressive
glacial troughs in Northern Arran.
A ‘U’ shaped valley is one that has been
overdeepened and widened by glacial erosion,
thereby severely modifying its pre-glacial form.
The slopes of the former fluvial valley are
considerably oversteepened by the glacier,
which occupies the entire valley rather than
merely the channel on the floor of the former
river valley. In addition, interlocking spurs are
planed-off to become truncated spurs, thereby
straightening the line of the valley as it is
transformed into a trough. The ‘U’ shaped valley
may terminate headwards in a trough end or it
may form part of a breached divide. Its flanks
may be marked by debris fans where tributary
streams descend steeply from neighbouring
corries or from hanging valleys.
Glenrosa Water and Abhainn Mór are examples
of misfit streams. This is a term given to streams
that are not proportionate in size to the valleys
they occupy.
Garbh Allt: A Hanging Valley
•
•
•
Garbh Allt is a small hanging valley
which enters Glen Rosa. It originates
in Coire á Bhradain (a corrie) to the
west of the main glacial trough.
Hanging valleys are tributary valleys
with an elevation distinctly higher than
that of the floor of the main glacial
trough. A river flowing down the
hanging valley will, therefore, descend
to the main valley as a waterfall.
It is generally agreed that the greater
effectiveness of the main glacier in
over-deepening the valley it occupied
is the main reason for the discordance
of the valley floors, but local geology
may be an important contributory
factor (as in Glen Easan).