Ard Bheinn is the northwestern part of what is known as the Arran
Central Complex; the eroded remains of a stratovolcano which
erupted nearly 60 million years ago at the time when the North
atlantic Ocean was just starting to open as a former super
continent split to form Euroasia and North America. This volcano
shows at least five phases of eruptions three of which are clearly
exposed around Ard Bheinn (to those with some familiarity with
Geology). After an first building a large cone of basalt lava flows
the volcanoe exploded forming a huge caldera (volcanic crater) 5
kilometres in diameter and at least 1 km deep, the evidence for
this is in vast chunks of older rocks including Cretatious chalk,
Jurrassic marls and Devonian sandstones that have fallen down into
the caldera (the largest are several hundered metres long and up to
50 metres thick). The mapping of this structure by Benjamin Peach
and Willian Gunn (1901) was the first direct evidence that
volcanoes had been active in the Britch Isles as recently as the
early tertiary.
The cache site is reminicent of a Dartmoor cache and is actually
rochs which formed during a pyroclastic flow on the side of a
smaller volcanic cone that built up on the floor of the caldera. If
you wander around within a couple of hundred metres of the summit
of Ard Bhein (the Prominent Mountain/Major Hill) you will
find evidence of ash falls, pyroclastic flows and large chunks of
underlying rock that must have been blasted out of the volcano.
There is no obvious route up the mountain which is why the
terain has been graded as quite tough. The best marked route is
probably up the Slochd track which starts opposite the Balmichael
visitors centre and then winds up through the forestry plantation
on tracks and rides before deteriorating into a quad bike trail,
then over a soulder of Beinn Bhreac before assending Ard Bheinn
from the coll to the east of the summit. Geologists may be
interested in climbing up to, or desending down to the forestry car
park at Glenloig as this allows a greater variety of lithologies to
be examined including the chalk outcrops on the west side of Binein
na h-Uaimh (smaller mountain of the cave).