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Staffa is composed of the earliest of the outpourings of basalt
lava, the Fingal’s Cave Lava of the ‘Staffa Suite’. This thick
lava flow solidified into large hexagonal columns which
characterise the isle. The lava is underlain by red ash which
can be seen from Fingal’s cave along most of the west coast.
The lava itself is divisable into a lower zone of massive
regular columns with sides varying in number between 8 and 3
though the average is 6, a middle zone of narrow wavy columns,
and a top zone which is mostly slag. Most people approach the
cave along the causeway where the horizontal joints which cross
cut the columns form useful steps. Count the sides of a number
of columns here and calculate the average number. At Fingal’s
cave the two columnar zones are well exposed. In the columns
here calcite, which is secondary in formation, appears to
‘cement’ the columns together. It was for this reason that when
first discovered it was assumed that Staffa was man-made (by a
race of giants) because the stones were cemented together.
The base of the lava, which is vesicular and brecciated can be
seen at Port an Fhasgaidh. The slaggy zone can be seen for 300
yards south of the Goat Cave. At Goat Cave, thin ash and
carbonaceous sediment separate the Fingal’s cave lava from the
chilled columnar base of an overlying flow.
Macculloch’s Tree:
A visit to the fossil tree at Burg makes another
excellent geological excursion if you have plenty of energy and
a long day to spare. The return trip is approximately 12 miles
and some of this is along narrow coastal goat tracks. (Strong
boots are essential.) The coastal scenery is spectacular and in
fine weather you will be richly rewarded. One part of the walk
includes the descent of a long metal ladder set into the cliff.
(This can be avoided by scrambling down the cliff before the
ladder on the outward journey, but this is only possible at low
tide.) Aim to reach the Fossil Tree on a falling tide. Park your
car at the National Trust for Scotland carpark at Tiroran and
proceed on foot to Tavool and thence on to the Trust property of
Burg. Walk on past the old farm buildings towards Dun Burg.
(Visit the cairns and monuments.) Continue on along this coastal
walk via the goat track for about 2.5 miles. The fossil Tree
stands in a recess in the cliff 150 yards north of the
waterfalls near Rudha na h-Uamha.
Macculloch’s Tree is the cast of a 40 foot high conifer which
was engulfed by an early lava flow. The remains of the trunk
consists of breccia and charred wood. The top of the trunk has
been capped with cement and as it is a site of special
scientific interest you should not hammer nor collect samples.
The cast of the tree rises vertically into the basalt lava flow
which cooled up against the large tree forming small columns at
right angles to the cooling surface. The roots of the tree are
not seen, but close by is a carbonaceous mud under which is a
prominant bed of red ash.
Further along the coast is a cave with two entrances. In the
overlying northern face of the main entrance is the remains of
another tree. This is a 10 foot long dicotyledonous tree which
was 6-8” thick. It is enclosed in a brecciated part of the lava
flow. It is likely that these trees were parts of a forest which
was engulfed by the molten lava flow in early Tertiary times. |
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