A Wee Isle’s Mighty Geology

The Isle of Arran in SW Scotland is very well known to generations of undergraduate geologists from Britain and Ireland. But maybe when caught up in the blur of mapping, bad weather and late nights on a field trip we can sometimes fail to appreciate just how truly special the local geology really is. That is certainly the case for the Isle of Arran – it is hard to think of somewhere in NW Europe that is more geologically diverse and crammed into such a small region. It is often called “Scotland in miniature” because it mimics the topography of Scotland as a whole – rugged, heather-covered mountains in the north, and rolling hills with lush green fields in the south.

Figure 1. Geological map of the Isle of Arran.
Figure 1. Geological map of the Isle of Arran.

The geology of Arran too is like a miniature Scotland. The mountainous north is comprised mainly of 600 million-year old metamorphosed sedimentary rocks (the Dalradian Supergroup), unconformably overlain by Devonian and Carboniferous sedimentary rocks. The Northern Arran Granite was intruded into these rock units during the Early Palaeogene 58.5 million years ago, which domed the Dalradian metamorphic rocks and its Palaeozoic sedimentary cover so that they now dip away from the core of the granite pluton. The northern and southern parts of the island are separated by the trace of the Highland Boundary Fault, a major Palaeozoic fault with a complicated history of subsequent movements. The south part of the island is comprised mainly of Permian and Triassic clastic sedimentary rocks, with the Early Palaeogene Arran central ring complex, a classic example of an eroded caldera, cropping out in the central part of the island. Early Palaeogene dykes and sills, including the spectacular Drumadoon sill, crop out throughout the island.

Figure 2. The Drumadoon sill in SW Arran. This c. 60 million year old sill is harder than the rocks it was intruded into and forms a prominent ridge in the landscape. It also shows spectacular columnar jointing. These six-sided columns formed when the sill cooled and contracted.
Figure 2. The Drumadoon sill in SW Arran. This c. 60 million year old sill is harder than the rocks it was intruded into and forms a prominent ridge in the landscape. It also shows spectacular columnar jointing. These six-sided columns formed when the sill cooled and contracted.

The diverse geology packed into a small geographical area makes Arran an ideal location for introductory undergraduate field trips, as students can be introduced to a wide variety of rock types and field techniques with minimal travelling. However, even allowing for its diverse and rich geological heritage, there are some special geological localities on Arran that are so outstanding that they are of international importance. These geological highlights include a fossil track-way made by a giant “millipede”, fossil lightning strikes and reptile footprints.

The fossil track-way is found in Lower Carboniferous rocks (c. 350 million years old) near Laggan in NE Arran. The trail was made by a giant “millipede-like” arthropod – a myriapod called Arthropleura. This creature was scarily up to 2m long with 23 pairs of legs! Myriapods, like insects, don’t have lungs, and instead rely on air flowing through a series of tubes through their bodies. Back then the atmosphere contained over 30% oxygen, compared to today’s air which is made up of 21% oxygen. This may have allowed organisms such as myriapods and insects to reach such large sizes during the Carboniferous.

Figure 3. The myraipod trackway near Laggan in NE Arran. Two sets of parallel tracks made by the left and right feet of the animal are visible and are over 30 cm apart.
Figure 3. The myraipod trackway near Laggan in NE Arran. Two sets of parallel tracks made by the left and right feet of the animal are visible and are over 30 cm apart.

How do we recognize fossil lightning strikes in the geological record, and how do they form? Pure quartz sand melts at about 1800°C. When lightning strikes sand, the intense temperature from the lightning bolt, can reach 30,000°C and causes the sand to be fused together into a tube-like shape. In the present-day, these-tube like structures, known as “fulgurites”, are usually found beneath the sand surface in environments such as deserts. On the Corrie foreshore in NE Arran, fulgurites have been found in Permian desert sandstones and consist of a pale tube of fused sand filled with oxidized red sand.  It is very rare to see such fossil lightning strikes preserved in the geological record.

Figure 4. Fulgurite (fossil lightning strike) in Permian sandstones in Corrie in NE Arran. Photo by Robbie Goodhue.
Figure 4. Fulgurite (fossil lightning strike) in Permian sandstones in Corrie in NE Arran. Photo by Robbie Goodhue.

Fossil reptile footprints have also been found on Arran near Blackwaterfoot on the SW coast. Following their discovery in the early 1990s, many student fieldtrips have been led to the fossil footprints in a loose block at the base of the old sea cliffs 300m NE of Cleiteadh nan Sgarbh. The species that made these is an ancient 4m-long lizard-like animal known as Isochirotherium, meaning ‘hand beast’, because its ‘feet’ are actually more like hands. Since then, many more sets of footprints have been discovered at several locations on Arran (Clark and Corrance, 2009). The lizard is thought to have been similar to a crocodile, except that it would have had a shorter snout and long legs that extended straight down from its body, rather than sticking out to the side.

Although the underlying geological structure of Arran was formed many millions of years ago, as it wandered the Earth’s surface, the landscape we see today largely results from the action of ice during the Ice Ages, the most recent of which ended about 10,000 years ago. But the description and interpretation of these landforms is entering the realm of the geomorphologist…

By Dr. David Chew, Trinity College Dublin.

References

http://www.scran.ac.uk/packs/exhibitions/learning_materials/webs/43/index.htm

http://www.arranmuseum.co.uk/what-will-you-see/geology/the-geological-history/

Clark, N. D. L. and H. Corrance, H. (2009) New discoveries of Isochirotherium herculis (Egerton 1838) and a reassessment of chirotheriid footprints from the Triassic of the Isle of Arran, Scotland. Scottish Journal of Geology 45, (1), 69–82.

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