Landscapes

Geology determines the landscape to a large extent. The nature of the underlying rock determines how it responds to various geological processes that are active at the surface of the Earth. At the most basic level, rocks which are hard and hence more resistant to weathering tend to stand out above rocks that are softer and more prone to weathering. A good example on Arran is granite, which forms the high peak of Goat Fell in the north (shown on the left), and also the small island of Ailsa Craig off the south coast of Arran (pictured on right).

Soft rocks weather rapidly and form rich soils, hard rocks are obviously more resistant, and produce less soil. This fundamental relationship is seen all across Arran, and a good example is along the southern coast where the hard resistant dykes stand out above the much softer sandstones into which they were intruded. The tidal zone shows this clearly, with swarms of dykes being visible (as demonstrated in the picture on the right). Inland, resistant sills often give rise to waterfalls such as at Auchenhew. Another good example if this is shown on the left at Levencorroch Burn (shown on left), where a relatively hard and resistant dolerite sill produces another waterfall as it prevents the erosion by water of the much softer Triassic sandstones beneath it (the description of the larger image explains how the waterfall forms).

Perhaps the biggest influence on the present day landscapes of Arran has been the Pleistocene ice-ages. In geological terms, the last ice-age happened very recently (i.e. 18,000 years ago) so it is not surprising that the landscapes of Arran so strikingly reflect the dramatic modifications produced by thick glaciers and ice-sheets. Valleys, such as Glen Rosa, have been gouged out by glaciers in the characteristic U-shape; clearly the tiny stream now present did not cut this beautiful valley. Apart from gouging out valleys, the ice was also responsible for moving large blocks of rock - the so-called 'eratics' that are visible in many parts of Arran (image on right).

The action of the sea on rocks also exploits some of their geological properties. The image on the left shows Black Cave on Arran, a modern sea cave cut into relatively hard columnar jointed lavas, where the weaknesses present along the joints has been exploiting by the action of the sea to produce a relatively tall but narrow cave aligned along the jointing direction. On the left is an image of King's Cave, in which relatively wide caves have been cut in much softer Triassic sandstones. Note also that King's Cave is a fossil - it was cut when the sea was around 10 metres higher that its current level. This was another prominent feature of the landscape on Arran produced by the relatively recent glaciation. The enormous weight of the ice pushed the land down ( a pheonomenon known as isostacy). At the end the ice-age the ice melted much more rapidly that the land could rebound, and so the sea was at a much higher level once the ice had gone. Since that time, Arran has been slowly rising again due to isostacy, and hence the early post-glacial coastline is now much higher up the coastline. Where ever you go on the coastline of Arran, keep your eyes peeled for the evidence of these massive land movements.

So although the ice has gone, there is ample evidence of its former presence on Arran, in the form of the raised beaches. There might be even more specific information available from a study of the landscapes on Arran. Corries indicate accumulation sites of glaciers, and Arran has numerous good examples which confirms that the peaks in the northern granite were sufficiently high to form a separate location of ice accumulation from that on the mainland. The fact that there is significant change of topography at approximately 750m in the northern granite has been taken by some scientists as indicating the depth of ice - above 750m peaks such as Goat Fell and Cir Mor are jagged and sharp, while lower peaks (again composed of granite) are rounded. The jagged peaks may have stood out about the ice-sheet as nunataks (being shattered by freezing and thawing) while the more rounded peaks were beneath the ice and being smoothes as it moved. Look carefully at the image of Goat Fell shown above (with the Waverley in the foreground) and you can see this feature. The image on the left also shows this feature if you examine the profile of Goat Fell in the background.

Geological boundaries (i.e. the contacts between different types of the rocks in the field) also have major effects on the landscape. In the image on the right, the prominent notch occurs along the line of the contact between the northern granite on Arran (on the left) and the Dalradian rocks (on the right). The granite is much younger, and has intruded into the crust when hot, and caused local metamorphism in the much older Dalradian rocks (see hornfels). This produces a zone of altered rocks that weather more rapidly, leading to the feature we see today (more details in description of the image on the right - click on it to read about this).

Landslides are another dramatic feature of the landscape controlled by underlying geology. A major landslide has occurred on Arran in relatively recent time, producing the 'fallen rocks' on the coast north of Corrie. Click on image on left for more information.