May 8th - Cavehill.


Roger Field (Botanical) and James Rutherford (Geological)


Botany


During a brief respite in a very rainy week, a large group of members gathered to investigate the spring flora of the Hazelbank area on Cavehill, which is situated to the rear of Belfast Zoo. As the name implies, this section consists mainly of a steep slope covered by hazel scrub, with a well developed herb layer.

Before leaving the car park, we were treated to a fascinating explanation of the rather complicated geology of the adjacent cliff cutting by James Rutherford, the Hon. Geological Secretary.


Growing beneath the vertical face was a large clump of Fallopia sachalinensis, Giant Knotweed. This closely resembles Japanese Knotweed, but does not appear to be as invasive. As we entered the reserve proper, it was apparent that many of the expected woodland plants were well past their best. These included Wood Anemone, Bluebell and Wood-sorrel, although some flowers were still present. Pyrola minor, Common Wintergreen, was seen here by me about thirty years ago, although the exact bank is now completely overgrown by Luzula sylvatica, Great Wood-rush.


As we progressed, we were accompanied by various raucous noises, which we discovered to be penguins. Eventually a halt was called, as it was growing dark, but not before several plants of an uncommon grass, Melica uniflora, Wood Melick, had been discovered. On turning round, we were a little disconcerted to find ourselves being closely observed by a lion, fortunately on the other side of the fence! 


A list of the plant species is available.


Roger Field.



Geology


Cave Hill displays one of the most striking mountain profiles in Ireland and provides a wonderful backdrop to the city of Belfast, giving stunning views over the city and north Down. The main area of study is the cliffs along the approach road to the zoo and the subject is the the relationship between geology and landforms and man.


The rock types that make up Cave Hill are of recent origin in geological terms. They consist of lias clay, the oldest, c.180 my.,  chalk, which is cretaceous c.130 my., followed by the basalt 60 my. and finally a thin layer of boulder clay, dating from the end of the last ice age. The characteristics of these rocks are fundamental to the moulding of the landscape.


The main lines of the cliff run nearly horizontally and nearly vertically. The horizontal lines result from the strong bedding in the chalk and the near parallel flows of the basalt; the vertical slopes reflect the numerous fault lines generated by the weak basement of the lias.

The weakness of the lias is accentuated by percolation of surface water through numerous cracks or joints in the chalk and basalt lubricating the liasic surface. This results in rotational slippage carrying down large sections of the cliff under the weight of the rocks themselves and the water, a process known as cliff recession. A series of these slump features lie between the cliff and the lough shore. Dredging of the navigation channel probably accentuated this process. As the slope is unstable there are problems for maintaining roads and housing in the district.


Opposite the zoo car park there is an interesting exposure of chalk and basalt in the cliff. The junction of the two rock types is exposed showing the outline of the chalk surface. Here there are cross-sections of clints and grykes, and minor dry valleys. There is also some evidence of the chalk having been altered by the hot lava. This contact results in hardening or marbleising of the chalk and also red staining from the chemical reaction with the iron rich basalt. The exposed cliff also reveals how the lava reached the surface. One vent, among many around Cave Hill, was clearly presented, with chilled margins and iron staining. The presence of Bernard Anderson with his expertise contributed to an interesting occasion.


James Rutherford (Hon. Geological Secretary)

 
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