Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Wednesday, 19 November 2014

Flat Holm Erratic Hunt (7): the geomorphology

 Flat Holm seen from the NW -- in this photo the west side of the island is on the right -- note the overall roche moutonnee form.

As mentioned in earlier posts, Flat Holm is a relatively low island, almost circular, and just over 600m in diameter.  There is a gentle slope from the more exposed western shore up towards the eastern cliffs -- and it is on the east that the highest land on the island is to be found.  The highest point is only 32m above sea-level, but because the tidal range is so large here -- up to 15m on some spring tides -- the island expands and contracts rather spectacularly every day.......

As noted in earlier posts, the island has an ideal roche moutonnee form which matches perfectly the passage of ice across the island, moving from west towards east on several occasions during the Ice Age.  However, it would be a mistake to attribute the form of the island entirely to glacial processes, since the main geological features are a series of pitching anticlines and synclines running broadly NE-SW -- and these features, above all else, explain "the lie of the land."  There are extensive exposed bedrock surfaces on the western side of the island between the tide marks.  Scrambling about beneath the cliffs is more difficult on the south coast, and more difficult still on the east side of the island, where cliffs and thick vegetation make exploring an occasionally hazardous occupation.

The western side of the island is a geological paradise, with abundant fissures, faults, folds, thrusts and anticlinal and synclinal structures easily accessible -- alongside many variations in rock type.  The rocks all belong to the Carboniferous series, but in addition to "classic" hard grey limestones there are also mudstones, cherts, oolites and also veins of calcite and post-Carboniferous weathering and residual products exposed particularly in fissures and gullies in the cliff face. 

The most famous surface features are the large ripple marks exposed on the shore platform to the west of the farmhouse.  They have been exposed as a result of the breaking up and stripping off of overlying strata -- partly by glacial processes and partly by wave action.

Some of the large ripple structures on the west side of the island.  A perfectly preserved Carboniferous sea floor......

 Steeply dipping bedrock (mudstones, shales and sandy layers as well as interbedded limestones) near the SW corner of the island.  The upper 2m or so of the bedrock exposure is broken up by periglacial and slope processes;  there are signs of frost heave features.  At the top of the cliff is a thin sandy loam incorporating windblown material.  This is probably Holocene.  In the gully in the middle distance there is a plug of reddish material which may be of Triassic age; but there is much debate about this.......

Around the whole coast of the island there are traces of a raised beach platform -- or more likely, several of them.  Structural controls are so dominant that these fragments are difficult to identify with any certainty -- but they occur at all sorts of altitudes from mean tide level up to about 3m above extreme spring HWM.  Trying to match these traces with the raised beach platforms of Pembrokeshire (for example) is very difficult, because of the vast tidal range here in the Severn Estuary.  The best preserved raised beach platform is about 2m - 3m above extreme HWM, on the cliffs at the SE corner of Coal Beach.  The platform extends towards Point Bay.  This is what it looks like:

Sid Howells on the raised beach platform which runs for almost 100m along the cliffs near the eastern extremity of the island.  The surface is irregular, but it is a well-pronounced 
feature up to 4m wide.

There is also a modern wave-cut platform which is occasionally more than 100m wide, cut by a combination of marine solutional processes and  abrasion as blocks and pebbles are moved about by waves and tidal rises and falls. Parts of this platform are well covered with rocky debris and pebbles, but occasionally it might be exposed, as in this photo of Point Bay, as seen from the clifftop near the Foghorn.

Because the whole island is made up of calcareous rocks with complex structures and abundant fissures, it is inevitable that there are tunnels, caves and solutional hollows and collapses all over the place.  Some of these have been used for the exploration of past inhabitants for lead and other minerals -- and indeed for the exploitation of mineral finds.    Above the raised beach rock platform between Coal Beach and Point Bay, there are two tunnel entrances.  It's thought that one of them is natural and the other man-made -- but to me it looks as if both might be natural but maybe enlarged by those in search of mineral wealth.  This is the entrance to one cave.  It looks very similar to the caves of Gower, South Pembrokeshire and Caldey Island -- and it is quite possible that prehistoric layers might be present and worthy of examination.

Castle Rock is a spectacular double stack with an old cave system at its centre.  It is being whittled away by marine processes, and at any moment -- probably during some storm bringing big waves in from the north -- the tops of both stacks will slide into the sea, along old bedding planes which can easily be seen from the beach below.

 Finally. a word about the solutional features which are particularly prominent around the southern shores of the island where the massive limestone beds of the Birnbeck Limestone are exposed.  We can refer to these features as "karren".  They incorporate jagged raised surfaces and complex and intertwining channels which are sometimes almost a metre deep.  mechanical abrasion does not seem to be very active in most cases-  these are straightforward solutional features.  The edges of the ridges are often razor-sharp -- capable of ripping boot soles to shreds, not to mention hands, if you are unwise enough to lose your balance when hopping around on the ridges........

Conclusion:  Plenty of geomorphology to get your teeth into here, not to mention classic geological features.  Interestingly enough, Sid, Chris and I did not identify a single in situ glacial or fluvio-glacial deposit on the island cliffs.  That confirms, in my mind, that Devensian ice probably did not reach Flat Holm.  But there are erratics everywhere, and in several places we see deposits that look like very ancient glacial deposits that have been eroded and modified over a long period of time.  The best guess is that they are Anglian in age, and that it was the Anglian Glaciation that carried those thousands of erratics from the far west into the vicinity.

No comments: