Geology Group Meetings held in 2007

Alum Bay – 22nd April

Alum Bay - Rare Fossil Fish

Alum Bay – Rare Fossil Fish

A small group of four gathered for a walk at Alum Bay led by John Winch. We first negotiated the stony beach which curves round south-westerly towards the cliffs leading to the Needles, and John pointed out the Reading Beds as we passed them. From that point on we were mainly clambering over rocks, and hard hats came into play owing to possible falling debris from the chalk cliffs above, a point reinforced by a National Trust safety notice – but though we observed the springs coming out of the cliffs, in the event nothing further transpired. John Walton recalled how as a young man in 1948 he had walked out most of the way to the Needles following this lower cliff route, and had passed two cannons on the way which had tumbled off the fort! This feat would be almost physically impossible today. As we continued on our route John Winch pointed out the iron sulphide deposits in the chalk, and how the various flint bands, now almost vertical, would originally have been horizontal but the heave in the rocks had pushed them up. There were several Belemnite fossils, at least 65 million years old and related to the modern-day squid, one-and-a-half inches in length and looking for all the world like little bullets. A few of the many sea-urchin fossils were chiselled out for us to take home. But despite a low tide our way was barred by deep water right in to the rocks, and we were unable to venture the original distance John had intended. The keen eyes of John Walton spotted a Peregrine Falcon overhead, and two Rock Pipits as we returned along the cliffs.

The prize find of the trip was, however, an extremely rare fossil fish, possibly 100 million years old, lying on the chalk face. John Winch had certainly never seen anything quite like it on the Island before, and local records are very few and far between. The rare species was duly chiselled out of the rock, and we made our way back.

For this non-geologist the whole experience had been a complete revelation!

Alan Phillips

Alum Bay

Alum Bay

Prospect Quarry, Shalcombe SZ384866 – 24th June

Waterproofs, hard hats and sturdy footwear were essential equipment for a rainy afternoon at this quarry, which exposes most the thickness of the Bembridge Limestone Formation. Only the lowest (oldest) few metres of the bed are concealed. Seven geologists led by John Winch were rewarded by a variety of features and fossils. John quickly spotted a giant land snail Megalocochlea pseudoglolobosa, in limestone blocks near the entrance gates, and was soon also showing us a large Ramshorn snail lookalike, Planorbina discus. We viewed an infilled Pleistocene? palaeovalley cut into the top of the limestone near the entrance ramp, and found good specimens of dark orange calcite crystals infilling cavities and veins. The ‘spider-egg cocoons’ proved more elusive, though John found some small specimens. Unlike most British limestones, which are marine, Bembridge Limestone and the slightly older limestones in the underlying Headon Hill Formation are freshwater deposits. Bembridge Limestone formed in an extensive shallow lake on lowlands close to sea-level. It was quite alkaline freshwater, unlike lakes we are familiar with. The small conical freshwater gastropod Galba longiscata (currently changing its name to Lymnaea longiscata) is locally common. There were also several specimens of Viviparus angulosus, a large freshwater mollusc with a body whorl similar in size to the rather rare Megalocochlea but a taller spire. Water levels fluctuated and the lake occasionally dried out, allowing subaerial karst weathering, so many fossils are only represented by casts and moulds where the original shell was dissolved away and later replaced by an infill of fine limestone. A narrow band of grey limestone high in the cliff face probably represents the lake bed during a period of muddy inflow or possibly soil formation. The top of the Limestone is immediately below the modern surface regolith and soil, forming a northward-dipping slope or cuesta from Thorley to Shalcombe. Elsewhere on the Island the Limestone is overlain by Bembridge Marls, a brackish water sediment of the Bouldnor Formation, indicating a marine incursion from the east. John mentioned the rare mammal bones sometimes found in Bembridge Limestone, which correlate with the gypse fauna at Montmartre in Paris.

H.J. Osborne White (1921) described Bembridge Limestone as varying from eight to twenty five feet in thickness, and recorded it as Oligocene in age. It is thus shown as Oligocene on the geological map, never updated, but the Eocene/Oligocene boundary has been much debated. Curry (1992) recorded Bembridge Limestone as the topmost bed of the Late Eocene (34 million years ago), when a
broad land isthmus connected England with France. A marine North Sea basin extended west almost to the present east coast of the Isle of Wight, followed by land westwards to Devon/Cornwall, and beyond that was a western marine gulf. The Island was 50 degrees North, and this was a period of declining temperatures although the average temperature was still ten degrees above that of today. The Early Eocene (55 million years ago) probably had over six times the present-day concentration of atmospheric carbon dioxide (Beerling 2007), and experienced the hottest climate of the entire Cenozoic (Tertiary). An abrupt decline in greenhouse gases occurred 40 to 30 million years ago, partly due to their chemical reactions with rocks exposed in the newly uplifted Himalayas. This was changing global climate to the modern ‘Icehouse-World’ by Bembridge Limestone times, as the first ice began accumulating in Antarctica.

Mike Cotterill