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Essex Rock & Mineral Society Field Trip Just two members of ERMS turned up to join 3 from Southampton Geological Research Group. We were met by the Quarry Manager, but then left to get on with our visit and to stay as long as we wished. The day started with broken cloud, and then became gloriously sunny in the afternoon. Colemans Quarry is an active quarry operated by Aggregate Industries, who work the Black Rock, Vallis and Clifton Down Groups of the Carboniferous Limestone. This limestone is both folded and faulted, and Triassic Dolomitic Conglomerate, Rhaetian and Lower Jurassic sediments fill pockets and fissures in the limestone surface, which is covered by a more extensive layer of Upper Inferior Oolite. Holwell came to fame in 1858 when a geologist named Moore purchased three tons of fissure filling, spent 3 years sifting and sorting and separated more than one million fossils including 27 small mammal teeth. The working quarry areas are approached via two roadway tunnels which lead down to the lower levels, and a large extent of fresh quarry face can easily be accessed. What is available to be collected, however, is totally unpredictable, depending on whether the faces being worked at the time of the visit contain many richly mineral-filled fissures. Repeat visits at future dates, therefore, would be expected to yield fresh opportunities to collect. The most plentiful mineral found was Calcite, with crystals up to 30 mm across present as the lining of cavities. Getting the crystals out, however, was not that easy, since the best cavity was rather like an iceberg, with 90 per cent of its mass buried under several hundredweights of boulders at the base of a freshly blasted cliff face which was far from stable! Nevertheless, both ERMS members obtained satisfying specimens. Some of the Calcite crystals found were associated with impressions of missing Calcite crystals in a brownish black microcrystalline mineral which looked strikingly like Sphalerite, and here the puzzle becomes more obscure. According to the mineral list for Colemans Quarry on the mindat.org web site, Sphalerite does not occur. However, this list is suspicious because it contains 24 minerals with names beginning from A to R, but absolutely nothing beginning with S to Z. It is almost as if a page has been omitted when the list was posted on the web. To resolve the enigma, a sample of the mineral in question is being analysed to find out exactly what it is. Some of the Southampton group found a few small fossils but I did not note what they were, but my recollection is that they were corals. The grand finale was a wonderful flying display by two Peregrine Falcons, who obviously use Colemans Quarry as part of their territory, at which point I bade my farewells and started the long uphill walk back to the car, dodging the sprinklers which had been turned on to dampen the dust on the roadway. Chris Swan |