BrownSea Island

14th May 2000

Leader Dr. Mike Cosgrove

I visited Brownsea over twenty years ago (in pre-geology days), and I was enchanted by it. The impression that remained with me was that of a magical place marooned on a sandbank in Poole harbour. I was not the only one captivated by its’ charm. After his visit in 1818, the Prince Regent declared "I had no idea I had such a delightful spot in my Kingdom". On this particular day, Brownsea was shrouded in a persistent sea mist which added an element of mystery for those who were newcomers.

We followed a geology trail, incorporating 3 DIGS sites, that have been marked and described by the DIGS group, with the cooperation of the National Trust. Dr. Mike Cosgrove was a member of the team that worked on this project and we were lucky enough to have the benefit of his guidance for the day, along with the excellent pamphlet they produced as a result of their work. Fortunately, Mike is a geochemist, which is just as well, because chemistry is an important aspect of Brownsea sediments and their exploitation.

As a whole, the structure of Brownsea is simple. The lowest layer is Parkstone Clay which is overlain by Branksome Sands and together these make up the Poole Formation of Eocene age (about 40Ma old). This is capped with Pleistocene gravels, a relic of the Ice Age, being deposited ONLY a few hundred thousand years ago. The unconformity represents millions of missing years. The beds dip gently down to the east, so the eastern end of the island is mainly sands and the western end has cliffs of Parkstone clay, about 15m high. Erosion is a problem on the island, particularly as all the beds are unconsolidated. The eastern sandy shores and cliffs are facing the harbour mouth and are not only subject to the effects of severe weather from the Channel, wind and also wash produced by shipping. Mike commented that he noticed significant changes had occurred between visits, despite the efforts of the NT to remedy this by using wire baskets filled with smashed pottery from the Brownsea pottery dump. Despite their position on the lee-side of the island, the Parkstone Clay cliffs are also retreating which was evident from the remains of trees littering the shore area, having fallen from the top.

As we followed the South Shore from east to west, we were able to see the beds as they dipped towards us. One of the best exposures of the Branksome Sands was, typically, in the ‘No Access’ area, fortunately the falling tide made it available to us. There was cross-bedding, current-bedding, erosion surfaces, laminae of varying resistance and a good example of a layer of conglomerate (clay lumps) mixed with lignite. There were two different sands here, the lower was a beach dune sand and the upper was a river deposit. Many of these structures were repeated further along the exposure. Near the Rose Cottage steps, a channel had cut down into the clay and the fill reflected these features in miniature. As we continued along the shore, the top of the Clay became apparent underfoot as the wet sand beach became sticky and squelchy.

The Clay layer that emerged was a dark colour that weathers to grey. Anoxic conditions in a marine lagoon or swamp had partially preserved organic matter and the reducing conditions had produced a pyrite-rich clay. There were yellow deposits on the weathered surfaces, not due to sulphur, but to Jarosite, a hydrated Fe/K sulphate, the sulphate being an oxidation product of the sulphide in the pyrite. Within the clay are thin layers and lenses of sand, and one of these had formed an outcrop near the Terrier Steps. This sandstone was well-cemented because groundwaters, rich in iron oxides, had passed through it leaving behind a deposit of Goethite (hydrated FeO).

The high iron and sulphur content of the Poole Formation has had mixed blessings for local industry. The production of copperas seems to have been quite successful, but the ceramics and brickmaking industries were doomed to failure. Copperas is hydrated ferrous sulphate which was used as a colour fixer in the dyeing industry, for tanning, ink manufacture and making Prussian Blue pigment. Brownsea was an excellent source for this. The abundant sulphate ions also form sulphuric acid within these sediments with the result that clay products are of poor quality. This was evident when you see the amount of rejected broken pottery that makes up the beach material near Pottery Pier and by the condition of the bricks of the walls near the castle. As Mike stressed, the sulphur is the key to success or failure. Apparently, the NT has been having trouble with the plumbing for years, they must have been horrified when Mike and his colleagues told them that the place is bathed in dilute sulphuric acid.

Brownsea is definitely worth a visit if you were not able to come on this particular occasion. The guide to the geology trail is called ‘The Geology of Brownsea’ and is available at the shop, but you have to ask for it. The tearooms are highly recommended! Many thanks to Mike for giving us such an interesting trip.

Finally, I know that my impression of the marooned sandbank was a bit off the mark, but I was right about the magic!