Lesley Dunlop led a group of 13 members and 2 visitors to look at the geology of Greenham Common on the morning of Saturday 15 November. Meeting place was the Control Centre car park. It was a misty morning, following a day of rain. Greenham and Crookham Commons lie on a long ridge between the Rivers Enborne and Kennet. The ridge consists of Eocene deposits of acid, sandy clays of the Bagshot Formation, overlain by Silchester gravels and seated on heavy impermeable clays of the London Clay, which were laid down about 50 million years ago. The Silchester gravels were freshwater deposits, laid down about 500,000 years ago at the time of maximum extent of the ice fields, the Anglian stage. At this time, the course of the Thames was diverted by the advancing ice sheet from a route to the north of the Chilterns, eventually becoming a tributary of the Rhine, to its current location to the south of the Chilterns, passing through the Goring Gap. The walk started out on a track which led eastwards to a large shallow pool which was full of water, following the previous day’s rain. The impermeable London Clay bedrock stops the water from draining away. The soils are a complicated mixture of deposits in which free draining soils dominate, but with clay pockets producing extensive seepage zones and springs. These springs give rise to streams creating the small, flushed and waterlogged valleys of Alder woodland. Lesley pointed out the line of the Chalk hills to the south. The various layers were laid down horizontally, but became folded when Africa collided with Europe. Continuing along the track, a small herd of cattle were grazing nearby. Snowy and Parrot Waxcaps were growing in the grass and Common Centaury was seen in flower. Birds seen included a Pied Wagtail, a Stonechat, 3 Fieldfares and a Siskin. The stones in the gravel track were mainly flint, but they were inspected carefully for quartz specimens. These originate from veins in granite in south-west England which were formed about 290 million years ago. They were washed out first to beds in the West Midlands, then to the older, higher gravel beds in the Thames Valley. The next part of the walk was supposed to be across Bury’s Bank Road and into BBOWT’s Bowdown Woods reserve, where streams carve out gullies and the wet woodland is dominated by Alders. Alder has hard wood which repels water and had many uses, including forming the base of wooden clogs, paint brush handles and scaffolding poles. Unfortunately, the safe road crossing point could not be located, so the group turned round and retraced their footsteps. One of the party had spotted a regular pattern of ridges and troughs to the side of the track, with acid-loving plants such as Bell and Common Heather and Dwarf Gorse on the ridges. The Dwarf Gorse was in flower at the same time as the common Gorse, a most unusual occurrence. According to the Collins Flower Guide, the common Gorse flowers December – June, while Dwarf Gorse flowers July – September. The search for quartz pebbles continued without success, but a Sea Urchin fossil with 5-fold symmetry was an interesting find. Also seen towards the end of the walk was a flowering Blue Fleabane plant.