{"id":4632,"date":"2020-01-19T09:44:12","date_gmt":"2020-01-19T09:44:12","guid":{"rendered":"http:\/\/rdnhs.org.uk\/blog\/?p=4632"},"modified":"2020-01-19T09:44:13","modified_gmt":"2020-01-19T09:44:13","slug":"geology-at-cookham-dean-11-january-2020","status":"publish","type":"post","link":"https:\/\/rdnhs.org.uk\/blog\/geology-at-cookham-dean-11-january-2020\/","title":{"rendered":"Geology at Cookham Dean – 11 January 2020"},"content":{"rendered":"\n

Julia Cooper stepped into the\nbreach on Saturday 11 January 2020, when the original leader of the geology\nwalk, Lesley Dunlop, was called away on urgent family business. Armed with\nLesley\u2019s map and notes and the Berkshire Geoconservation Group\u2019s \u2018Geological\nSites to Visit in Berkshire\u2019 booklet, Julia led 14 members on a walk around\nWinter Hill and Cookham Dean on a mild grey morning. While the group gathered,\na mixed flock of Redwings and Fieldfares landed briefly in the trees below,\nbefore flying on. Looking north across the valley of the Thames from the Winter\nHill viewpoint, a series of gravel terraces could be seen on the far side. The\noldest and highest terrace, the Winter Hill gravel, was laid down about 500,000\nyears ago. Lower down, the Taplow and Shepperton terraces could be seen, with\nthe most recent deposits laid down between 20,000 and 100,000 years ago. The\ngravel terraces were laid down by braided river systems in the warmer\ninterludes between ice ages. During the colder phases, sea level fell and the\nriver cut downwards through the Chalk bedrock. Changes in gradient of the A404\ndual carriageway on the far side of the valley mark out the succession of\nterraces. The next section of the walk followed a footpath to the Woodland Trust\u2019s\nQuarry Wood. Hazel catkins were fully out and a Song Thrush was singing. Quarry\nWood has deposits of Gerrard\u2019s Cross gravel, laid down more than 500,000 years\nago. The oldest gravel deposits include pebbles of Quartz and Quartzite,\noriginating from a time when the Thames catchment area included parts of the\nMidlands. These were easily found in places where the gravel was exposed, such\nas round the roots of fallen trees. Far below, Marlow church could be seen,\nbuilt on a terrace just above the present floodplain. Other interesting\nsightings in the wood included a branch of rotting wood which was stained\nblue-green by the Green Elfcup fungus and an unusual growth of the Turkeytail\nfungus on a fallen twig. While the twig was still in the tree, the lower surface\nof the Turkeytail was pointing downwards. When the twig fell at an angle, the\nTurkeytail responded by growing a rosette of smaller fungi, all growing with\nthe lower surface pointing in the new downwards direction. The walk then continued\ndown a narrow lane to Cookham Dean Chalk Pit. The Seaford Nodular Chalk was laid\ndown about 85 million years ago in the Cretaceous period. The Chalk was made\nfrom the bodies of tiny coccolithophores deposited in warm tropical seas and it\nhas been estimated that it took about 1000 years to deposit a thickness of\n10cm. The quarry face has narrow bands of flints at intervals of about 0.5 \u2013 1\nmetre. These represent changes in seawater chemistry when more siliceous\norganisms such as sponges were deposited. The quarry also has small patches of\npink material, known as Phosphatic Chalk. The next part of the walk was through\nthe village, then along a footpath which climbed from the Chalk up a slope which\nwas capped with Winter Hill gravel. On the final stretch back to the Winter\nHill viewpoint, Stinking Hellebore flowers were noted in the woodland below the\nroad.<\/p>\n\n\n\n