Geology and Hydrology
The physical attributes of the site have defined the biological interest. The proximity to the Thames, the underlying geology and the hydrology have interacted with land use to provide a mix of wet woodland, ponds, small springs (flushes), scrub and undisturbed grassland.
Geology and Soils
The underlying geology is represented by a series of lime-rich strata of the Cretaceous period (about 100 million years ago). Gault clay is the lowest and oldest strata, forming the gently sloping base to the two hills. Here the soils are very heavy, poorly drained and seasonally waterlogged. The steep sides to the hills are composed of the much harder Upper Greensand, rather like a very dirty chalk in appearance. The uppermost parts of the hills are composed of LowerChalk. Both these latter rocks are porous and their soils tend to be well drained with a tendency to summer drought.
Drift deposits
Over parts of the reserve more recent 'drift' deposits overlie the 'solid' geology described above. Both hill tops are covered by a thin layer of ancient river gravels, dating from a period before the present day Thames Valley had been formed. Other river gravels of much more recent origin cover the north-eastern corner of the woodland. These have been deposited by the Thames in the last few thousand years. Modern floodwaters have deposited a narrow strip of fertile alluvium adjacent to the river.
Hydrology
A spring line occurs at the boundary between free-draining Upper Greensand and the underlying impervious Gault clay. This gives rise to two main springs and a number of smaller flushed areas within the woodland. One spring feeds water to the two large woodland ponds, the other flows along the eastern boundary of the woodland. There are also a number of other ponds and temporary pools (scrapes) scattered across the reserve.
Tufa springs
Having passed through the lime-rich Chalk and Greensand the spring water issuing in the east of the wood holds a considerable amount of dissolved lime. On reaching the ground surface much of this is deposited, rather like a crunchy 'limescale', over twigs, stones or even leaves that have fallen into the water. A less well developed series of tufa depositing flushes also arises from below the river gravel terrace along the banks of the Thames. Tufa depositing flushes are a rare feature in Britain and those on the reserve support a number of rare Soldier Flies.
