Cock Marsh is a beautiful area of unique wildlife habitat of flat water meadows and steep chalk hillsides near Cookham village, alongside the River Thames. It covers 19 ha (46 acres) and has been common land used for grazing since 1272. It was bought by local villagers and given to the National Trust in 1934. Cock Marsh is a designated a Site of Special Scientific Interest (SSSI). It is home to many rare plants, notably Cyperus fuscus (Brown Galingale), which is found in only a handful of sites in Great Britain.

 

The whole area is grazed by cattle and horses and locally by rabbits. The outstanding feature of the site is the marshy grassland at the foot of the slope containing a series of four silted pools and supporting a rich plant community including water violet Hottonia palustris, marsh arrowgrass Triglochin palustris, marsh stitchwort Stellaria palustris, lesser persicaria Polygonum minus, marsh pennywort Hydrocotyle vulgaris, water hemlock Oenanthe aquatica and many other species including one specially protected under Schedule 8 of the Wildlife and Countryside Act 1981. The ponds are surrounded by emergent vegetation and each is different in character, the plant communities ranging from those characteristic of bare mud to willow Salix scrub. The whole low-lying area is subject to periodic flooding and occasional drying out. This fluctuation in water levels is an important factor in maintaining the botanical richness. The site is locally important for breeding and wintering birds.

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It has four burial mounds that are scheduled monuments which are thought to have been constructed in the early Bronze Age with later Anglo-Saxon use.