Are the dew ponds on the Wiltshire downs fed by dew and how old are they?
Dew ponds are actually fed by rainwater and they are normally situated in a slight depression so that there is a reasonable sized catchment area for rain. The amount of dew falling in a year is around half an inch whereas the rainfall will be between 30 and 40 inches a year. The water retention properties of the dew ponds lay in their making and the appreciation of the margins of the pond as a catchment area. The bottom layer of the pond was puddled chalk or clay, which was normally covered by straw, laid as it would have been on a thatched roof. This was then covered by a mixture of loose materials such as chalk rubble, sand, flints or gravel. Most existing dew ponds date from the 19th or early 20th centuries, although a few may be 18th century. The only apparent ancient one is Oxenmere on Milk Hill on the downs to the north of the Vale of Pewsey. A Saxon charter of 825 refers to this pond as marking the boundary of Alton Priors, which it still does. It is possible that a pond has bee