Ancient periglacial ponds with a beetle species surviving from the last ice age.
Zoom in to the Google map to see the points of interest.
Thompson Common is a classic site for seeing periglacial ground-ice depressions over 10,000 years old. These form a cluster of natural ponds on the Common dating back to the last Ice Age. Because the physical landscape here has been unchanged for so long it also has provided extraordinary habitat continuity for wildlife. The site is designated a Site of Special Scientific Interest (SSSI).
One of a group of truly ancient ponds.
Water violet Hottonia palustris makes a fine display in early summer. [Image courtesy The Landscape Partnership]
The ponds formed where blisters of ice developed in the subsoil in periglacial conditions. The frost mounds expanded in winter then collapsed in summer, gradually widening. It is difficult to classify these ancient landforms. They may be relict lithalsas, where ice lenses formed from water bodies in gravelly subsoil, or perhaps pingos of ‘closed system’ type, which are formed by pressure of water trapped under frozen ground; some may be palsas, formed by ice that developed in blanket peat. There are more examples nearby at Stow Bedon Common (see map).
Aerial view of an active palsa fieldin Lapland. [Image courtesy Dentren at English Wikipedia [CC BY-SA 3.0 or GFDL license] from Wikimedia Commons]
A pingo or palsa field at Nunavut, Canada. [Image courtesy William W. Shilts, Prairie Research Institute, USA]
A developing lithalsa. [Image courtesy RG West (‘Pleistocene Geology & Biology’, Longman, 1977; Fig. 5.7)]
The ponds at Thompson Common have a rich biodiversity, and are unusual for containing many rare and notable species. Some beetle species, such as this Hydroporus glabriusculus are more typically found in Scandinavia, suggesting they may be a relict population dating back to the last Ice Age. If so, the Common may have continuously provided habitat for over 10,000 years!
The tiny water beetle Hydroporus glabriusculus (about 3 mm long). [Image courtesy Dr Lech Borowiec, Wroclaw University]
Thompson Common is marked on Ordnance Survey Explorer map no.229 ‘Thetford Forest in The Brecks’ (c. TL 934 958). However it is private land, and is only accessible by designated footpath (see map). The ponds on Stow Bedon Common can be seen from the disused railway line footpath (see map). There is a handy car park just off the A1075 at TL 940 966 (see map).