'Breckland stripes' and the story of a vanished mighty river.
Zoom in to the Google Map to find the points of interest.
Knettishall Heath is a tract of Brecks heathland on the edge of the Little Ouse valley. It is developed on chalky, sandy and peaty soils, which have influenced the variety of plant-life. It is a good place to see distinctive ‘Breckland stripes’ related to periglacial processes active in the subsoil during the last Ice Age. Two small gravel pits have geological stories to tell stretching back over a half a million years.
Most of the Heath is developed on sandy and gravelly sediments of glacial origin, and support acidic heathland with heather and bracken. These sediments can be seen in two gravel pits in the middle part of the Heath, not far from Hut Hill. Take a look at the easterly one (site A on map): it has an abundance of tough, quartz-rich pebbles of brown and purplish coloured quartzite, and there are also vein quartz pebbles. These distinctive pebbles once rolled along the bed of the mighty Bytham River that flowed through the Brecks a half million years ago, with its headwaters in the West Midlands. It was destroyed when the Anglian ice sheet arrived, about 450,000 years ago.
The Eastern Gravel Pit.
Brown quartzites from the Bytham River.
The western gravel pit (site B on map) lies at a slightly lower level, and has far fewer quartzose pebbles; it is rich in flint. The sediments here are likely to be outwash sands and gravels from the Happisburgh glaciation. You can see sand beds here, likely to have been laid down by a meltwater river, and a clay-rich till deposited beneath the ice sheet. The mixed gravelly material lying on top of them is probably the result of sludging of superficial deposits down-slope during the last cold period. The outcrop of sand is peppered with small holes. These were probably made by the ground-nesting solitary bee species Andrena, which favours warm, dry environments.
An outcrop of current-bedded sands. [Photo courtesy Gary Battell]
Chalk bedrock comes close to the surface at the western end of the Heath; it is covered by a superficial layer of sand. This is the place to see ‘Breckland stripes’, where frost action in the last Ice Age churned the subsoil into alternating chalky and sandy patterned ground. Contrasting calcareous and acidic plant communities are growing here side by side on gently sloping ground south of the road. They are best seen in May and June (site C on map). A trench has recently been dug across some stripes, revealing the geological patterning in the subsoil. This can be seen at the western end of the Heath. Please do not disturb ground-nesting birds on surrounding land; keep dogs on a lead.
Studying the chalk and sand interface.
The Little Ouse has a wide floodplain here, underlain by dark, peaty soils, the remains of former wet woodland and fen. It is flanked on either side by flat stretches of sandy land, e.g. west of the main car park. This may be the remains of a river terrace, a former floodplain deposited during the last Ice Age when the river flowed at a higher level. Another interpretation sees it as the remnants of a former lake bed, formed when meltwaters backed up in the Little Ouse valley due to an ice sheet occupying the Fens.
Flat ground in the middle of the Heath, as seen in 1909. [Photo courtesy Holt-Wilson family archive]
The site is on public access land - see Ordnance Survey Explorer map no.230 ‘Diss & Harleston’ (c.TL 954 805). It is signposted from the A1066, and managed by the Suffolk Wildlife Trust with a ranger service, waymarked trails, car parks and restroom facilities.