The area of Breckland is a unique landscape of heaths, conifer plantations and farmland on part of the chalk plateau in south-west Norfolk and northwest Suffolk. The Devil's Punchbowl is one the distinct features of this unique landscape.
Over the millennia, Breckland has undergone transformations from both tropical seas and arctic tundra. The bedrock is Cretaceous Middle and Upper Chalk, deposited as a pure limestone in tropical seas between 100 and 65 million years ago. This is covered by thin, sandy, glacial drift left behind from when the Anglian ice sheet covered the area around 400,000 years ago. During the last glacial period (100,000 – 12,000 years ago) the tundra-like conditions left layers of free-draining wind-blown sediments covering the chalk, forming an unconsolidated sandy sediment (or glacial till) with areas of boulder clay between chalk and sand. The soil is a mix of chalk, sand, silt, clay and flint which makes it free-draining and allowing the underlying chalk to be easily eroded by acidic groundwater. Several sites of special scientific interest (SSSI), in the Breckland natural area such as Thetford Heath and Grimes Graves reflect the importance of the Ice Age and its part in the evolution of this unique landscape. The whole area is well worth a visit and there are numerous travel and visitor guides available online, as well as many geological articles and studies.
Breckland is also renowned for its fluctuating meres or lakes, with twelve semi-permanent meres recorded, the most famous being Ringmere, Langmere and Fowlmere, the latter situated just across the road from our target feature “The Devil’s Punchbowl".
The Devil Punchbowl, East Walton Common, near Croxton, is a classic example of another geological feature of this type of landscape where the active process of the effect of acidic water on the chalk bedrock has resulted in the formation of so called karstic depressions or swallow holes, which may be evident on the surface or hidden beneath the glacial deposits. It consists of a cone-shaped hollow, a single, very deep, subsidence doline (depression), formed by the collapse of Pleistocene glacial sands and boulder clays into a space created by the solution of the underlying chalk. A survey of the surrounding heathland area in 1941 estimated over 50 oval or circular “swallow holes” as a result of either solution of the chalk or possible collapse of underground cavities.
The Devil's Punchbowl is significant as the particular conditions allow it to be one of the few meres that supports certain flora and fauna. The information board gives some interesting details about this.
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Tasks
1. Estimate the diameter of the Punchbowl.
2. What has caused the concentric circles visible in the vegetation within the Punchbowl itself? Give an explanation.
3. There are a number of objects at regular intervals running outwards from the centre of the punchbowl. Explain what they are for.