Geology Site Account

Site name: Latton Ice Wedge Polygons

Grid reference: TL 472078

Brief description of site:

Fields west of the A414, south of Potter Street, reveal a remarkable pattern of cropmarks during hot, dry summers.

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Summary of geological interest

These fields are situated on the London Clay with a thick covering of boulder clay (Anglian till) which was laid down by the Anglian ice sheet about 450,000 years ago. These features, however, date from around 50,000 years ago, during the coldest part of the Devensian or last glacial stage.

During this time the ground was frozen to a considerable depth (permafrost) and the shrinkage of the ground surface created a network of deep cracks which filled with ice during the winter. During each brief summer the ice melted causing sand and gravel to collapse into the cracks and during the following winter the cracks would reform, water would again turn to ice and therefore enlarging the cracks. This is a phenomenon common in the High Arctic today. During the glacial periods the process continued for thousands of years.

Today, the fossil ice wedges consist of more unconsolidated sediment than the surrounding compacted clay and gravel, allowing crops to put down deeper roots. The cracks therefore reveal their presence as crop marks, often creating a colourful network of huge polygons that can only be seen from the air.

Ice wedge polygons revealed in cropmarks in fields west of the A414, south of Harlow. The photograph was taken during the hot summer of 1976. Photo: Cambridge University Collection. Copyright reserved.