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Secret boulders in Saffron Walden and beyond

Ice sheets and glaciers can drag rocks and boulders long distances as they slowly flow downhill. When the glaciers melt, the rocks get left behind and can tell us about the direction of ice movement from hundreds of thousands of years ago.

Glacial Erratics

Boulders that are dragged to new areas by glaciers and ice sheets are called 'erratics' because they do not match the rock types in the place they end up.

The Gibson Boulders

The Gibson Boulders in Saffron Walden are a mound made of at least 25 glacial erratics, built by George Stacey Gibson in the gardens of his house on High Street as a place to put his summer house. The boulders almost certainly came from farmland he owned in the area. The mound is still on the corner of Margaret Way and Gibson Gardens. You can read much more about them on the Essex Field Club website:

Septarian nodules

The biggest boulder in the Museum grounds is a septarian nodule, so called because it is a lump of rock (nodule) that contains walls (septa). The septa are made from the mineral calcite, which formed in cracks in the limestone nodule as it slowly hardened and dried, shrinking from the centre outwards, buried under a Jurassic seabed.

If the nodule is exposed to air or water, the rock will erode. Limestone erodes more quickly than calcite, so you can sometimes see a honeycomb pattern of the calcite septa raised above the main surface of the rock.

The septarian nodules found around Safforn Walden were brought here by the Anglian Ice Sheet which covered most of Britain from 450-430,000 years ago.


The limestone boulder formed around 300 million years ago during the Cardboniferous Period, and probably came from Yorkshire or the Peak District where this is the main rock type.

The medieval coffins are made from Limestone from a later Period, the Jurassic, around 170 million years ago. This Jurassic limestone would have been dragged here from Lincolnshire by the Anglian Ice Sheet.


The basalt boulder dates from 60 million years ago, during the Paleogene Period, and was formed in from cooling volcanic lava in Scotland.

Local rocks


Coming soon.

Rocks from a lost time

Coming soon.

Sarsen stone

Coming soon.


Coming soon.

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