A fragment of a fossil clam shell in the Fan Bay Deep Shelter tunnels at the National Trust’s White Cliffs of Dover site.
The following photo shows fragments of the clam Cladoceramus undulatoplicatus in the roof of the tunnel above the sheet flint in Photo 2 of “How were flints in chalk formed?”
Image reproduced by kind permission of the National Trust: https//www.nationaltrust.org.uk
It is rare to find such a well-preserved example of a fragment of this clam shell, which is also referred to as a Snowshoe Clam (Everhart, 2017, p. 44).
The name Snowshoe Clam is appropriate. These clams were large, up to 1 metre across, and they lived on the seabed which was a soft chalky ooze. To prevent sinking into the ooze, they had thin shells compared to many clams, which minimised their weight. But they also adopted similar features to an old-fashioned snowshoe which is used to aid walking on soft snow without sinking into it. The snowshoe and the clam both employed an undulating underside folded like a fan (plicated). The undulations increased the surface area applied to the soft chalk ooze. This reduced the applied pressure and therefore the amount of settlement into the soft material.
The undulations are also reminiscent corrugated metal sheet (“wriggly tin”) which is formed to provide rigidity to an otherwise flimsy sheet material – also applicable to a large thin-shelled clam.
Normally these fossils are seen in section on vertical exposures, where they show as unremarkable thin wavy lines. If they are seen in plan, it is usually on a bedding plane exposed on a fallen chalk block at the base of a sea cliff. These exposures are short lived as they are rapidly broken up by ensuing high tides. This example is protected from the elements and has been exposed in its current state of preservation since 1941 when the tunnels were excavated.
This fossil species is found in chalk spanning a relatively short period of about 85.7 to 83.6 million years (“probably no more than a couple hundred thousand years” has also been suggested, (Everhart, 2017)). The thin layer of rocks that this represents makes it a useful marker fossil. It is used internationally to define the Coniacian/Santonian Stage boundary in the Chalk.
Andrew Coleman
Rev. 28/06/2025
Reference:
Everhart, M. J. (2017). Oceans of Kansas: A Natural History of the Western Interior Sea (2nd ed.). Indiana University Press.
Geomusings 