Team Dorset makes a cryptic discovery

Sherborne, England — It was a good day for Team Dorset. Cassidy Jester (’17) is shown above in Coombe Quarry near Mapperton, Dorset. She is standing on an erosion surface between the Comptocostosum Bed (Aalenian) below and Horn Park Ironshot (Bajocian) above. These are beds 2d and 3a in the local stratigraphic system, and ammonite zones Scissum and Discites. There is a considerable disconformity here, meaning a significant hiatus of unrecorded time, several ammonite zones worth. The snuffboxes we’re interested in are found jut above this boundary.

Tim Palmer picked up the above rock as we started our measurements and descriptions. He deduced right away that he was looking at a cross-section of a burrow now filled with light brown sediment. The darker layers above are ferruginous (iron-rich), serpulid-bearing laminae like those that make up the snuffbox cortices, and they are hanging pendently from the roof of this burrow into the original cavity beneath. At one time this burrow was an open tunnel with cemented walls and the iron-rich layers grew from the ceiling like stalactites. Tim demonstrated with this single specimen that the iron-rich layers grew in dark, cryptic spaces, strongly supporting the hypothesis of Palmer and Wilson (1990) that the equivalent snuffbox layers accumulated on the undersides in gloomy darkness

Cassidy and I then recognized that the iron-rich “stromatolites” we had seen on our earlier visit to the quarry were actually these iron-rich layers filling Thalassinoides burrow systems that are truncated by the erosion surface. In the above image you are looking down on the erosion surface at a branching burrow filled with iron-rich layers. These are not stromatolites but cryptic burrow fills.

Later in the afternoon we returned to the Sherborne Stone quarry yard and looked at Thalassinoides burrow systems in the Sherborne Building Stone cut by giant saws. We see here a view parallel to bedding showing a box work of tunnels filled with a darker sediment. This matches the pattern seen in the Coombe Quarry erosion surface.

This is a cross-section of the same kind of Thalassinoides burrow in the Sherborne Building Stone. We see the vertical connections to the surface and the lateral tubes. These burrows formed the cryptic spaces for iron-rich layer deposition as seen at Coombe Quarry. Or at least that is our hypothesis! Tomorrow we will test it by examining the burrow systems associated with the snuffboxes at Burton Bradstock.

As usual, we ended our day with more historical architecture and stonework, this time at nearby Sherborne Castle, a 16th century Tudor mansion sitting on magnificent estate grounds. Much of our work is on land owned by this estate.

The format below is a bit messy, but here is a download of our GPS data for the localities on this expedition: