Wooster Geologists

Sherborne, England — Cassidy Jester (’17), Tim Palmer and I today finished our fieldwork. Cassidy is now set for her Senior Independent Study project with plenty of specimens, observations, photographs and ideas to last the next 10 months. This morning we visited the Burton Bradstock beach exposure of the snuffboxes, meeting our great colleague Caroline Buttler (Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cardiff) and her husband Simon for lunch on the outcrop. It was great fun, and Caroline had additional discoveries for us, including the exfoliated snuffbox layer shown above with serpulid worm tubes.

We had time to look for more fossils associated with the snuffboxes. Above you see a gastropod on the left and an ammonite on the right, with snuffbox bits scattered about.

We also found many examples of burrow systems with cryptic pendent iron-rich layers, including those shown above. (I rotated the image 180° because the block we studied on the beach is upside-down.)

On the way back to our lodgings near Sherborne we stopped by the Iron Age hill fort Maiden Castle, a portion of the massive earthen ramparts of which are shown above.

Our collecting, measuring and describing is done. Most of the work for this project, of course, will be in the Wooster geology labs. We will have delightful memories of our sunny days in Dorset, and the invaluable assistance of our colleagues Bob Chandler and John Whicher. I am personally most grateful for the geological and navigational skills of Tim Palmer, our wonderful companion and astute advisor. Without him none of this could be done.

The Dorset crew in the Whicher Museum. From the left, Bob Chandler, Mark Wilson, Tim Palmer, John Whicher, and Cassidy Jester (’17).

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:

Sherborne, England — Cassidy Jester (’17) now has a Senior Independent Study project: Origin and paleoecology of ferruginous oncoids (“snuffboxes”) from the Middle Jurassic (Bajocian) of southern England and northern France. (We’re not going to France; I have specimens I collected 20 years ago there.) Pictured above is a nice collection of these snuffboxes on the Dorset coast near Burton Bradstock. More on them below. Today Tim Palmer, Cassidy and I had a great time starting our data collection.

The first thing we did this morning, though, was visit the astounding fossil collection of John Whicher, one of our new citizen scientist friends. He has a spectacular collection of exquisite fossils, most from the Inferior Oolite and all meticulously curated. His preparations are amazing, especially when you know what a fossil looks like when first collected.

Tim and Cassidy are here admiring some of the Inferior Oolite ammonites in John’s display cases. Each specimen is numbered and has full locality and stratigraphic context.

John has a workshop that would be the envy of any university, along with storage for those specimens awaiting his patient preservation. Here we see our other new friend Bob Chandler cutting a rock for us. Bob has his own equal collection. These indefatigable amateurs are making extraordinary contributions to science.

At noon we started our own work along the coast at Burton Bradstock, Dorset. We depended upon cliff falls like this one where the rocks of the Inferior Oolite at the top of the cliff crashed to the beach below.

This gorgeous block is an example of the snuffbox bed fallen into our hands on the Burton Bradstock beach. The long part of the measuring stick is one meter. We are looking at the base of the snuffbox-bearing unit, so the block is upside-down.

Cassidy is here studying that above block, with the English Channel in the background and brilliant sunlight.

This is one of the snuffboxes with a shell fragment as a nucleus. The shell has many borings that were excavated before it started accumulating the layers of iron oxides.

The snuffboxes have all sorts of details, from the compositions of the nuclei, the structure of the cortices, the fossils found encrusting them, and their overall shapes. Many have “horns” in cross-section like the two above. Note also the iron ooids (rusty red dots) between the snuffboxes. Their origin is another mystery.

We ended the day with a visit to the ruins of Cerne Abbey in Cerne Abbas, which was founded in 987. The remaining buildings are considerably later but still incorporate remnants of the old. This is now a romantic ruin on a small estate.

Tomorrow we continue to study the snuffboxes in other localities. We hope again to avoid the rains that have affected much of the country this week.

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