19 Apr 2016

Finally, a spring day with some of nice fossils. (Updated)

We did a quick safari to the beach to find the almost up tide up (this pic is 2 hours before high water), so little natural history to be done. So we had to content ourselves with a very quick rampage amongst the rocks to see what fossils we could find.

As it happened, not a bad haul:
Right to left:
1. A 2cm diameter ammonite, juvenile or adolescent Arnioceras semicostatum (the specimens in the field guides, which I assume to be adult are 4 cm across). Andy from Andy's Fossils contacted me in comments to suggest that this is actually Hilderocas bifrons (likely to be found in Runswick) rather than Arnioceras which is not) for which many thanks. He's probably right.

2. At the bottom is a Gryphaea, a fossil oyster which died c. 185 million years ago. I and my field assistants find lots of these but this one is in a shingle-smoothed block of dead, fossilized, smashed together molluscs. Probably the result of a mudslide sweeping these creatures from their muddy home into an anoxic (oxygen-deprived) basin (I've been swotting up on the Jurassic conditions hereabouts, thank you Mr. - soon to be Dr. - I hope - Johnson) which killed them en-masse.

3. Coral or a sponge. Jurassic, much smoothed so hard to identify.

4. Star of the show. I think you're looking at a fragment of mid-Jurassic plant, probably a cycad. We found this on the beach at the foot of some cliffs where we know cycads have been found before. The place is a Site of Special Scientific Interest so off limits to fossil collecting but not off limits to 'measuring a section*', which is what my smart wife (a natural science graduate from the University of Toronto) suggested we do. You see, the rock matrix a round the fossils isn't what you'd expect on our beach. Up the cliff we went, looking at the changing geology as we did. After much scrambling, hanging onto blades of glass, scrabbling dirt away from the underlying rock we hit rock similar to that encasing our beach fossil. A glance at our geology field guides at home and it was bang in the geological horizon for the Jurassic cycads.

* Measuring a section means you find a specimen at the foot of a cliff (or, if you're unlucky, mountain) and traipse up the thing looking at every stratum to see where the thing came from.

1 comment:

AndyS said...

You´re off by a few million years there with Arnioceras...
It is in fact unlikely, but probably not impossible (someone might loose one from their rucksack, or glacial drift) to get an Arnioceras at Runswick Bay.
This is most likely a small Hildoceras.
All the best,