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Lower Greensand: Isle of Wight, UK

Blocks of Perna bed (oxidising to a brown colour) litter the beach at Atherfield point...

The marine Lower Greensand (mid Aptian to early Albian: ~115-100Ma) overlies the lagoonal Vectis Fm, and is a richly fossiliferous unit that records deepening marine conditions. It is subdivided into the Atherfield clay (lowermost), Ferruginous sands, and Car-stone / Sandrock (uppermost).

The Atherfield Clay
The erosive base of the Lower Greensand is marked by the 'Atherfield bone bed': a thin sandstone yielding infaunal bivalves and reworked vertebrate material: hybodont spines & teeth, fish debris, occasional pterosaur teeth, and more rarely, dinosaur and crocodile bones. The Atherfield bone bed is actually rather thin at Atherfield itself (pictured, top left), but gets up to a foot thick a few miles northwest at Compton Bay.

... but usually they are covered by sand. This is not so bad really, since the few blocks that are permanently above the sand level have an encrustation of barnacles, and in the summer, seaweed, which obscures the fossils. The above photo shows the after effects of a storm where the seaweed has been ripped off the Perna bed of Atherfield ledge, this tends to happen in spring, making it a good time to prospect here.

A particularly large bowling-ball sized specimen of Eucymatoceras plicatum that I found in 1997, note the double-zig-zag ribs (compared to the more common Cymatoceras).
Inset: the haphazardly shaped bivalve Perna, after which the Perna bed is named

The Atherfield bone bed is overlain by the Perna bed: a bluish-coloured calcareous sandstone named after the burrowing bivalve Perna, which is a rather large and an odd shape: consequently, it is very difficult to find a specimen that has not been damaged in some way. The bed is more resistant than surrounding rocks so it survives as blocks scattered on the beach, and as a ledge at Atherfield point itself. The bed records a shallow water softground and is richly fossiliferous: dominated by burrowing bivalves like Panopea. Any hard surface (including the shells of other creatures, both living and dead) was quickly colonised by encrusters like serpulid tubeworms, and oysters like Exogyra. The attractive golf-ball like scleractinian coral Holocystis elegans can also be found here, along with occasional nautilus (Cymatoceras radiatum and the rare Eucymatoceras plicatum, pictured), & very rare ammonites (Prodeshayesites: a precuror to the ubiquitous Deshayesites found higher in section).

Holocystis elegans, the golfball coral (image from here)

A large Thalassinoides burrow in the Perna bed, probably made by a crustacean

Crustacean bodyfossils are rare in the Perna bed. This nice specimen of Glyphea sp. was found by my dad in 2007

This hoploparid lobster was found by my dad in 2006, in the usually rather barren clays between Atherfield point, and the lower lobster bed.

A radial Thalassinoides burrow in the Atherfield clay that I collected in 2005. These are quite common in the bluish clay below the lower lobster bed. If you click the image for an enlargement, you can see the textured surface, probably due to infilling by fecal pellets of the burrower. These burrows are about the right size and morphology to have been made by Mithricites.

The small crab Mithricites vectensis (carapace up to 2cm across) from the lower lobster bed. These are especially cute when you find the claws and legs still attached. Plate from Gould (1859).

An ammonite Dehayesites sp. from the lower lobster bed. Upon death, the shell sank and settled on the soft seabed, attracting serpulid worms which encrusted it before eventual burial..

Also from the lower lobster bed this ammonite (Roloboceras sp.) was encrusted with small bivalves instead of serpulids. These specimens always remind me of this doctor fun cartoon.

A nice specimen of Meyeria magna, a small lobster common in the lower lobster bed. Sometimes these are encrusted with very small bivalves, and have been found preserved in simple burrows.

Overlying the Perna bed are a thick succession of massive blue-brown clays, mostly devoid of fossils, with some notable exceptions. First is a grey-blue clay which generally preserves only badly preserved Meyeria lobsters, but occasionally you find good Thalassinoides burrows and excellent hoploparids (see photos).

Above this lies the lower lobster bed, notable by a shift to slightly siltier sediment, less blue or brown than the underlying clays. The lower lobster bed is famous for its superbly preserved fossil crustaceans Meyeria, (very common), Glyphea sp., hoploparids, and Mithricites. This horizon often also yields encrusted Dehayesites and Roloboceras ammonites, various gastropods, bivalves, and occasional brittlestars.

Immediately above the lower lobster bed is a ~5m thick silty fine sandstone containing large phosphatic concretions called "the crackers". This is a somewhat comical name, since anyone who has ever tried to crack one would testify that they would better be named "chisel bouncers". The crackers vary in size from about 50cm in diameter, to car-sized behemoths, and are very solid, terribly heavy, and even more hard. Should you be lucky enough to find one with a "soft sandy core" (the term is relative) these are usually crammed with fine fossils: often spectacularly preserved in orange calcite.

In fact, the crackers are so-called because of the wily old boys' way of cracking them. Fossil collectors of yesteryear had a ingenious technique which would probably be illegal these days. First selecting a suitable cracker that looked as if it might have a soft core, they collected piles of driftwood from the beach, and built a mighty pyre around the rock. Then once the rock heated up, seawater was thrown onto it. The resultant sudden cooling created stress fractures through the tough exterior: hence "crackers". Needless to say, I haven't tried this technique, and I am sure the coastguard are happier for it.

It's worthy of note that the shift to a more silty-sandy facies seems not to have suited the Meyeria too well: the crackers beds yield far more Glyphea sp. preserved in black iron phosphate. The overlying upper lobster bed is similar, with more hoploparids.

Cracker core as found in a mudslip. This was full of gastropods and a fine Dehayesites ammonite

The same specimen after a bit of cleaning.

My brother looking northwest from Whale Chine through the Ferruginous sands. You can just about make out the chalk cliffs of Freshwater in the distance. People collect the semi-polished shingle here and at Atherfield for use in fishtanks.

This is the mouth of Whale Chine. Note the wonderful range of plastic bottles that wash in from the English Channel

Our old dog Alfie (now sadly deceased) used to love the beach. Here he is halfway up Whale Chine steps barking encouragement to my dad and me.

The Ferruginous sands
Our last stop is Whale Chine: famous for the large heteromorph ammonites you can find here, and a whale that was washed up at some point in the 19th century (hence the name). The series of iron-rich sandstones that overlie the upper lobster bed record even deeper water. Initially, the Gryphea beds yield a typical bivalve fauna, with occasional ammonite and nautilus fossils, but these tend to be very brittle. You can also find large basketball-sized "nests" of the terebratulid brachiopod Sellythyris sella. These contain hundreds of individuals, with growth stages from 5mm juveniles to 4cm adults. If you look closely you will also find small rhynchonellids in the same clusters.

Eventually you get to the beds from which the famous large heteromorph ammonites are found. The lobsters (hoploparids) are bigger here too, the hybodont sharks are very large indeed, and there are also occasional ichthyosaur remains (although I never found any).

I don't tend to collect above the Ferruginous sands (Carstone and Gault) as it is very rare to find anything at all, and what you do get is very fragmentary (the Gault on the mainland is much better).

My dad proudly poses with a Tropaeum sp. heteromorph ammonite he found at Whale Chine. Of course, I was the one who spent a day cleaning the thing up. If it looks heavy here, imagine how heavy it is with the same amount of matrix on it again. Now imagine carrying it a quarter of a mile along a gravelly beach, then up Whale Chine. Fun. It's no wonder we all get bad backs in this business.

Here's a couple more of the rather grand looking even-more-heteromorph ammonites (pictured on display at Dinosaur Farm Museum). These are Australiceras sp., from slightly higher in the succession. If you want to know more about these specimens Raymond Casey has published many monographs on these and other ammonites of the Lower Greensand.