The Jurassic flora of North Yorkshire
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A bit of geology
Following the impressive cliff path along the North Yorkshire coast from Middlesborough via Whitby to Bridlington, the layers are getting younger and younger, with some local exceptions. Roughly spoken the layers go through the time scale from the Lower Jurassic to the Upper Cretaceous. In most cases the depositions are marine, but there are also a couple of terrestrial sediments, or better deltaic sediments. And that's where the plant fossils occur.
These strata date from the Middle Jurassic and they have been formed by rivers flowing in eastern direction. These brought in so much silt and sand, that deltas came into existence. From time to time creeks got blocked and many plant remains got embedded in the mud. Because of the fact that the complete area was lowering slowly, this proces could continue during a long time, but from time to time the delta was inundated and got covered with a layer of sand. Thus it could happen that some of the blocked creeks petrified totally, plant remains included. In the open river arms and in the estuaries also plants were deposited, but these were so heavily damaged that only chopped plants have remained.
The three depositions with plant fossils are (from old to younger):
the Saltwick Formation, the Gristhorpe Member and the Scalby Formation. The
Gristhorpe Member is part of the Cloughton Formation. See scheme. The layers
alternate with marine depositions. And just fossils from these marine layers
are used for dating. This is not possible with the fossils from the delta
We (my wife and I) have collected mainly at four places: Whitby (with the Whitby Plant Bed), Hayburn Wyke (also with the Whitby Plant Bed but with different content and age), Scalby (Scalby Plant Bed) and Cayton Bay (Gristhorpe Plant Bed). We have also found nice fossils on the beach between these places, e.g. near Burniston, but in the first mentioned places the fossils are still in the layer and not rolled.
For the exact location of the sites I refer again to Van Konijnburg-van
Cittert & Morgans (1999) in which booklet a precise description is given
how to find the places. Recently the sites have been placed under protection:
they are now Sites of Special Scientific Interest (SSSI). Permission of the
owner is required, but as far as I understand modest collecting of fossils
is still possible. In the booklet another place has been mentioned, i.e.
Hasty Bank, inland. For entering of this site permission of the Head Forester
is required. We didn't visit this place.
rock with the fossils is often hard clay, sometimes sandstone and by exception
ironstone. It is necessary (except at Hasty Bank, which is situated inland)
to collect at low tide. Thus the first thing to do is to buy a tides table.
1. Mind the tides. The upcoming water could surprise and enclose you.
The fossil seed plants from Yorkshire have preserved their cuticle. This is special because in many other sites this is not the case. This waxy protection layer includes a print of the cell structure of the epidermis and is very important for the scientists.
The black layer on the fossils consists of coal and is enveloped by
the cuticle. With a special treatment including calcium chlorate and
nitric acid, called maceration, the coal can be dissolved. The cuticle remains
and can be used for making a microscopic slide. With these slides often species
can be identified with certainty. Some plants are resembling each other very
much, while the cuticles can be very different. On the left the cuticle of
the cycas Nilssonia compta.
In the impressive standard work of Harris the flora of North Yorkshire
is extensively described in five books. If possible the cuticles of the plants
are also depicted. The book parts came out respectively in 1961, 1964, 1969,
1974 and 1979.
In the Jurassic the modern flowering plants were not yet evolved: they
took over in the Early Cretaceous. During the Jurassic the gymnosperms were
dominant, but the ferns were also abundant. The following groups can be
coastal area of North Yorkshire is one the most important sites for plants
from the Jurassic. The standard work of Harris proves that very convincing.
Isn't it exceptional that plants which were growing 150 million years ago,
show their cell structure and can be studied in detail? But a lot of effort
is needed to extract the fossils from the rock and good fortune is needed
to find rather complete specimens.
Near Burniston large footprints of dinosaurs occur. Excursions are organized to the (protected) place where they are found. Ask the Tourist Information about it. We only found one small footprint and we were childlike happy with it.