This week we have a guest Blogger. Star is on work experience for the week and has an interest in Geology and as we have an abundance of archives & books relating to this subject seemed an interesting subject to write about.
I have been looking at ‘green box 30 (WHIT/E1/E1.8/1)’ that is based on Geology, specifically fossils. There was a lot of information in the box relating to marine fossils like crocodiles, corals and other sea creatures, but I decided to focus on the booklets and guides relating to ammonite fossils.
It is very common for Whitby and the surrounding areas to have exposures of rocks that are rich in ammonites and other marine fossils, though there is a lot of variation between specific series of strata. The rock series that I have focused on is called the Corallian Beds, this includes the Upper Calcareous Grit, the Osmingtan Oolite Series, the Middle Calcareous Grit, the Hambleton Oolite Series and the Lower Calcareous Grit.
The Upper Calcareous Grit has very few exposures, and can mainly be observed though pebbles, loosened and transported by erosion, that are rich in ammonite and coral fossils, so these strata can be very hard to analyse. However, there are two very small exposures at Pickering Quarries, one was only 2ft wide while the other was 6ft wide, where data can be gathered from. A booklet, named A stratigraphical guide to the fossil localities of the Scarborough district by P. C. Sylvester-Bradley, composed of notes and observations from many geologists, included information about the fossils found at the two exposures in the Upper Calcareous Grit. That booklet stated that the 6ft wide exposure was abundant in fossils of large ammonite species such as Didecipiens. This 6ft bed was composed of very fine-grained argillaceous marl called Throstler Clay. The 2ft wide exposure located at Pickering Quarries, east side of Newtundale was just as rich in ammonite fossils at the 6ft exposure had been. However, the 2ft exposure is composed of finer sandstone and shale, which provides better conditions to form fossils in, due to how much more compact finer grained sediment can be, which makes it unlikely for the shells to be broken or lost once they are buried.
Below the Upper Calcareous Grit is the Osmingtan Oolite series. This is composed of a very coarse-grained limestone, which forms when debris, like a small grain of sand, rolls around in a warm, shallow, high-energy sea and gets coated in layers of silica. Those grains are then buried and a, usually calcite, cement seeps through the pores, gaps, left open by the coarse grains and holds them all together. This means that oolitic limestone is very bad at preserving fossils. The grains are too course to hold enough detail to show a clear mould or trace fossil, and the large pores means a cast fossil is likely to be damaged or broken down. This is proven as the booklet stated that there were no ammonite fossils in the limestone section of the series. However, above that oolitic bed, there was a bed created by deposited sediment in a coral reef. This formed an impure bioclastic (fossil rich) limestone, and a dark, fine grained mudstone where fossils could be more easily preserved, though what fossils did form there are likely to be fragmental as the high energy waves and current would have broken up the fossils. In the limestone in the Osmingtan series, there was a lot of ammonite and coral fossils. There were some ‘giant’ ammonites, such as C. cardioceras, preserved in the strata 6ft below the deposits made by the coral reef. There was also a huge number of corals preserved in that limestone, such as the colonial coral Thamnasteria and the solitary coral Thecosmilia. In total, the Osmingtan series is a 48ft exposure, composed of mudstone and limestone, in Pickering Quarry.
The next bed was the Middle Calcareous Grit, which is made of two main types of rocks, gritstone at an exposure in Filey, and impure limestone at an exposure at Forge Valley, west to Pickering. However, there are also some smaller exposures to the north and south of Carr Naze and Brigg. Breaking up those strata was very thin, calcite rich beds of shelly creatures. Those beds contain a wide ranger of species of coral, lamellibranchiate, brachiopods and gastropods, but very few ammonites due to the small size of the beds. The most noticeable species that could be identified in those beds was Trigonia literata, a species of lamellibranchiate. Not much is stated in the booklet about this set of strata due to the lack of exposures with noticeable or identifiable fossils. This may be because what few exposures that are visible are too coarsely grained to have been able to preserve fossils in any amount of detail that would make them easily recognisable.
After the Middle Calcareous Grit is the Hambleton Oolite series. This is a huge exposure, with a total of 101ft of visible strata located at Wydale Quarry. This is very well exposed at the Hackness district and is made of 30-50ft of oolitic limestone, 11ft of a reef deposit and 40ft of Greystone. Just like in the Osmingtan series, there are no fossils in the oolite section of the series due to the coarse-grained nature of the oolites that make up the limestone. The reef deposit below this is a lot more abundant in fossils, especially coral and ammonites. There is a wide range of ammonite species found in the reef deposit and the very top of the Greystone beds (also stated as Passage Beds in the booklet), such as C. cardioceras and Peltocerecas, though these species are not found in the greystone below the reef deposit. The only species of ammonite found deep in the Greystone was called Cardioceras Costicardia, and even then, very few specimens were found, making it clear that the environment the Greystone formed in was either lacking in life, so there was no animals that could die and be fossilised, or the sediment that formed the Greystone was so unsuitable for preserving fossils that it wasn’t able to, other than the few fossils that did manage to form.
The last bed in the Corallian Beds is the Lower Calcareous Grit. This was a very thick exposure at High Red Cliff, Cayton Bay. Despite its size, the exposure was very hard to access without ropes due to the extreme height and sheerness of the cliff, meaning there is very few opportunities to observe and take samples from this exposure. This means that the cliff face hasn’t been explored as well as it should have, meaning a lot of fossils could go undocumented. Fossils are very rare at the uppermost top of the High Red Cliff exposure, most likely because what fossils were there were so exposed to weathering and erosion (mainly by rain) that they were dislodges and fell out of the cliff into the sea, or was destroyed and therefore unrecognisable as fossils. Due to the fossils falling out of the cliff, there is a lot of fossils that have been found at the beach under the cliff. Unfortunately, as they are not embedded in a rock when found, their exact locality is not known. However, there was a lot of fossils found to the north side of Carr Naze, specifically the ammonite species C. cardioceras. Some small quarries scattered around the area expose some of the Lower Calcareous Grit, which helped to get a larger understanding of what the strata are made of. There is a thick, 25ft wide bed made of brown sandstone, a 49ft bed made of gritstone then a 14ft bed of oolitic limestone called Ball Bed’. There are some specimens of C. cardioceras ammonites found in the Lower Calcareous Grit strata, however more species could be found and identified if the huge exposure on High Red Cliff was able to be explored more.
With so many species of ammonites, it is easy to misidentify some, which, as recorded in another booklet, named The Ammonites called A. serpentinus by C. Thompson, I found in ‘green box 30′, led to three species having six different names.
This happened because some geologists were looking at some samples of ammonite fossils, for example, Young and Bird looking at an ammonite fossil and called it A. mulgravius. However, another geologist had looked at another sample of the same species, and identified it as a different species, giving it the name A. serpentinus, making it so that the one species actually had two names. This happened again with another geologist, eventually leading to the species having six different names, Caecilia, Capellinus, Strangwaysii, Falcifer, Mulgravius and Serpentinus. This confusion happened due to how the shells of the ammonites changed as it matured. All the identifying features on the shells changed from having overlapping, foliated suture lines when the animal was young, to having clear, defined suture lines as it grew into adulthood. Another appearance trait that changed as the animal matured was the size and shape of the umbilicus (the point where the shell started to grow from) and the size and shape of the ribs. When the animal was young, it had a wide, deep umbilicus as it had a smaller shell, due to it not having had the chance to grow very big yet, and therefore had a very big umbilicus, but as the animal grew the umbilicus got smaller and shallower as the shell had the time to grow bigger, therefore making the umbilicus seem smaller compares to the rest of the shell. The ribs also changed, as when the animal was young, the shell was smaller so it only needed thin, round ribs to protect it, but as it grew, the shell got bigger and so needed more protection and structure, leading to broader and flatter ribs. The samples the geologists looked at also varied in size, with one sample having a radius of 71mm, and another 58.5mm.
This variation is what led to the confusion when trying to identify each species. However, one geologist noticed this conflict between the names and did some research into the ammonites. After gathering his results, he discussed them with some other geologists and they eventually came to a conclusion about what had happened and had created a solution.
They discovered that the variation was due to different ages and maturity. They came to the conclusion that instead of six, there was three species of ammonite between their samples. They named them Falicifer, which was the younger form of the species, Mulgravius, which was the older form, and Serpentinas, which was its own distinct species.
This is only a very small snippet of all the archived information available, there is many more resources about Geology on shelves E 1.7 and E 1.8, in boxes 25- 31. More archived documents can be found on shelves C 2.3, 2.4 and 2.5 and books can be found in cabinets 30-33 in the library. Also, in our Frank Elgee archive are documents and scrapbooks featuring arachnological discoveries, which are fascinating to look through.
If you are interested in accessing more information about Geology, contact [email protected]
By Star Benson, sixth form student at Prior Pursglove College.
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