Yorkshire has some of the finest fossils in the United Kingdom, from the Carboniferous plant fossils of the Aire Valley, to the fossil corals of Littondale, and the brachiopods of upper Nidderdale, but what if you are not able to get to these places? Well, you can see some geology and fossils in the urban centre that is Leeds, and in a place that is out of the wind, rain and sun that we all encounter in the great city.
Please be aware that this earthcache is only accessible during the opening times of the Leeds Central Library. Leeds Central Library Calverley Street, Leeds LS1 3AA.
Opening times :
Mon – Wed 09:00 - 19:00.
Thu 09:00 - 18:00.
Fri 09:00 - 17:00.
Sat 10:00 - 17:00.
Sun 11:00 - 15:00.
The library has many fine features, but what we have come to look at is a column on the second floor on the library, at the top of the stairway.
The column is formed of polished Devonian limestone, from a geological period 416 to 354 million years ago. Limestone is a sedimentary rock, and is composed of the accumulation of the shells of dead sea creatures over time. Though this is a special type of limestone, as unlike the great scar limestone of the Craven Dales, it has been subjected to other processes as well as sedimentation.
Devonian limestone, was first formed in shallow marine basins, which had lagoons and coral reefs, and as the sediment built up, so the limestone was formed, but then tectonic movements of the planets crust, caused the limestone to be lifted up, where it was cracked, split, fissured and weathered, then further plate tectonics caused it to be covered with layers of mud and sand. This was an ongoing process, and the tectonic movements caused the limestone to be subjected to the pressures of heat and compression, so that partial crystallisation took place, but never at a deep enough level to change the limestone to marble by a process of metamorphism. The process of transformation of the calcium carbonate in the limestone is known as diagenesis, and when marble is formed the crystallisation of the rock tends to destroy any fossils, but since this has not happened at a deep level, fossils are preserved in Devonian limestone, which is also known as Devonian marble, but in reality is still a type of limestone. In some places the limestone as also soaked by underground water channels, known as aquifers, with iron rich water some of which was oxygen rich (oxic) and at other times lacking in oxygen (anoxic), this caused the limestone to become reddened in a process known as phreatic haematisation soaked the limestones in a mix of oxic and anoxic water to redden and alter them, though at times not totally, so the limestone can show blocky areas of reddened haematite, and streaks of white calcite.
Since the limestone has not been transformed into marble, it is still possible to see fossils and here on this column are examples of coral fossils. Though not all corals are the same. There are different types.
Types of Coral.
Rugosa. These were a type of coral which is now extinct, and was also known as Tetracorallia. Rugosa could either be solitary, with one polyp, or living in colonies. Each individual coral. was formed of a coralite, in which the coral polyp lived, and could retract into. These appear as small circular marks, with inward facing lines.
Lithostrotion vorticale. Is a form of Rugosa, but appears like a honeycomb.
Rughelse Coral. These appear as branches extending outwards.
Siphonodendron Coral. These appear as long noodle like fossils.
This being an earth cache, in order to log your find, you need to answer some questions.This being an EarthCache, in order to log it, I ask that you complete the below tasks. Please send the answers to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. Looking at the column, what colours can you see?
2. Looking at the column, do you think that phreatic haematisation has taken place, if so what is the rationale for your answer?
3. Looking at the fossil coral, what type or types are here?