The lower rock bed layers were again limestone, the rocks were harder with small, fine crystalline grains. The beds were constructed of big boulders of limestone. Once again towards one side of the quarry was a dip which was approximately 30 – 40º. On the ground there was vegetation and some trees.
Below is a field sketch of the second location:-
The dip and strike at this location was calculated to be 040/30º/North West.
Our third and final location was Tedbury camp quarry. This was different to the first two locations. At this position, at one point we were stood on a wave cut platform looking down on beds that have dipped and flattened horizontally. The rocks here were found to be grey in colour and quite hard. At this location there were a number of the same circular “polo” shaped fossils present that were present in the first location. Also visible were the bands of chert, and the inferior oolite. There was also vegetation and trees present at this location. The dip and strike was calculated and recorded to be 61/45º/North – North West at this location.
At each location it was possible to perform a simple test to determine whether the lower rocks were limestone. Limestone should react when an acid is poured onto it by producing carbon dioxide and effervescence. At each location a small volume of acid was poured onto the rocks and these effects were seen.
By visiting and investigating these locations it is possible to compose a possible theory of how, where and when it was formed. The lower carboniferous limestone was formed by firstly the deposition of the sediment. This sediment was then subject to lithification through compaction – the increased pressure and temperature compressed the sediment and eventually this fine grained sedimentary rock was formed. The next process was the tectonic activity, which was responsible folding, uplifting and tilting of the rocks. The top of these rocks were then eroded. This flat surface became the base for the newer rocks to be deposited. Below is a diagram of this process:-
The lower beds of rocks are carboniferous limestone. There are many fossils present, which suggests that these rocks were formed in a tropical, warm environment. The sediments would have been deposited underwater on a carboniferous limestone seabed, and there could have been tropical coral reefs present. In order to date the rocks fossil content can also be analysed. The principle of fossil succession can be used to do this. The “polo” shaped fossils found here are crinoids. These fossils are found in more than one location in the area. These fossils suggest that a possible position for the formation of the rocks could be just south of the equator, and that would correspond to formation at approximately 330 – 340ma. This process of aging is used to differentiate between the different types of rocks present in this unconformity. There is a difference of approximately 200ma between the carboniferous limestone of the lower beds and the oolitic limestone of the upper beds.
The upper beds that were deposited at a much later date than the lower beds appear to be oolitic limestone. These rocks would have been deposited approximately 200ma after the carboniferous limestone, and would have been somewhere just north of the equator towards the Bahamas. The lower carboniferous limestone would firstly have been deposited, lithified, a wave cut platform would have formed and this oolitic limestone would have been deposited on top of this wave cut platform. The structure of this rock is different, by observation it can be seen that it is not as cemented as the carboniferous rock, with sand/calcareous rounded spaces. It is a different colour – yellow/tan, and was formed in a much more turbulent environment.
There are a number of fossils present in this rock. There are shell fragments, ooids, and oyster shells. The oyster shells are present on the surface between the two rock types. This oyster habitat on the surface suggests that there was once shallow water environment. The oysters present are Jurassic, which have bored down into the rock below them, using acid. This shows a different sea bed environment than before, when the carboniferous limestone was deposited, it was a solid hard ground sea floor where the animals encrusted themselves and created a habitat, which is evidence that the surface was exposed without deposition for a long time.
The difference in both rocks proves that there was a gap between deposition and formation. The middle shows a missing period in time. The unconformity shown is angular, and the diagram overleaf shows how this was formed. The missing periods in time are most likely to be Permian, Triassic and the bottom of the Jurassic.
The discovery of unconformities such as this one has greatly affected our view of geological time. Before these discoveries the earth was thought to be much younger than is thought now. Many different theories had been proposed before James Hutton gave the theory of the earth. The first volume of reports on the geology of Britain was produced around 1856.
One of the earlier estimates of the age of the earth was by James Ussher who stated that the earth was created 4004 b.c. Charles Darwin estimated that the earth was at least 30ma, that would be the time required for his theory of evolution. There is a considerable difference in time between rocks in unconformities with long periods of time missing; this led to the concept of deep time (John McFee). In 1669 the law of original horizontality states that nearly all sediments are deposited horizontally, leading on from this is the principle of stratigraphical superposition, which states that the layers are deposited from bottom to top, the oldest layers being at the bottom and getting younger as you go up. Nicolaus Steno proposed the law of stratigraphic succession in 1665. This was not an effective way of estimating the age of the earth because the large missing periods of time in unconformities are not taken into consideration. Lyell and other scientists proposed that it is possible to determine ages of the rocks, numerically, by using the stratigraphical record but at this time it was assumed that rate of sedimentation was constant and that all strata were formed without interruption, that they were all conformable. From 1840 – 1850 saw the beginning of the following of the evolution of animals to date – biostratigraphy. Lord Kelvin calculated in the 1890’s that the earth was between 20 and 400ma old. He based his theory on the cooling rates of the earth, but he didn’t account for a major factor – radioactivity. In 1899 John Joly estimated the earth to be 90ma old, by analysing the salt in the earth, but one of the problems with this was the fact that the sea and land were not created at the same time.
In 1812 William Smith, whilst laying canals, discovered the faunal succession. This led to the eventual means of worldwide correlation. By using this we can analyse the fossils found in unconformities to discover the relative dates of the strata.
James Hutton (1726 – 1797), who is often referred to as the father of geology proposed the theory of uniformitarian theory of geology, which states that all earth processes affect the surface of the earth, and had been at a constant rate for a very long time. He based a large majority of his work in Scotland nearly 200 years ago, at Siccar Point and at Newton Point, Arran. His exploration led to the theory of deep time, and he is known to have quoted that “the result, therefore of this physical enquiry is that we find no vestige of a beginning, no prospect of an end”.
Bibliography
Earth system science EA01050 – “A trip across the planet “handout.
Geology today – understanding our planet (Murck/Skinner) 1999
Earth – an introduction to physical geology (Tarbuck & Lutgens) seventh edition 2002
