Barr Beacon Quarry is a disused quarry which hasn’t been worked since the 1920’s, and was quarried for sandstone required for local buildings. The quarry consists of 2 different rocks laid down horizontally on top of one another.
I aim to find out the environments of deposition of each rock, for example desert conditions, shallow marine water, deep marine water, and river conditions.
I think that both rocks had different deposition environments and one rock water deposited in water and one on land.
1. I first studied a photograph of the quarry to select the areas that I am going to study in closer detail.
2. I will draw sketches of the whole quarry and of smaller section in greater detail in order to obtain a greater knowledge of the rock.
3. I will describe the rocks in multiple places using sketches to aid me.
4. I will collect samples of each rock that I will sieve in the laboratory.
5. I will describe the sedimentary structures and textures present and will use sketches to help.
6. I will conduct a pebble survey on the scree slope at the base of the cliff by walking down it backwards and collecting the stone in front of me, measuring its size and shape and then repeating that until I have 20 samples.
7. I will conduct an analysis of the pebble layer in order to determine its environment of deposition by selecting a random sample of 20 stones and recording their maximum length and the orientation of that length.
8. Finally I will sketch some sedimentary logs of the sandstone to help me with identifying its environment of deposition.
In order to prevent injuries while on this field trip, you must follow the geological Code of Conduct.
1. Collect the very minimum of samples to avoid unstable cliffs collapsing.
2. Wear all safety gear, safety hats and stout boots.
Climbing the sandstone cliff in order to get to the pebble layer may prove hazardous and therefore shoes with good grip are necessary.
Method of Sieving
Using stacked sieves of different grades I can work out the composition of the sandstone in grain sizes.
Pour sandstone sample into top (coarsest) sieve and shake until all possible grades have been penetrated.
Remove each sieve by turn and weigh the contents and record the value.
This data can then be put into a bar chart table in order to classify the sandstone.
Classifying the sandstone will help me to identify the environment of deposition of the sandstone, as certain environments contain only one grade of sand, be it medium grained for desert, fine grain for deep water or coarse grained for rivers.
Red-brown to orange-brown in colour, very well rounded grains, comprises of small < 2mm grains of sand in thin layers, occasionally thin dark clay layers and occasionally angular stones, 2-20mm across. The clay can only be deposited in water, so water must have passed over the area more than once. The sandstone layer is about 2-3m thick, with lots of cross lamination, a sure sign of desert conditions, with occasional mudstone between bedding planes. The rock has many joints in it, and no mica is present, another sure sign that this was deposited in a desert. The sand sample in location 1 that I sieved is a very regular medium grain graph, as is the sand in location 2.
Poorly sorted, very hard, mainly quartz layer. Well rounded with occasional igneous rocks of many sizes, 1mm to 20cm, all facing in generally the same direction. The pebble survey of the scree slope reveals that the majority of stones are rounded and of a large size, an average of 5.53, with the larger stones at the bottom and the smaller ones at the top.
The pebble survey of the layer shows that all of the stones face the same direction that indicates that the layer was deposited in a river. I have drawn a rose diagram to show these results.
I have identified results that prove my hypothesis correct, for example the colour, lack of mica, the round grains, the lack of fossils, the uniform grain size and the cross lamination on the sandstone layer proves that the sandstone was deposited on land and in a desert, and the uniform direction of the pebbles and the medium to coarse grain size in the pebble layer proves that the pebble layer was deposited in water in a river.
Sampling the sandstone and the pebble layer creates problems because sampling relies on the layers being completely uniform, which they obviously are not. One anomaly that I found in the sandstone layer was the angular stones, which don’t fit in a desert and throw off the cross lamination that indicates that it was formed in a desert.
The procedure used had to rely on sampling the area, as it was too big to cover accurately. To improve it you could take more samples of the sandstone from different heights, as this would represent different times in geological history.