GC8DWP1 St Austell Bay Geology - Raised Beach Deposits (Earthcache) in South West England, United Kingdom created by spooky

GC8DWP1 ▼

A cache by spooky_luke Message this owner

Hidden : 9/21/2019

Difficulty:

Terrain:

Size: Size: other (other)

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Geocache Description:

This tide-dependent EarthCache is located on a publicly accessible beach to the west of Spit Point in St Austell Bay. Access to GZ is available via the South West Coast Path and then across the sand on the beach. Free parking is available at the additional waypoint provided.

To avoid being cut off by the tide, or not being able to access GZ, you should plan to visit this EarthCache at Low Tide.

Tide times can be found here

The outcrop that you are looking for

Most of the rocks exposed in St Austell Bay (including the ones forming the basal unit at GZ) are metamorphosed sediments known as the Meadfoot Group. This name comes from Meadfoot Beach in Torquay, where the same rock types were first described in detail.

The Meadfoot Group is further sub-divided into 2 other Formations and the rocks in this bay belong to the Bovisand Formation. Comprising layers of mudstones, siltstones, some volcanics and limestone, these rocks were created under relatively shallow marine conditions around 400 million years ago during the Devonian Period. The layers contain lots of finely divided pyrite (iron sulphide) which gives them their dark-coloured appearance and also suggests that they were laid down under fairly anoxic (low oxygen) conditions. Following their deposition, the sediments were compressed, dewatered and subjected to a process called diagenesis which converted them to hard rock. Subsequent major Earth movements throughout Devonian and Carboniferous times affected much of SW England and caused the rocks in St Austell Bay to become folded, faulted and metamorphosed into rough heavily-fractured slates and sandstones, with abundant quartz-filled tension veins.

The feature exposed at GZ and the main subject of the EarthCache is a distinctive set of raised beach deposits.

As the name suggests, raised beach deposits are formed from beach sediment when the sea level in the past was higher than the present day.

During the Quaternary Period (2.6 million years ago to approximately 12,000 years ago), Britain experienced an Ice Age. Throughout this time, temperatures alternated between colder and more temperate conditions.

Although Cornwall (unlike some other parts of the UK) was never completely covered by glacial ice, it was subjected to periglacial conditions as a result of being at the margin of a very cold area. The extremely low temperatures caused the ground surface to freeze at times and some of the soil and bedrock to crack and shatter. Contraction (shrinking) of the ground during these extremely cold conditions created voids known as ice wedges that later got infilled as shown below:

Infilled Ice Wedge
Seasonal variations in temperature and longer periods of temperate times (known as interglacials), allowed the frozen ground to partially thaw and flow under gravity.

Another consequence of the warmer conditions during interglacial times is that much of the neighbouring glacial ice melted and caused the sea level to rise well above present day levels. These elevated sea levels eroded the pre-existing rocks and deposited new layers of sediment. A relatively well-preserved example of these raised beach deposits with some interesting sedimentary structures is visible at the headline coordinates.

At the base (present day beach level) of the exposed section, 400 million year old Devonian mudstones outcrop.
Resting on this Devonian bedrock are Raised Beach Deposits A and B that started forming around 120,000 years ago. The boundary between the bedrock and the raised beach deposits therefore marks a considerable difference in time that geologists call an unconformity. During the intervening time, more rocks would have formed at this location, but have since been eroded away and replaced by the raised beach deposit.

Raised Beach Deposit A displays a distinctive sedimentary structure (hidden by the yellow box in the outcrop image on this cache page) created as a result of flowing water.

You will need to identify which type of structure it is from the following:

Cross Bedding

Imbrication

As water flows, it can transport grains of sediment and cause them to build up into small piles. Once the angle on the downstream side of the pile becomes too steep, the sediment cascades down and creates a tilted layer that gets preserved as cross bedding. If the direction of flow changes, so does the orientation of the layers, giving rise to intersecting sets of cross beds.

Flowing water can also cause elongated clasts (pieces of broken rock) to rotate so that their longer axis is aligned parallel to the direction of flow. If a sediment contains many clasts, each align with a similar sense sometimes giving rise to a stacked appearance.

Raised Beach Deposit B is different to Deposit A as it was formed under different conditions.

You will need to observe and compare both types to try to identify the conditions under which each deposit formed.

At the top of the exposed section (but somewhat obscured by vegetation), there is up to 1m of very fine grained buff-coloured silty material. It was formed approximately 15-20,000 years ago at the very end of the last glacial (colder) period. This sediment is a wind-blown deposit known as loess. Sea Level was at a minimum and sand blew around on the exposed land, decreasing in grain size as the grains collided with each other and became worn.

In order to claim this EarthCache, go to the published coordinates - You should be standing in a gully in front of an exposed section of cliff.

Please message or email me your answers to the following questions through my profile at the top of this page or via this link:-

1) Observe Raised Beach Deposit A. Identify the sedimentary structure visible near the base of the section - It is located in the area hidden under the yellow box in the photograph in the cache description above. Once identified, suggest how the sedimentary structure formed.

2) Compare Raised Beach Deposits A and B. Describe their appearance, for instance their colour, texture and grain size – Are they both the same? Suggest the nature of the conditions under which you think each deposit formed and explain your answer. For instance, "I think that Deposit A was formed under higher energy conditions than Deposit B because it is coarser material and contains much larger clasts that would have required more energy to transport them.

3) The largest clasts visible in the raised beach deposits are made from quartz. Find the largest clast that you can see and say whether it is in Raised Beach Deposit A, B or somewhere else. Suggest where you think the quartz most likely came from and justify your answer.

Feel free to log your find without having to wait for confirmation from me. However, if your answers are not correct or incomplete etc., I will contact you to verify your find.

EarthCache guidelines changed in June 2019 and allow me to request a photograph of you or a personal item at GZ to prove that you actually visited, but I appreciate that this is not always possible. If you can and would like to upload a (non-spoiler) picture at GZ, then please feel free to do so.

Thanks for taking the time to visit this Earthcache and I hope you enjoy your visit.

Additional Hints (Decrypt)

Frr vzntr naq vasbezngvba va gur qrfpevcgvba.

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)