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Royal Marine Commando Training #13 Hidden Geology
Mission Background
For the first time in the RMCT series, there is no physical container for you to find here. You will however have to visit the posted co-ordinates and complete some tasks in order to complete this mission. Specialist equipment required for this mission is a measuring tape.
Mission Summary
Your Royal Marine Commando Instructor requires you to visit a prominent rock, along with another hidden geological feature at the posted co-ordinates which is accessible at low tide. Tide times can be checked here. Park your car at N 54° 33.926' W 5° 38.417' then hug the edge of the shoreline from N 54° 33.923' W 5° 38.429' out to GZ. This is a walk of approximately half a mile each way. The terrain can be slippy in places and may require wading in parts so good footwear is essential. Once at the posted co-ordinates, make the required observations which will involve the use of your mesauring tape, answer the required questions and take the required photo to successfully complete ths mission. Send your answers to me via e-mail or message centre using the link on my user profile at the top of the page. You may post a found it log (along with the required photo) without receiving a response from me. Any found it logs without a corresponding email or message will be deleted and any with significant errors will be contacted for further checks.
Strangford Lough
Strangford Lough is a sea inlet, which emerged from under the melting ice-sheets of the Ice Age and is for the most part less than 10m in depth. It is approximately 30km long, covering an area of 150km2 and is the largest sea inlet in the British Isles. About one third of its seabed is exposed at low tide including the vast northern sand flats. The enclosed sea lough has approximately 70 islands and numerous pladdies, which are submerged at high tide. The islands are drowned drumlins in various stages of erosion. Numerous glacial erratic boulders also litter both the edges of the lough and the lough itself.
Feature you will pass walking out to GZ
Types of Rocks
The three main types of rock are sedimentary, metamorphic, and igneous.
Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments which accumulate as sediment in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may crumble easily. It is usually the only type that contains fossils. An example is limestone, shale and sandstone.
Metamorphic
Metamorphic rocks are formed under the surface of the earth under intense heat and pressure. These rocks often have ribbon layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. An example is marble, slate and gneiss.
Igneous
Igneous rocks are formed when molten rock (magma) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. An example is granite, obsidian and basalt.
The Ice Age and Erratics
An ice age is a period during which the temperature of the Earth persistently falls resulting in ice formation. The ice sheet over the British Isles reached down from the north as far south as Cardiff and was three miles thick in places.
The most recent ice age (ie the most recent period of repeated glaciation) occurred in the Pleistocene Epoch - the time period that began about 2.6 million years ago and lasted until about 11,700 years ago. when temperatures began to rise and the glaciers retreat. We are currently in a warmer interglacial period called the Holocene Epoch.
Geological evidence for ice ages comes in various forms, including glacial erratics.
Glaciers
A glacier might look like a solid block of ice, but it actually is moving very slowly. It moves because pressure from the weight of the overlying ice causes it to deform and flow. As it flows it will gouge and carry rocks from in its path.
With greater snowfall and lower temperatures the ice will build-up and the glacier will grow.
If the temperature rises the glacier will begin to shrink. As it shrinks the encased rocks are freed to fall to earth.
Glacial Erratic
Glacial erratics, often simply called erratics, or erratic boulders, are rocks that have been transported by ice and deposited elsewhere. The type of rock (lithology) that the glacial erratic is made from is different to the lithology of the bedrock where the erratic is deposited.
For example, in the diagram above, an erratic could be a boulder of sandstone is picked up by a glacier, transported, and deposited on top of a limestone bedrock. Some erratics are useful to scientists because they are of a distinctive rock type, which means that their source outcrop can be identified and located. Glacial erratics are therefore useful in reconstructing past glacier flow directions, the timing of glacier retreat, and even the type of glacier flow.
Erratics can be huge boulders or little pebbles. They can be any shape but are often are often very rounded because the rough edges are eroded while the erratic is transported within the ice.
View From GZ
Mission Tasks
1. Examine the tall boulder in front of you.
- a) Using your measuring tape, calculate the maximum height, width and depth of the boulder.
- b) Assuming that the Specific Gravity of this boulder is 2.9, estimate the total mass (weight) of the boulder in tonnes. You will have to work out the volume of the boulder in cubic metres first. Do this by multiplying the height, width and depth of the boulder that you estimated above together (e.g. 2 x 2 x 2 = 8 cubic metres). For each cubic metre of rock, assume that the mass is 2.9 tonnes (e.g. 8 x 2.9 = 23.2 tonnes).
- c) Describe the colour, shape and texture of the boulder.
- d) Is the bottom quarter of the boulder the same colour as the top three quarters? Why do you think this might be?
- e) Using your measuring tape, at the lough / west side of the boulder calculate the average high tide height. Describe how you calculated this.
- f) What type of rock is this boulder, igneous, sedimentary, metamorphic and specifically, what type of igneous, sedimentary or metamorphic rock is it, i.e. granite, limestone, marble etc?
- g) Examine the boulder carefully. Do you see any evidence of the ice flow having been in contact with it (scuff marks for example)? If not, explain why this might be. If so, describe what you see.
2. Examine the rock underneath (east / Portaferry Road side) the boulder.
- a) Describe the colour, shape and texture of this rock.
- b) Is this rock igneous, sedimentary, metamorphic? Give a reason for your answer.
- c) More specifically, what type of igneous, sedimentary or metamorphic rock is this, i.e. granite, limestone, marble etc.
- d) During your walk out to GZ, did you see any other rock like this?
3. Given your answers above:-
- Explain why you think the tall boulder is either just a boulder or a glacial erratic boulder.
- Where do you think the tall boulder came from?
4. Upload a photo of either yourself or a personal item at the tall boulder which must be taken from the shore. Artistic creativity is welcome, example photograph below.
Miscellaneous
I wish you all the best on your mission to find this cache. Who knows, if you successfully complete same then you just might have what it takes to become a Royal Marine Commando!
Other caches in The Royal Marine Commando Training series:-
1) Royal Marine Commando Training #01 The Low Crawl
2) Royal Marine Commando Training #02 Problem Solving
3) Royal Marine Commando Training #03 Observation
4) Royal Marine Commando Training #04 Confined Spaces
5) Royal Marine Commando Training #05 Agility
6) Royal Marine Commando Training #06 Tool Up
7) Royal Marine Commando Training #07 K9 Support
8) Royal Marine Commando Training #08 Darkness
9) Royal Marine Commando Training #09 Always Prepared
10) Royal Marine Commando Training #10 Reverse Wherigo
11) Royal Marine Commando Training #11 Obs Post (OP)
12) Royal Marine Commando Training #12 Coming Together
13) Royal Marine Commando Training #13 Secret Geology