Nigg Bay EarthCache

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Thathanka: Thank you for all you visits. I hope you enjoyed this geologically amazing piece of land.

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Hidden : 9/11/2012
Difficulty:
3 out of 5
Terrain:
3 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Scotland is well known as one of the few areas of the world that possesses a range of rocks stretching back to the beginning of the Earth.

For north east Scotland the oldest rocks date back to around 500 million years ago. The cliffs of Nigg Bay are an excellent place to learn how the landscape around Aberdeen evolved during the last Ice Age. I would like to invite you for a walk along Nigg Bay shore and finding some interesting geological objects.

Please be prepared: the walk may take 2-3 hours, it's 3-5 km long, sometimes you will have to hike or almost climb (depends on what way you chose). You will also need a measure (about 1 meter long).

Although it is possible to fulfill all the tasks any time of the day the best will be to try during low tides. You may find tides tables on the Internet.

All the objects in random order (click on the pictures to enlarge):

A. Granite and Quartz
In one of the places you will find granite block with a quartz vein going through.
As the magma cooled to form granite and some fractures in the solid rock appeared, fresh magma, usually of a lighter color was injected under high pressure into these fractures, cooled, and crystallized.



B. Erosion in action
In next place of our walk you may find a rock with visible signs of erosion - destructive force of nature. You can notice that smaller pieces of rock fell off from the main block.
This object will be subject of one of your tasks, so please have a closer look.



C. Geo
Walking along the coastline you may notice bigger breaks in sea shore going straight inside the land. They are Geos - linear clefts in a sea cliff, effect of victorious fight of sea over the land. Geos may have their source in wave erosion but they can also form through roof collapse of narrow sea caves.
In our case it seems that sea found a weakness in a rocky cliff and used huge energy of waves to create this spectacular view.



D. Layers of the cliff
The sheltered position of Nigg Bay, between two headlands, and a gently sloping seabed offshore has resulted in a well developed sand and cobble beach. From the car park look right towards the cliffs at the back of the storm beach. The cliffs show sediments that choke an abandoned channel of the River Dee.
"Window" with 3 layers: 1 - head with deposits, 2 - red clay layer, 3 - gray/light brown clay layer:

Closer look at the head and red clay layer:

The former river course is cut down to -40m below present sea level and formerly was occupied by glacial rivers that drained from an inland ice sheet. The land level was lower than today because the weight of ice pushed it down but since the ice melted the land gradually risen, like a cushion regaining its shape after being sat on. The infill of sediment, a part of which is shown in the cliff sections, dates to the last Ice Age, long before there were humans in Scotland and at a time when glaciers were forming in the Grampian Mountains. The lowest sediments consist of gravel and boulders - some of which appear to have originated in Scandinavia and to have been transported here by glacier ice. The overlying glacial sediments appear to have been derived from farther south down the coast, the reddish colour of the sediment due to the former passage of ice across the Old Red Sandstone rocks of Angus.



Six distinctive beds have been recognized in the exposure (beginning from the top):
* Head - very top of the geological succession, originated during Devensian Glaciation (about 115-10 thousand years ago). It’s built from sand and gravel and contains deposits shown on the photo above.
* Gravels (1–3 m);
* Red sands with laminated silts and clays (1–2 m);
* Red clays (till 2–3 m);
* Grey clays (till 10 m);
* Sands and gravels (3–6 m).

E. Pink granite with xenolith
On the rocky shore you may find some pink granite with xenoliths of country rock embedded in it.  Xenolith (literally, "alien rock") probably broken off not far from its current location, dropped into the already-cooling magma, and left embedded in the granite for the past 450 (or so) million years. Have in mind that all action happened far underground and thanks to processes like folding which took place sometime later it’s available for us.




F. Pothole
A pothole usually is a hole in the bedrock at the base of waterfalls or in strong rapids. Once the initial cavity is done by a stream, moving water spins sand, gravel and rock fragments in any small indentation in the bedrock what accelerates the process of enlargement of the pothole and after some time, the sand and stones carve out an elliptical hole. Potholes may also form under or near the edge of glaciers by the action of glacial meltwater what is shown on the picture below.





Picture based on one by Pennsylvania Department of Conservation and Natural Resources

G. Sea stacks
Sea stacks are quite common on the Aberdeen’s shore. As many interesting geological objects they are an effect of erosion caused by sea waves. They are alone rocks cutted out horizontally from larger rock formations, and they provide a spectacular type of coastal landform. Among numerous sea stacks there are some in a shape of a bridge but the ones you will find have a form a separated rocks.



H. Stone tree
Interesting object of “disneylandisation” of geology. ;-) Fake petrified wood with remains of resin extraction.
In fact tree is petrified when all organic materials have been replaced with minerals like silicate, while retaining the original shape and structure. “The petrifaction process occurs underground, when wood becomes buried under sediment and is initially preserved due to a lack of oxygen which inhibits aerobic decomposition. Mineral-laden water flowing through the sediment deposits minerals in the plant's cells; as the plant's lignin and cellulose decay, a stone mould forms in its place.” (quote from Wikipedia)



I. Folded layers of metamorphic rocks
Metamorphic rocks are the rocks formed in a process of intense changes due to high pressures and high temperature. The pressure and temperature may be caused by tectonic processes such as continental collisions but also by the intrusion of hot magma from the Earth's interior.
Examples of metamorphic rocks are gneiss, slate, marble or schist.
The resulting rocks may have a very characteristic "layered" or "folded" look what can be noticed on Nigg Bay coast in the one of our research places.





I hope you enjoyed the stroll and you found it interesting. My intention was to show you how geologically rich may be small piece of land very close to the city like Aberdeen. I hope you feel encouraged to make your own discoveries - please share them with geocachers' comunity in your log entry.

Literature
The text above is based on many articles but mainly a few listed below and there you can find wider and much more advanced description:
1. Nigg Bay, J.E. Gordon
2. Nigg Bay to Cove. Coastal Footpath. Geology and Geomorphology, Prof A. Dawson, University of Aberdeen
3. The Balnagask Headland & Bay of Nigg - Aberdeen, Peter Robinson
4. The Aberdeen Coastal Path Guide. Aberdeen City Council


Tasks
To count the Earthcache please answer following questions. Replies should be sent via the contact form. Please do not publish them in your log. You do not have to wait for my permission to log your visit; if your answers will be wrong I'll contact you.
1. Match at least 7 objects from description above (points A - I.) to the waypoints below (points 1-9). I suggest that you walk around the waypoints in numerical order, matching the points as you go.
2. Measure "diameter" of the pothole.
3. Explain in own words what is the reason of erosion of rock shown in point B.
4. In one of the points you may find "stone tree" made of granite. Explain why from geological point of view it cannot be real.

Additional Hints (No hints available.)



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