This is one of a series of EarthCaches placed at sites within the Westmorland Dales area which the Cumbria GeoConservation group considers as being good examples of various aspects of the local geology.Our geolgogical research has been made possible by the Westmorland Dales Landscape Partnership Scheme, jointly led by the Friends of the Lake District and the Yorkshire Dales National Park Authority and has been grant funded by the National Lottery Heritage Fund.
This is one of three treefolds designed and built in 2017 by Harriet and Rob Fraser, with the help of Andrew Mason, an expert waller. You can find out more about them here. The tree within this treefold will replace a nearby tree – the Dowly Tree – which has died and is now only a stump. The Dowly Tree, when alive, was well known in the area as a significant feature of the landscape and was probably a boundary marker. In the Westmorland dialect, ‘dowly’ means sad, lonely or melancholy. The rock for the treefold has come from a little quarry to the NW of Orton village. This geocache is located on Little Asby Common owned by Friends of the Lake District. The Common is important for its geology, landscape and nature conservation.
Limestone is a rock composed primarily of calcium carbonate, which is white. The calcium and the carbonate ions come from seawater and are used by organisms living in the sea, to make their shells. This limestone is about 350 million years old and at this time, known to geologists as the Carboniferous Period, the area that is now Britain was just north of the Equator and the seas were teeming with life. When the sea creatures died, their remains fell to the sea floor and collected as pale particles of lime, which were later compressed and cemented together to make the almost white limestone we see now. Grey limestone has a tiny amount of mud in it.
Under some circumstances, limestone can appear slightly yellowish or brownish. This is an indication that the original limestone has been altered by the movement of fluids through it, after it had been buried by other sediments, possibly thousands or millions of years later. These fluids may have been similar to concentrated seawater, with a higher proportion of magnesium in it than normal, and some of the calcium in the rock was then replaced with magnesium, forming a mineral composed of calcium magnesium carbonate, which is called dolomite. This produces the yellowish colour.
Sometimes, when limestone is converted to dolomite, holes can appear in the rock. Geologists call these ‘vugs’ and they often contain crystals. Magnesium ions are smaller than calcium ions, so when calcium carbonate (limestone) is converted into calcium magnesium carbonate (dolomite), the volume of the rock is reduced. One possible explanation for the vugs is that they were formed as a result of this volume change. The crystals in the vugs were formed as more fluids percolated through the buried rock and deposited their minerals in the holes.
Questions:
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1. Describe the differences between the rocks making up the treefold and the typical rocks at the surface of the ground nearby, concentrating particularly on the colour. What name would geologists give to the treefold rocks?
2. Describe any vugs that you can see. How big is the largest one? What different colours of crystals are present in the vugs?
3. An extract from a poem is carved around the treefold. Above the carved word ‘circle’ is a relatively dark grey stone, containing a distinctive coral fossil. Describe this fossil.
4. Look around the treefold rocks and describe the appearance of two other fossils in these quarried rocks.