This EarthCache involves getting into the limestone pavement. DO NOT TRY AND APPROACH FROM ABOVE. IT IS UNEVEN AND WILL BE SLIPPY WHEN WET.
So, here we are at Middle Scar. Middle Scar is a limestone pavement, with some special features, namely grykes. Now these are also spelt grikes in some literature, for the purpose of this EarthCache we shall use grykes.
Limestone is a sedimentary rock, initially starting as lime lime mud or accumulation of the shells of ancient sea creatures. Over time sediment build up, more layers were added, and then eventually over millenia, limestone was formed. It is basically calcium carbonate. When the glaciers came to these parts, the glaciers scoured the land and left bare limestone pavements. Over time, erosion occured, and whatcwas a flat surface became crossed with various lines and shapes.
Erosion and weathering have a variety of forces which can affect limestone. These forces go all the way back to glacial times.
The glaciers as they moved over the land, scoured the land underneath. Now, some sources say that calcium carbonate is more soluble in cooler temperatures. Glaciers are not just moving bodies of ice, they have meltwater channels in them. Limestone is very porous, that means that water can filter through it easily. In doing this it can actually pick up some of the calcium and this can lead to cracks and fissures starting to form. When the glaciers retreated, they left limestone pavements, but they also left what is known as glacial till. This is clay, rocks and soil overlying the ground. Now, as water percolate through soil, it becomes slightly acidic, and this can further weaken limestone. So, erosion can start even when limestone is not exposed.
The exposed limestone pavements, had small cracks in them. Over time, rain and ice weakened the limestone, rain made smells cracks into fissures. Ice widened and fractured rock , as when ice forms it expands. So, crack and fissures got deeper, and then grykes formed, they are the deep linear features in limestone pavements.
It is said however that gryke formation may actually be older than the time of the glaciers. Limestone was formed during the Carboniferous period, this lasted millions of years, during which in some locations, landforms changed, with some locations bring covered by ancient woodland. This meant that groundwater was more acidic, thus already starting the process of erosion.
Grykes can be classified by their size.
Shallow, Wide Grykes. These are generally found at higher altitudes and are associated with thinner limestones.
Mid-Range Grykes. These have depths between 0.5 and 1 meter and are common in upland limestone pavements.
Deep, Narrow Grykes. These are often found in lowland wooded limestone pavements and are characterized by shallow soils.
Giant Grykes. These can be up to 7 meters wide and 33 meters deep, with some extending into underground caves.
Deep Grykes. Grykes which are more than twice their width are classed as deep grykes.
We have already talked about erosion and weathering, though some sources relate the depth of grykes being related to the size of bedding planes. The larger the bedding plane, the larger the size of the grykes.
In geology, beds are layers of rock which have been laid down. Bedding planes are the features separating these beds, and can represent change in depositional sediment, pause in sedimentation, or a change in the marine current conditions leading to less or more sediment being deposited. Now many people get confused with cracks in limestone. Bedding planes are always horizontal (unless tilted by fault action), whilst vertical features are known as cracks.
So, we been brought to this location, and it being an EarthCache in order to log it, I ask that you answer the below questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1. At the location you are amidst the limestone pavement, using the above information, please define the type of gryke.
2. What is the rationale for your decision?
3. What contributed to the formation of this dyke?
4. Is the dyke straight and the same width or does it taper? You will have to walk the length of the dyke.
5. How would you describe the beds of limestone in the gryke?
6. Please have your photograph taken, or that of your device, pointing towards one of the holes in the dyke walls. This is known as 'doing a kinnear'. The below picture gives an indication of what expected, however, please do not reveal any answers.
