This is one of those great places to watch sunset, I just love it. I often come up near here and watch the Northern Lights as well.
It is also a place to fully appreciate the geology of the area, as you can see Dry Rigg quarry with its curving and folded rocks, whilst high above is the limestone crag of Moughton.
You will notice that the coordinates have taken you to a sandstone boulder. To be more specific, it is Austwick Formation Sandstone. It is a sedimentary bedrock formed between 433.4 and 427.4 million years ago during the Silurian period. During this geological period, the area was a deep ocean floor, known as the Iapteus Ocean, sand and silt was deposited, and earthquakes meant that marine landslides occured. Over time, the sediment build up, and the sea dissapeared, and eventually rock was formed.
What is sandstone?
You would think sand, but there is more to sandstone than just sand. First of all it depends where it comes from, as to what it is composed of. It is basically a recycled rock, everything which makes it has come from terrain that has been eroded down to sand sized grains. So, this boulder could be made up of sand sized grains that originally was an ancient mountain millions of years ago, who knows?
The main constituent of sand is quartz and feldspar, these are both hard minerals. Now, there can be other ingredients such as clay, mica, calcite and iron oxide. These all bind together to make the sandstone, but each ingredient has a different resistance to erosion. Sandstones are commonly brown, yellow and tan in colour, and are found throughout the world.
What are we here to look at?
We are here to look at a geological feature on top of the boulder called a pan hole.
So, what is a pan hole?
Well, it's a feature on a rock caused by erosion. It presents as depression or basin in flat or gentle inclined rock surfaces, and whilst this pan hole is in sandstone, they can occur in different types of rock.
The question is, how has it formed?
Holes can be formed in river beds, by the action of water moving sediment and rocks about, thus scouring and gradually eroding the rock. Once a rock is confined to a hole in the rock, water agitates it, and a circular motion can occur, eroding the hole further, which is then deepened and widened. This is what has happened in the above picture, which is at Stainforth Foss.
Holes can also form as the result of the erosive power of water. Here there is a small waterfall, directly where it falls can produce a plunge pool, as the result of the river bed, and underlying geology being eroded. The above picture is of the River Ribble at Helwith Bridge.
There is a lot of limestone in the area, just look over to Smearsett Scar, Smearsett Copy, Moughton and Penyghent, all limestone rock. Holes can be found in limestone. Limestone is formed of calcium carbonate, now when it rains, water gains carbon dioxide. This produces a weak acid called carbonic acid. The rain water then dissolves the calcium carbonate in the limestone, and dissolution weathering occurs, as the limestone is eroded away. In this case it forms shallow holes or basins. They can hold water, and eventually an overflow channel occurs, as erosion occurs.
Holes in limestone, near Ribblehead.
Now, that all explains the actions of water, but there isn't a river or stream here is there? When it gets cold, and snow and ice forms, cracks in rocks can get filled with ice. Ice expands thus exerting pressure on the rock, and can cause cracks to widen and lengthen over time.
Winter in Upper Ribblesdale, when snow and ice can be present.
Cracks and weaknesses can occur in rocks, some as minor features to a microscopic level. These can be as a result of different sediment, cracks from damage to the rock and from the scouring action of glaciers. On a level or mildly inclined rock surfaces, if there is a crack it will hold water. Over time, water can erode many rocks though very slowly, due to the water being slightly acidic. The longer the water stays there, the longer erosion occurs.
Sandstone can be eroded by the presence of water, as when in contact with it, it can slowly affect the ingredients making it up. As said above, some are more resistant and others less so. Acidic water can affect the less resistant ingredients, and in time they dissolve. This then leaves fragments of stone, which can further erode the rock by being being abrasive when moved in water.
This 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. Please describe the pan hole in terms of shape, and any specific features.
2. After reading the cache description, please explain in your own words, how you think the pan hole formed.