Earl Crag sits above the Aire Valley and the village of Cowling, it is one of the gems of Yorkshire. A fine gritstone escarpment, with wide ranging views which include Pendle Hill, Ingleborough, Pen-y-ghent, Skipton, Sharp Haw, Rombalds Moor, and lots of tother places. The views are there for all to see, but there is more to this crag, than just pure stones and rocks. Somewhere hidden in the crags is the Nose, a natural feature, and a good opportuntiy to learn and understand some geology.
NOW A WORD OF WARNING. DO NOT APPROACH FROM ABOVE. APPROACH FROM BELOW.
The rock that you can see is known as gritstone. Gritstone or grit is a hard, coarse-grained, siliceous sandstone, and is often known as millstone grit, which is an informal term for a succession of gritstones which are found in the Peak District and Pennines of northern England. These sediments were laid down in the in the Carboniferous period, in deltaic conditions. The area that is now northern England lay in a subsiding basin between high mountain ranges. Rivers carried sediments which compressed under pressure of overlying rocks to give a rock sequence which is about 1700m thick. Deltas of sand built out over deeper waters in which clay and mud was deposited. Sea-level fluctuated because of global temperature changes, so alternating beds of mudstones and sandstones are found.
As already stated, Gritstone is a type of sandstone. Sandstone is a clastic sedimentary rock, mainly of sand sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are: tan, brown, yellow, red, grey, pink, white, and black.Sandstones are formed from cemented grains that may either be fragments of a pre-existing rock or be mono-minerallic crystals. The cements binding these grains together are typically calcite, clays, and silica.
Many sandstones and gritstones have been classified, and what you can see is Kinder Scout Grit. Yes, you are still in Yorkshire before you question your location. As the name suggests, the resistant sandstones of the Kinderscout Grit form the Kinderscout Plateau, but it is also found in Lancashire and Yorkshire. Kinderscout Grit is dominated by medium- to very-coarse grained, cross-bedded sandstones. It has a relatively high content of feldspar and frequently contains layers of rounded pebbles.
There is a substage known as the Kinderscoutian, which was part of the Namurian age.
At the most basic level, because grit is strong, it tends to form the highest ground, as we can see here. Because it is has highly variable layers, it tends to form cliffs - Edges - when the erosion of weak layers undermines stronger material above and the resultant overhang breaks off. A variation on this occurs where shales layers interleave grit or similar sandstones. A process which is still active today (sometime visible on old gritstone gate posts etc.) is the development of crenellations in pinnacles of gritstone. These are vertical flutes which are at their deepest near the top of exposed features, and run vertically down, irrespective of the angle of the local strata, but gradually fade after a metre or two. Erosion is clearly due to rain running down the exposed rock, but the mechanism is unclear - erosion is greatest where the volume of water running down the rock is least and vice versa. It is likely that minerals are being transported by water moving through the pores in the rock in such a way that vertical lines of weakness develop.
Fluted rocks
Gritstone, like all sandstones is a sedimentary rock. Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or galciers. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.
Stratification is the way sediment layers are stacked over each other, and can occur on the scale of hundreds of meters, and down to submillimeter scale. It is a fundamental feature of sedimentary rocks.Over time the different sediment types will be stacked on top of each other and the migration of the shoreline will produce superimposed layers (stratification) of different types of sedimentary rock. If we assume that strata are layers we would think they would be straight, but that is not always the case. Think of the different layers of a cake, each layer could be called a strata.
This being an earthcache, in order to log it, I ask that you answer some 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) Now, geocaching, and earthcaches are there for us to enjoy and learn, and they are not meant to cause worry or put you off doing them. After all, remember that we are in friendly Yorkshire, so as part of this earthcache, lets have a bit of fun. Please look at the nose, as much as you can. One perspective is better that others and that is looking east.
(a) Using the below diagram, please describe the type of nose.
(2) Please feel the nose, what does it feel like, is it rough or smooth, is it composed of all the same material?
(3) What colour is the nose?
(4) Now next to the nose, is a larger piece of stone with lines on it, these are examples of strata. Look at the strata, are they horizontal, tilted, curvy or some other direction?