To start with the GPS bounches about around here. So I have included a picture of what you need to look for to start the journey.
Now Earl Crag is made out of Gritstone, namely Kinder Scout Grit, which is a type of coarse grained sandstone. Loved by many climbers, it is a sedimentary rock type that started out life on the floors of wide river deltas where the grit deposits were typically laid down, with every tide or flood dumping yet another layer of sediment. Mostly, these layers would be only as thick as the shifting currents allowed but, occasionally, conditions would change for a short while and a thick layer, many metres in some cases, would be laid down. These ancient sediment happened some 300 million years ago and over long periods of time and under great pressure these gritty sediments were eventually transformed into rock. The final piece of the jigsaw slotted into place when the areas were uplifted and then eroded to expose the resilient and magnificent bands of gritstone as the edges and tors we recognise today. Some exposures of gritstone are incredibly uniform and free from lines of weakness, a quality that made the rock type particularly useful for the quarrying of the distinctive millstones that were used to grind cereals. Elsewhere the gritstone contains strata that create chinks susceptible to weathering. Once the process starts these blips grow everdeeper into breaks and later cracks, both horizontal and vertical.
Rock types weather down quite differently, both by chemical and physical processes, during exposure to water and extremes of temperature. Quartz grains, found in sandstones, are made of silica which does not break down chemically. Sandstones are resistant to chemical weathering when held together with a quartz cement, but wherever the cement is either clay or iron rich or missing, the sand grains become loose and will be washed out. Variable types of cement result in differential weathering, producing deep, yellow cavities or a honeycomb pattern on some rock faces. Other materials such as feldspars and muscovite found in the millstone grit will eventually break down into clay particles. Flaggy sandstones with numerous bedding planes will decompose much more readily than a massive quartz sandstone which is resistant enough to produce exposed cliffs and benches. Freeze-thaw weathering occurs when temperatures fluctuate around 0°C. Water present within pore spaces, along with bedding planes and in joints, expands on freezing, causing blocks to fall from cliff faces. Angular blocks form scree slopes which are found below many gritstone edges. Freeze-thaw conditions continue to break down sandstone blocks into quartz grains or fragments.
Weathering normally takes place along natural joints, and bedding planes.
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. At times though, the strata on a rock is not horizontal, this can be due to various reasons, but one is cross-bedding. Cross bedding can appear as diagonal lines on a rock in its most basic form.
Beds are seperated by bedding planes, cm to m thick units of sedimentary rock that were deposited approximately horizontally (beds) and are separated by horizontal planes (bedding planes); the rocks typically weather more along these planes. Beds are usually fairly uniform or change gradationally in composition. Bedding planes usually represent breaks in sedimentation. When sediment is initially deposited, it is laid down in horizontal layers, with the oldest strata at the bottom. Horizontal bedding usually indicates that little or no structural deformation has occurredto a sedimentary succession.
A joint is a break (fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface (plane) of the fracture. Although they can occur singly, they most frequently occur as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.
Types of joints
Joints can also be classified according to their origin. On the basis of their origin, joints have been divided into a number of different types that include tectonic, hydraulic, exfoliation, unloading (release), and cooling joints depending on the specific author and publication. Also, the origin of many joint sets often can be unclear and quite ambiguous. Often, different authors have proposed multiple and contradictory hypotheses for specific joint sets and types. Finally, it should be kept in mind that different joints in the same outcrop may have formed at different times and for different reasons.
Tectonic joints are joints that formed when the relative displacement of the joint walls is normal to its plane as the result of brittle deformation of bedrock in response to regional or local tectonic deformation of bedrock. Tectonic joints often reflect local tectonic stresses associated with local folding and faulting.
Hydraulic joints are joints thought to have formed when pore fluid pressure became elevated as a result of vertical gravitational loading. In simple terms, the accumulation of either sediments, volcanic, or other material causes an increase in the pore pressure of groundwater and other fluids in the underlying rock when they cannot move either laterally of vertically in response to this pressure. This also causes an increase in pore pressure in preexisting cracks that increases the tensile stress on them perpendicular to the minimum principal stress (the direction in which the rock is being stretched).
Exfoliation joints are sets of flat-lying, curved, and large joints that are restricted to massively exposed rock faces in an deeply eroded landscape. It consists of fan-shaped fractures varying from a few meters to tens of meters in size that lie sub-parallel to the topography.
Unloading joints or release joints are joints formed near the surface during uplift and erosion. As bedded sedimentary rocks are brought closer to the surface during uplift and erosion, they cool, contract and become relaxed elastically. This causes stress buildup that eventually exceeds the tensile strength of the bedrock and results in the formation of jointing.
Cooling joints are columnar joints that result from the cooling of either lava from the exposed surface of a lava lake or flood basalt flow or the sides of a tabular igneous, typically basaltic, intrusion. They exhibit a pattern of joints that join together at triple junctions either at or about 120° angles. They split a rock body into long, prisms or columns that are typically hexagonal, although 3-, 4-, 5- and 7-sided columns are relatively common.
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.
This earthcache is about getting into the geology so lets get on with it. The questions are staged as you go along. First look at the rock around you take note of its colouring. You will notice a hole in the rock face. You need to climb up into it, and then enter the tunnel. You will need to crawl and watch your head.
1. Please look at the rock around the tunnel, is it is the same all round, or are there differences. What do you think has caused this tunnel to be formed? Is there any evidence of jointing, if so what type?
2. Please measure the dimensions of the tunnel, what is its height and width?
So crawl along the tunnel towards the daylight, and crawl out, you will come out onto a large rock shelf. Be careful though it has an unguarded drop. Above it mentions that the kinderscout grit is a coarse sandstone. Have a look at the rock face.
3. What does the rock look like, is it is the same colour as the rock before you entered the tunnel? How big are the seperate grains, what colour are they? Is there just sand grains, or evidence of any other rocks, if so, what types?
4. There are two large initials, what are they?
This is not yet the end of the journey, follow the rock face round the corner and upwards, be careful here, it can be steep and slippy. You will notice in the rockface, where it has been weathered, as there are small holes. Continue upwards, until you can see the Pedestal, a large piece of rock standing proud, flat, slighty tilted. Please look at it. It has a smaller base compared with the top.
5. Look at the rock, is it all the same? Are there different layers? Do you think that each weathers the same, if not why?
6. What do you think will eventually happen to the larger flat, slightly tilted rock?