The Cheesden Valley is now a quiet, relatively remote valley between Bury and Rochdale and to the north of Heywood, though look beyond what you can at first see, and you will notice many an industrial relic. This valley was once a hive of industry, filled with mills and habitation, though now all there is are moss covered ruins, bumps and lumps and a chimney. Its history goes far beyond the era of man though. At the time of the last ice age, when the ice front began to melt about 8,000-10,000 years ago it produced some dramatic changes in the local landscape. A series of huge glacial lakes were formed in the region, stretching across the Irwell Valley, Whitworth Valley, Naden Valley and Wardle. These lakes changed size, depth and drainage direction as the ice thawed. Lake Irwell extended as far as Ramsbottom, and its main drainage channel ran down Cheesden Valley into Lake Rochdale. Meltwater streams can run with high pressure from glaciers, carrying debris with them that gradually cuts away at valleys. This process deepened the existing valley, carved gorges, and gouged out the Cheesden Valley between Deeply Vale and Ashworth Valley as we know it today. The modern stream is a comparatively insignificant remnant of this once-powerful waterway.
Go further back, and the rocks that now underplay the area were being laid down. This was during the Carboniferous Period, a time when the rocks that lie underneath the valleys and hills of Rossendale were formed, when the whole of what is now the North of England was covered by huge river deltas and lagoons. Sediments, mainly sands, silts and muds, were eroded from hills in an area that now includes Scandinavia and Greenland and were swept into vast river deltas and lagoons in a central basin in a position now occupied by the Pennines. The sediment settled to the bottom as the water slowed down in the deltas and lagoons. The nearest equivalent sediments of today are forming in huge river deltas such as the Mississippi delta.
All Rossendale’s rocks are layered or “stratified”. This is because over the years conditions changed leaving different layers in the sediment. Sometimes the sediment would be mud, whilst at other times it would be sand. There were even times when pebbles were washed down to form a layer of pebbles mixed with sand. The different layers eventually turned into rocks with different properties. Coarse grained sedimentary rocks, perhaps containing pebbles, are known as gritstones, but in the past have been called ‘grits’ leading to the name Millstone Grit; medium grains equal ‘sandstone’; finer grains give rise to ‘siltstones’. The finest grained sedimentary rocks were once mud and are often dark coloured. Formerly known as ‘shale’ they are now referred to as ‘clayrock’. Typically the rock sequence is alternating layers of strong brown sandstones, softer dark shales (clayrock) and occasional gritstones.
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.
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. These situations are quite common in sedimentary basins and in regions flanking active mountain belts. Sediment derived from the erosion of the mountains is laid down in successively younger layers according to the Law of Superposition.
Attitude, Strike and Dip
The simplest form of rock deformation is inclined bedding, which occurs when strata is subjected to stress (either compression, tension or shear) and they become tilted relative to their original (horizontal) disposition.
While the concept of tilting is easy to visualize, describing the orientation of the inclination is not so easy to do. Consider that the beds may be inclined in any direction and in any amount from 0 to 90 degrees. Beds may even be overturned. The term attitude describes the orientation in space of rock strata, but it is not acceptable to simply conclude that beds are inclined. You must give some indication of the direction of tilt and the amount of inclination
Geologists long ago came up with a means of orientating bedding that has been deformed. Two components are required in this scheme; (1) the direction of the inclination (strike), and (2) the amount of inclination (dip). The strike isusually reported as a measurement of compass direction. Due north is considered to be 00° , due east is 090° , due south is 180° etc.The dip is reported as degrees measured downward from a horizontal plane. By convention, the dip of an inclined bed cannot exceed 090 °. Beds with dips of 090° are said to be vertical (or to have a vertical attitude). Beds that have been overturned have technically been rotated more than 090 ° , but their dips are still reported. Dip refers to the downward angle at which the tilted beds now lie. A dip of 90 degrees is vertical.
So why have I brought you here? Well first you have to get here, you are not looking for a massive rockface here, just an outcrop at the side of the stream, you may need to cross the stream to get here, be careful it is wet and muddy around here. Here there is a fine example of inclined strata, one which you may not expect.
So it being an earthcache, there are some tasks to do, please do not include them in your log.
(1) Please measure the angle of dip of this rock outcrop, what is it in degrees? Remember, the dip is reported as degrees measured downward from a horizontal plane.
(2) Is this sedimentary rock, why?
(3) Now tell me how wide any layers are?
(4) What does the rock feel and look like, is it all the same?