As you walk around Manchester, there is a lot of stone to see, most of it in the form of buildings, and you would not really think that there is much to see underfoot. But go to some places and you can actually see the native bedrock of the area. Talking about bedrock, you won't see Barnie or Fred or even Wilma here, we are talking about a different type of bedrock. The co ordinates take you to a spot near the city centre where you can see, and touch the native bedrock.
So what can you see, well it is sandstone.
Sandstone (sometimes known as arenite) is a clastic sedimentary rock composed 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. Since sandstone beds often form highly visible cliffs and other features, certain colors of sandstone have been strongly identified with certain regions. The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation,either from water (as in a stream, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water or ground surface (e.g., in a desert). Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.
Here we can see Collyhurst Sandstone, a type of soft red sandstone. It is a fine to medium grained sedimentary rock, created from desert sands blown into dune formations during the Early Permian period when the area which now constitutes the UK was within the desert belts to the north of the equator.
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, other forms of bedding or stratification occurs:
(a) Cross Bedding is a feature that occurs at various scales, and is observed in conglomerates and sandstones. It reflects the transport of gravel and sand by currents that flow over the sediment surface (e.g. in a river channel). sand in river channels or coastal environments. Cross-bedding can also be produced when wind blows over a sand surface and creates sand dunes.
When cross-bedding forms, sand is transported as sand-dune like bodies (sandwave), in which sediment is moved up and eroded along a gentle upcurrent slope, and redeposited (avalanching) on the downcurrent slope (see upper half of picture at left). After several of these bedforms have migrated over an area, and if there is more sediment deposited than eroded, there will be a buildup of cross-bedded sandstone layers. The inclination of the cross-beds indicates the transport direction and the current flow (from left to right in our diagram). The style and size of cross bedding can be used to estimate current velocity, and orientation of cross-beds allows determination direction of paleoflow.
(b) Graded Bedding, this is when the the grain size within a bed decreases upwards. This type of bedding is commonly associated with so called turbidity currents. Turbidity currents originate on the the slope between continental shelves and deep sea basins. They are initiated by slope failure (see diagram below), after sediment buildup has steepened the slope for a while, often some high energy event (earthquake) triggers downslope movement of sediment. As this submarine landslide picks up speed the moving sediment mixes with water, and forms eventually a turbid layer of water of higher density (suspended sediment) that accelerates downslope (may pick up more sediment). When the flow reaches the deep sea basin/deep sea plain, the acceleration by gravity stops, and the flow decelerates. As it slows down the coarsest grains settle out first, then the next finer ones, etc. Finally a graded bed is formed. However, decelerating flow and graded bedding are no unique feature of deep sea sediments (fluvial sediments -- floods; storm deposits on continental shelves), but in those other instances the association of the graded beds with other sediments is markedly different (mud-cracks in fluvial sediments, wave ripples in shelf deposits).
The underlying geology of Greater Manchester is dominated by rocks from three main periods from the geologic time scale :
1. The Carboniferous, which is a period that extended from 358.9 million years ago, to 298.9 million years ao. The name Carboniferous means "coal-bearing". During this time, sediments, mainly sands, silts and muds, were eroded from hills in an area that now includes Scandinavia and Greenland and were swept in to 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.
2. The Permian ( which extended from 298.9 to 252.17 million years ago), is when the world at the time was dominated by a single supercontinent known as Pangaea, surrounded by a global ocean, and Triassic (which extended from 252.17 to 201.3 million years ago), when the global climate was mostly hot and dry, with deserts.
3. The Quaternary, which is the current and most recent geologic period, and iss typically defined by the cyclic growth and decay of continental ice sheets.
Most of Manchester, and its suburban fringe to the south, is located on Permian sandstones and red Triassic sandstones and mudstones, mantled by thick deposits of till and pockets of sand and gravel deposited by glaciers at the end of the last glacial period some 15,000 years ago. The oldest rocks, from the Upper Carboniferous period, are sandstones and shales of Millstone Grit present as outcrops and uplands in the north-east of Greater Manchester, such as the upland moors of the Dark Peak and South Pennines to the east and northeast of Rochdale, Oldham and Stalybridge. These rocks are overlain by shales, mudstones and thin coals of Coal Measures upon which the towns of Oldham, Rochdale, Bolton and Wigan are located.
This being an earthcache, I ask that you complete a series of tasks in order to log the cache. Please do not include your answers in your log.
1. Please describe to me, what the stone feels like, and its colour (is it the same through out).
2. How high is this area of natural stone?
3. How do you think that this stone was depositied, what would have the environment been like?
4. Look at the stone, is it all the same, or is there evidence of other stones, what are there, why do you think that they are there?
5. Can you see evidence of stratification? If so, is the strata all at the same level? If not what angle is it at?
6. Is there any evidence of cross bedding, what is the rationale behind your decision?
7. From which geological period is the stone?