Many people drive along the A56 either north and south, and do not bat an eyelid about what they pass. Most of the time they are eyes to the road, foot to the floor. But to the side of the southbound A56 is an old quarry. It is known as Crutchmans' Quarry. On the 1848 OS map, there is a quarry shown here, with the area to the south known as Black Moss. To get to the cache site you need to enter the old quarry, you can either access it over a stile from the layby on the A56, or from the bridleway known as the Kings Highway. DO NOT APPROACH THE SITE FROM ABOVE, as there is a large shear drop into the quarry bottom.
The bedrock here 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.
The sandstone that we see here is known as Milnrow Sandstone, also known as Crutchman Sandstone. It was laid down 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.
To be more specific this sandstone is from the Langesttian substage of the Carboniferous Period. It was part of the Westphalian Stage, a stage between roughly 313 and 304 million years ago, which is a subdivision of the Carboniferous Period. The Westphalian is named for the region of western Germany where strata of this age occur. The Coal Measures of England and Wales, are largely of Westphalian age. The Westphalian is divided up into four regional substages. From oldest to youngest these are Langsettian, Duckmantian, Bolsovian, and Westphalian D.
The sandstone 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.
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.
Planar Lamination is variation in the colour, composition, grain size in surfaces parallel to bedding, but within a bed They typically represent variations in flow velocity, sediment supply, sediment composition.
Bedforms, When sediments get deposited from turbulent flows, the sediment interacts with the geometry of the flow. Depending on the flow speed, turbulence, and sediment characteristics, different structures or bedforms develop.
Bed Geometry and Flow Separation, Until now, we have been implicitly assuming that the beds are flat, but they are not. If you start with a flat, smooth bed of sand and increase water speed above it, irregularities form from irregularities in the flow and develop into ripples. First, a few grains pile up. Once the height of the pile is several grains high, there is a flow shadow down stream of them, and the viscous sublayer detaches from the base of the flow. The water has enough momentum that it does not hug the bed surface and instead, goes shooting out over the top. This point is called the separation point. The water flows forward and downward and reconnects with the bed at the attachment point. At the attachment point, water is flowing directly towards the sediment with a lot of force. This force moves the grains and causes erosion. In contrast, the area between the separation point and the attachment point has very low flow. Thus, sediment transport is very irregular along the bedding surface at a local scale.
Sediment Transport Over a Ripple, sediment grains are mobilized at the attachment point - more so than in normal flow because the water is shooting directly into the sediment- and the grains are moved downstream by saltation and traction. As the flow becomes parallel to the sediment surface again, its ability to transport sediment decreases. Thus, the grains tend to pile up and a new mound forms. This gives a periodic chain of mounds - the beginnings of ripples. As flow continues, grains roll and saltate up the stoss (upcurrent) side of the ripples. Once they pass the crest, they reach the low flow on the lee side of the ripple. The larger grains settle out and roll partway down the slope; this is the site of net deposition. As the process of deposition on the less side followed by erosion on the stossside continues, the ripples migrate downstream. If there is net deposition of sediment, the ripples leave behind distinctive dipping layers between two erosional surfaces that can be preserved in the rock record. These layers slope downstream and are one type of cross lamination.
Bedforms and Flow Velocity, the size and shape of subaqueous bedforms depends on flow strength and grain size and can be used to interpret ancient flow characteristics in a depositional environment from looking at sedimentary rocks. The minimum flow for ripples is determined by the minimum velocity for sediment transport. Once this flow speed is reached, ripples form if the sediment is transported as bedload. The maximum flow speed for ripples depends on the location of the attachment point on the stoss side of the ripples. As flow gets faster, too much erosion occurs at the crests of the ripples - the point of attachment is too far up the stoss side of the ripple- and the ripples flatten out.
Planar/Flat Lamination, planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed.There is a zone of planar lamination in coarse grained sediment at low flow.
Bedforms and Grain Size, Bedforms also vary with grain size. Very fine sand and silt are very easy to transport and erode. They form nice ripples, but do not form dunes when transported by water. Instead, ripples transition into planar laminae. Coarse sand and larger sediment is too hard to transport and erode to get ripples.
Why are the rocks at an angle?
Look around you, and you wll notice that the rocks have a definite downward slope, so what has caused this? It is known as Fault Drag.
What is a Fault?
A fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates. Energy release associated with rapid movement on active faults is the cause of most earthquakes. A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is the intersection of a fault plane with the ground surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault. Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane. The two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the hanging wall occurs above the fault plane and the footwall occurs below the fault. An unnamed fault has been mapped trending north -eastwards from New Houses to a point where it terminates against an unnamed fault. The fault throws up to 36 m to the south-east, throwing the Milnrow Sandstone to the south-east down against the Icconhurst Sandstone to the north-west in the Warmden Clough area. Warmden Clough is the stream which flows through the quarry site, and under the A56.
Before talking about Fault Drag, it is important to understand Folds.
A geological fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Many folds are directly related to faults, associate with their propagation, displacement and the accommodation of strains between neighbouring faults.
Drag folds are one of the simpler types of fold. They occur in conjunction with faults, and they represent the bending of rock before it breaks.It may seem, intuitively, that the rock in a drag fold is bent by being dragged along the fault surface—hence the name—but in fact drag folding must precede the actual breakage of the rock in a fault. Like everything else, rocks prefer to bend first before they break. In most cases, the direction of folding is toward the direction of movement on the fault.
So it has been a lot to read, but it being an earthcache there is a couple of questions to answer. Please send me the answers and do not include them in your log.
(1) What is sandstone commonly composed of? What type of rock is it?
(2) During what geological period, stage and sub stage was the rock here laid down?
(3) Please describe the different layers of strata, what is their minimim width and maximum width here?
(4) Are there bedding planes here? If you think so, what is the rationale for your answer?
(5) Is the rock different in seperate stratas, or is it the same, why?
(6) Please look at the grains in the sandstone, how will their size affect the appearance of the strata, and at what speed do you think the sand was laid down, can you see evidence of rippling, or planar lamination?
(7) At what angle is the strata tilted at?
(8) Do you think that the fault is to the west or east, why?