This earthcache takes you to an old quarry on Penistone Hill, which sits high above Haworth in West Yorkshire. Haworth is famed for the Brontes, but look around and there is much more to see, and much further back in time. West End quarry was worked from the 19th to 20th century. The rock that you see 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.
This type of sandstone is Woodhouse flags, which was laid down during the Marsedian age of the Carboniferous period.
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.
What have we got here?
The rocks you can see on the cliff face in front of you are made of sandy sediments deposited in a tropical river delta when the Haworth area was located near the equator on a large continent. Layers of sediments were laid upon one another and then compressed over a long period to create solid rock.The lines on the rock face run in two directions. The horizontal lines are called bedding planes and show that there were times when the river flow slackened and sand deposition temporarily ceased. Between some of the bedding planes are other lines which slope at a gentle angle and these represent sand banks in the river channel. This feature is called cross-bedding.
Cross-bedding is commonly formed in a river channel in which sand grains are being rolled along the bottom by fast flowing water.
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). 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.
Cross beds can tell geologists much about what an area was like in ancient times. The direction the beds are dipping indicates paleocurrent, the rough direction of sediment transport.
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. Please estimate the height of the rock face.
2. Where can you see bedding planes which are near horizontal (top, middle or bottom)?
3. Where is the cross bedding more evident (top, middle or bottom)?
4. How does the cross bedding appear, is it straight or bending in places?
5. Do you think that there is planar lamination here, why?
6. What does the sandstone feel and look like?
7. In what direction did the paeloflow go, was it right to left or left to right?