THIS IS A REMOTE CACHE, WHICH TAKES EFFORT TO GET TO.
IT IS ROUGH UNDERFOOT.
DO NOT APPROACH FROM ABOVE.
Easegill, and the area of Leck Fell is known for its caves, and limestone features such as Easegill Kirk and Cow Dub, but there is much more to see, and learn about. This EarthCache takes you to a sandstone cliff at the side of a waterfall. You need to be on the north side of the waterfall. You will need to cross the stream to get to the location. You will get wet feet.
DO NOT ATTEMPT IF THERE ARE HIGH WATER LEVELS IN THE STREAM.
So what is sandstone?
It is a sedimentary rock.
The formation of sandstone involves two principal stages:
(1) 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).
(2) 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 most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried. Colours can be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colourant is iron oxide, which imparts reddish tints ranging from pink to dark red, with additional manganese imparting a purplish hue.
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 laid down, and can occur on the scale of hundreds of meters, and down to submillimeter scale. It is a fundamental feature of sedimentary rocks.
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.
Now you would think that strata would be laid down in a flat horizontal manner, with level layers, but it is not always that simple, at times the strata can be tilted, folded or cross bedded.
Here, at the EarthCache site we have an example of cross bedding.
So what is cross bedding?
Well first of all look at the rock face,you will notice lines on it, which are not all going in the same direction, and some are not flat. Where the strata is not level or horizontal, 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.
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.
Bed Geometry and Flow Separation. As decsribed above, we cannot implicitly assume that beds are flat. 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.
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 how high the rock face is at the EarthCache location.
2. Please describe the rock face to me, the colours present, any structures visible, any fossils present, and the grain size.
3. Is there any planar lamination evident? If so, what would that tell you about the current flow?
4. At what angle does the cross bedding lie at in degrees, from the horizontal?
5. Look at the cross bedding, what direction was the paleoflow, if this cliff face was a perpendicular cross sesction through the cross bedding, please explain the reason for your answer?
6. At what height above the floor is there cross bedding on the rock face?