The Gap in the Edge
This is an EarthCache. There is no container to find at the co-ordinates given. However, to claim this EarthCache, you must first visit this location, read the earth science lesson below and complete the tasks. This EarthCache is concerned with the gap in the Lincoln Edge formed by the River Witham.
The River Witham cuts through the limestone escarpment at Lincoln
The
Lincoln Edge, also known as the
Lincoln Cliff is a remarkable stretch of high ground that runs through the districts of Lindsey and Kesteven in central Lincolnshire. It is a prominent landscape feature in a generally flat portion of the county. It runs for 50 miles, its from close to the Humber and down to Grantham. However, on a larger scale the Lincoln Edge is a continuation of the Cotswolds and then continues on into the North Yorkshire Moors. Its height varies but is typically 200 feet above sea level, but it still towers over the fenland to the east. The Lincoln Edge forms a natural boundary between the two sides of Lincolnshire, between the fen and marshland out to the North Sea coast, and the farmland merging into the Midlands up to and beyond the River Trent. It is believed that an ancient, possibly prehistoric, route ran along the top of the edge, which has been labelled "the Jurassic Way". More can be learned of this edge by completing another local EarthCache,
'Lincolnshire Edge'
Earth Science Lesson
This EarthCache is not so much concerned with the edge itself, but rather the gap in it, which you are standing at the lip of. The City of Lincoln stands where the River Witham punches a gap in the Lincoln Edge, which explains the city's steep streets, running down to and up from the riverside.There are actually two gaps in the Lincoln Edge, at Lincoln and at Ancaster. They indicate the former course of a primitive river which became the Trent. The river flowed from the Nottingham area towards the North Sea via the Vale of Belvoir and the gap in the ridge at Ancaster. At a later stage it used the 'Lincoln Gap' before assuming its present northerly course to join with the Yorkshire Ouse to form the Humber. The watercourse which remained flowing this route is what we now know as the Witham.
The Lincoln Edge itself is formed of a series of relatively resistent sedimentary rocks, mainly Lincolnshire Limestone, which dip gently to the east. However, the River Witham has slowly eroded the limestone edge, forming this gap. There are several different types of limestone, with varying degrees of resistance to erosion by water. Where water flows over or through a weaker rock, it cuts into it, forming a groove, leading to a river valley. This same process is also what causes waterfalls to form, with the stronger rock remaiming in place and the weaker rock being eroded to form a step in the river bed.
Types of Limestone
There are many types of limestone to be found, each with it's own qualities. here are some examples:
- Chalk: The famous White Cliffs of Dover consist of chalk, a type of limestone. The skeletons of small algae called coccoliths, deposited over thousands of years, became the white mud chalk that made the cliffs. Even though you can't see these small skeletons without the help of a powerful microscope, upon careful examination you might find skeletons and fossilised remains of ammonites, sea urchins, shells and sponges.
- Animal Shell Limestone: Animal shell limestone can include crinoidal limestone, as well as samples with broken or complete shelly remains. Crinoidal limestone comes from crinoids, an ancient form of marine life called sea lilies because of their resemblance to a flower. Even though they resemble a plant with a long stem, consisting of disc-like pieces stacked atop each other and attached to the sea floor, and wavy leaf-life arms, they represent another form of marine life whose fossilised remains became limestone. You can often find segments of the crinoid's stem in this type of limestone, looking somewhat like a backbone.
- Oolitic Limestone: This stone is from a marine sediment formed in a warm shallow sea and made up of sand-sized egg-shaped “ooliths” in a calcite matrix. The ooliths form around small grains, such as shell fragments which gather concentric layers of calcium carbonate (by precipitation) as they are rolled around the sea floor. Their colour varies from pale cream/buff, pink (like streaky bacon) and blue-grey. When you look closely at the limestone with a hand lens or loupe, you should be able to make out the tiny spheroid particles held tightly together. While some rocks are packed with ooliths, others may have very few, or even none in a particular small sample.
The Landscape
From where you are standing, the landscape slopes sharply away from you towards the river itself, and then rises again beyond the river valley as the edge resumes its shape beyond the bounds of the city. The City of Lincoln thus sits on one slope of the eroded edge, and the village of Washingborough on the other. As the running water of the Witham eroded the limestone edge, it cut a gap, growing deeper and deeper over geological time, forming a sharp V shape. This V shape is distinctive of the cross section shape of a river valley, as opposed to one formed by glaciation, a U-shaped valley. Erosion processes such as Hydraulic Action (the force of the water) and Abrasion (where the river rubs stones that are being transported against the bed of a river thereby breaking it down) are what widen and deepen the valley.
EarthCache Tasks
In order to claim this EarthCache you must first complete the following tasks and questions, and submit your answers to me. You can claim it before awaiting a respinse from me. However, if I do not receive a timely submission, I may delete your log. I ai to respond, however briefly, to all submissions.
1.(a) In which approximate direction does the Lincoln Edge run?
(b) Where it cuts through the edge, in which approximate compass direction does the River Witham lie? (Eg North to South, West to East)
(c) With the North Sea to the east, and the peaks of the Pennines out of view in the west, as you look at it, will the Witham be flowing to the left or to the right?
2. Look across the gap to the higher ground where the edge resumes. Use your GPSr, smartphone app, or other suitable device to meaure the difference in height between where you are standing and the River Witham.
3. In the park before you, look for samples of rocks. Look carefully at one of them.
(a) Examine its colour, weight and texture; is this rock crumbly and brittle? Can it easily be broken or marked?
(b) Using the decriptions above, what type of limestone is this?
(c) Using your observations about its hardness, tell me why the river was able to cut a gap in the edge at Lincoln?
4. (Optional) Post a photo of yourself (or part of yourself), your GPSr, or a sign of your own making, which shows at least one lip of the gap in the Lincoln Edge.