Chevin Forest Park GeoTrail #9: Cross-Bedding
The 9th cache in the series and the 4th of the 8 EarthCaches, is located towards the top of the northern slopes of The Chevin in a fascinating area of tumbled boulders and crags revealing some fascinating surface features such as those depicted on the moss-covered carved Geology Trail marker stone #5 at the published coordinates.
Please refer to GCB7RA7 for the background information on The Chevin's geology, which explains how and when the various rock formations were formed.
To Log the Cache please complete the following EarthCache tasks:
Location 1: proceed up the slope to N 53 53.602 W 1 42.194 just below the ridge where you will be standing in front of a moss-covered Doubler Stones Sandstone boulder with some prominent ridges on it highlighted by the moss. Look at the rock face and . . .
a) Describe its appearance and explain briefly how the lines/ridges are formed.
b) Estimate the angle of the ridges to the horizontal.
c) What was the direction of flow of the river?
Location 2: return directly down the slope to N 53 53.609 W 1 42.194 at the side of a smaller isolated boulder. Examine the top moss-covered surface.
a) Describe its appearance and estimate the angle through which this rock has rotated from its original position.
b) Describe briefly where the rock came from and how it ended up here.
Location 3: now look some 15m west to another isolated similar sized boulder @ N 53 53.608 W 1 42.203 with an interesting east-facing (towards you) rock face.
a) What are the lines you can see and what is their angle to the horizontal?
Please submit your answers to me using the GC messaging service or by email to forshaw.chris@gmail.com – thanks!
The gritstone layers forming The Chevin were deposited by a river with strong and shifting currents. This produced distinctive cross-bedding which can be seen in many gritstone outcrops such as the excellent example here.
This was formed in a fast-flowing river channel– such as during flood flows which are able to carry large amounts of sediment (mud and sand) which at times of lesser flow leads to the formation of ripples or dunes.
Cross-bedding is formed by the downstream migration of such ripples or dunes in the river channel. The fluid flow causes sand grains to move up the stoss (upstream) side of the bedform and collect at the peak until the angle of repose is reached.
At this point, the crest of granular material (sand grains) has grown too large and will be overcome by the force of the moving water, causing it to tumble over the edge and fall down the lee (downstream) side of the ripple or dune. Repeated avalanches will eventually form the sedimentary structure known as cross-bedding, with the structure dipping at the angle of repose in the downstream direction of flow.
Each set of cross-bedding has subsequently been eroded and washed away by another flood of water, so the top of each sand bank has been removed, this truncating (cutting off) part of the cross-bedding and leaving a clear boundary in the rock strata.
Links: See here for a short video of cross-bedding in Harrogate sandstone. See here for an excellent well-illustrated paper on Fluid transport processes and sedimentary structures related to fluid flow. See here for a great video on sedimentary structures formation.
In this area there are many fallen blocks of sandstone below the ridge which most likely became detached when The Chevin lay below ice during the last glacial maximum around 17,000 years ago. Water under the ice sheet penetrated into cracks and joints in the rock and widened as it froze, weakening the rock structure.
When the ice finally melted at the end of the ice age around 14,000-12,000 years ago, the then unsupported rocks were let down on to the underlying slope. Depending on the shape & position of a boulder, the water content (affecting slipperiness) of the underlying rock layers and the steepness of the slope, many then slipped, slid or even tumbled down the slope to eventually come to rest in various different positions compared with their starting orientation.
