Lucky 7 - 6. Earth EarthCache

This EarthCache is sited in the famous Sherwood Forest, on the edge of one of the finest examples of the typical old forest (now a SSSI). The reason that the forest here still exists is due to its unusual geology. The rock under the surface is what used to be known as Bunter Sandstone - now known as the Sherwood Sandstone Group.

Old Sherwood extended from Nottingham in the South to within a couple of miles of Worksop in the North, to the outskirts of Chesterfield in the West and to Southwell and Laxton in the East. It was always a scattered forest punctuated by broad tracts of heath. The infertile sandstone underlying the area has been largely responsible for the salvation of the woodlands, as it prevented them being cleared for agricultural use.

Large tracts of sandy heathland, dominated by heather, were once typical of the Forest of Sherwood and remnants are found within more open areas of the SSSI and Nature Reserves just north of here. The nightjar is often heard 'churring' eerily across the forest heaths at dusk during the summer, whilst the tussocky grass-dominated heaths are particularly important for ants and spiders and are favoured feeding areas for green woodpeckers.

According to Natural England;

"Soils are derived mainly from the underlying Sherwood Sandstones and in part from glacial deposits resulting in nutrient poor, light pebbly soils which are mainly acidic and excessively free draining and often only comprise a very thin humus layer overlying the mineral deposit. The sandstone is free draining to precipitation and surface water which is able to percolate quickly to the Sherwood aquifer some depths below. Surface water is not found on site and the water table is currently 15-20m below the surface."

The sandstone here is very loose and crumbly, hence the ability to drain water quickly, and quite a deep red colour.

Sandstone is often considered one of the weaker rocks. When you rub it, grains may come away or in some circumstances it can crumble very easily - this depends on the cement that holds the grains together. When sandstone is used for buildings or carvings it often gets weathered quite easily. The sandstones in this area would be likely to be very weak and crumbling, so this would be a good way to spot a sandstone here.

There are outcrops of Sherwood Sandstone in Mansfield and along the Robin Hood Way at Welbeck, but there is limited sign of it at the surface here.

At various places close by (Creswell Crags and Pleasley Vale for instance) there are prominent outcrops of an often crumbly, fissured, and sandy rock that looks like a type of sandstone, but is in fact Magnesian Limestone. This sand-coloured stone has been quarried locally for use in buildings; many local buildings but most notably the Houses of Parliament (the stone was from Anston Quarry, just to the north west of here). When quarried this has a smooth, fine grain but is susceptible to weathering - hence the crumbly and cracked appearance in the outcrops and the tendency for the cracks to make homes for plants. Further west, in the Peak District, there is also a great deal of sandstone. But much of that is that is Gritstone, a hard, coarse-grained, siliceous sandstone, more grey in colour and much more compact - so much less permeable to water. The Peak District Gritstone is also known as Millstone Grit; due to its rough hardness it was found very suitable for millstones (for grinding wheat in watermills or windmills).

At the coordinates is a large block of stone, serving as a focus for the EarthCache logging questions (and also to prevent illegal vehicular access to the track!).

Logging Requirements (Questions to Answer)

All the information you need to answer the questions can be found in the listing (and by studying the boulder), and they don't require any previous knowledge of geology. Please send in your answers (by email or using the geocaching Message Centre) before (or soon after) logging your find - I only ask you make a reasonable attempt. If no attempt is made, you risk your log being deleted. There are four questions but the answer to each should be short, making them quick and easy to answer. They don't have to be correct; the idea is to make you think about the subject.

Question 1 - General overview

Look at the boulder and try and describe the make up of the rock. Mentioning, for example, any fossils observed, the size of the grain, dominant colours (ignoring vegetation or lichen).

Question 2 - Permeability

Would you expect water to soak through this rock, or mostly sit on the surface?

Question 3 - Hardness

Rate the hardness of the rock on a scale of 1 to 10 where 1 is very soft and crumbly, 10 is very hard. This is a visual question, you will be able to answer this question by looking.

Question 4 - Final analysis

After reading the information in the listing and making your analysis by answering the previous questions, do you believe this rock to be Sherwood Sandstone, Magnesian Limestone, or Gritstone?

Please feel free to include a photo taken nearby, but don't give away any answers to the questions. Thank you for visiting this EarthCache.