2  Limestone Pavement

Over stile and turn right

After crossing the stile turn right to Task Site 2 and the limestone pavement. The route through the rock here was opened up by blasting of the rock around 1833 to form a carriageway, as part of an elaborate landscaping of the hill undertaken by the ironmaster and MP Richard Blakemore, the then owner of the Wyastone Leys estate and Deer Park. His activities caused much damage to the Iron Age hillfort site.

 

 

Route to pavement

Task site 2 requires you to look at the low rock face beside this path and then carry on to the limestone pavement. Stop and look at this small vertical face. Notice how many small fractures there are in it, that run both vertically and horizontally. These are not faults, but joints in the rock. Limestone easily cracks near the ground surface and it also dissolves in rain water, so the cracks get wider and wider. Remember, you are still beneath a shallow warm sea. The limestone here is called Gully Oolite the same as that at the caves.

Rain water is a weak acid. As shown in the next picture, acids attack limestones. The chemical reaction between the acid and the limestone, creates fizzing. The fizzing is due to escaping bubbles of carbon dioxide gas. The higher the calcite content of a rock the stronger the fizzing. On a rock like sandstone which contains no calcite, there will be no fizzing. Some sandstones will fizz, this will be due to calcite acting as the cement between the sand grains.

 

 

Limestone reacting to acid

A drop of weak hydrochloric acid applied to a piece of limestone. The fizzing is due to the escaping bubbles of carbon dioxide gas, produced by the chemical reaction between the acid and the limestone.

Rain water is a weak acid, much weaker than the hydrochloric acid used in the picture. With rain water, the process of attacking the limestone is just slower, but over time it can have a big effect. When the rain water drains through the gaps it can form underground rivers which wear away the limestone even faster and caves can form.

 

Fractured rock

This is a close-up of a typical area of this fractured rock. The fracturing occurs when the rock is subjected to tectonic forces, when the Earth’s crust is put under enormous pulling-apart forces caused by the endless slow movement of the Earth’s tectonic plates.

Up on the limestone pavement you can see how gaps like this have been significantly widened. Mainly be the action of acidified rainwater. Rainwater is acidified by dissolving carbon dioxide from the air, which converts it to weak carbonic acid, and by mixing with the acids produced by plants growing in the thin soils covering the beds of limestone.

 

The limestone pavement

Carry on walking along the path and up into a field on the top of the low the cliff. Here you can see the surface of the limestone with weathered joints. This is an example of a limestone pavement. The inset picture shows a much larger example of a limestone pavement in Yorkshire (above Malham Cove). Zoom the picture to see it fully.

The limestone is dissolved away along the joints not only by the weakly acidic rainwater, but also by acids produced as vegetation decays.

 

Clints and Grikes

The chunks of limestone not dissolved away are called clints, while the dissolved away places are called grikes.

Limestone beds pass under large areas of the Forest of Dean and the Wye Valley and numerous underground cave passages have been explored and others thought to exist. All these cave passages are produced by the gradual dissolving of limestone by acidified waters, which start attacking the limestone along the cracks of joints and faults.

Grike enlarging

Notice how plants are growing in the grike. The acid produced in the soil accelerates the limestones dissolution.

It is not difficult to imagine grikes like this forming into larger hollows, which could be entrances to a network of underground caves. In some parts of the Forest of Dean the hollows in the Gully Oolite (old name Crease Limestone) have been filled by iron deposits. These deposits were extensively mined in the past leaving behind strange pitted landscapes where the hollows are called scowles. A good example can be visited at Puzzle Wood near Coleford. The Puzzle Wood scowles have featured in several films.

Cleaned limestone pavement

This section of the limestone pavement was cleared of excess vegetation and cleaned in October 2019 by a group of volunteers with the permission of the Woodland Trust. The site provides an excellent place to closely examine the surface features of the limestone. The joint is forming a large grike.

 

Fossil coral

Coral visible on a limestone surface.

 

Entrance to Task 3 Site

Go back to the track return towards the stile and follow the broad pathway around into Task Site 3. The rock you are on now and at Task Site 3 is called Gully Oolite. Have you checked on the environment at the time? What were the threats? What factor suncream would you need here 351 million years ago?

 

Task Site 2 Question

What are big flat areas of jointed limestone called?

a) Fossil Pavement
b) Limestone Table
c) Wave cut platform
d) Limestone Pavement