4 Limestone cliff
Approaching Task Site 4
Task Site 4 is a cliff of Black Rock Limestone. The conditions here around 350 million years ago were marine, with a coastline several kilometres to the north similar to that of the Red Sea coast of today. Look at the ‘plan’ view (return to map screen) and the ‘then’ section. The land to the north at this time has been named the Midland Platform.
Massive limestone beds
This limestone is known as the Black Rock Limestone. Black Rock Limestone has and still is widely quarried in the area. It forms many of the steep cliffs you see along the River Wye. The parallel units of rock are called beds. The rock beds towards the base of the cliff are very thick.
In the Forest of Dean this limestone has been named the Lower Dolomite and is currently extracted mainly for roadstone in large quarries just west of Coleford and at Drybrook. Specifically the British Geological Survey now names this rock the Barry Harbour Limestone, which is in turn a member of the Black Rock Limestone Subgroup, which in turn is part of the Pembroke Limestone Group – bit like dividing a book into pages, sections, chapters, etc. It’s complicated! But don’t worry, just remember that here this rock is called Black Rock Limestone.
Thinner beds ?
Towards the top of the cliff the beds appear to become much thinner, but this is probably just jointing (cracking of the rock) caused by weathering near to the soil surface.
The joints in limestones are opened up over time by the action of water dissolving away the limestone. Rainwater absorbs carbon dioxide from the air, to form a weak acid (carbonic acid). As the acidified rainwater runs over and drains through the limestone, the acid will dissolve the calcium carbonate of which much of limestone is made. You have seen this effect at site 2 – the Limestone Pavement – on a bigger scale.
Is this rock a limestone?
Weak hydrochloric acid dropped on this rock, shows no fizz. The rock is called a limestone – the Black Rock Limestone – so what is happening? The limestone here is actually a dolomite. Dolomite formed when over time the calcium of calcium carbonate is replaced by magnesium. You would also be less likely to find fossils in dolomite, as their structure is often destroyed as the calcium is replaced by magnesium.
In the picture notice the sugary texture, which is a characteristic of dolomite. The pink staining is another characteristic of the dolomite in this area. The official British Geological Survey name for this rock is the Barry Harbour Limestone which is a member of a ‘group’ called the Black Rock Limestone Subgroup (‘Black Rock’ after a place in the Mendips). Here in the Forest of Dean and Wye Valley area, publications will often still use the original name of Lower Dolomite for this rock.
Spot the lone fossil
Although we have just said fossils are normally destroyed in dolomites due to the replacement of calcium by magnesium. In the rock here there is a fossil visible – it is very difficult to spot, you will need to look above head height. The fossil is a type of echinoderm. They are rare, but see if you can find anymore.
Echinoderms include the ancient crinoids and creatures we find today like star fish and sea urchins. This fossilised Carboniferous echinoderm had a mouth at the top of the tubular body, with two paddle like appendages that directed food into the mouth. You can see these ‘paddles’ on the left and right sides of the main body a few millimetres below the head end.
Beds you will be crossing
This shows the rock beds you will be crossing in the rest of the Voyage. You will be moving ‘backward’ in time by around 58 million years from the time of the Black Rock Limestone you are on at the moment. Notice that as you walk to the west, you are moving onto older rocks, because the beds dip eastwards. If instead the beds dipped to the west and you walked eastwards, would you be walking back or forwards in time?
Gaps in time.
The beds you will be walking over do not necessarily include beds that represent all the time over that 58 million years. In fact there is a gap of several million years between the top of the Brownstones and the bottom of the Quartz Conglomerate. Such a time gap is called an unconformity (or disconformity as the beds are at about the same angle), when either no sediments were deposited in the area, or if they were, they were eroded away before the Quartz Conglomerate was deposited.
Dip of rock beds
This diagram shows a group of rock beds dipping downwards. The angle the beds slope down from the horizontal is called the angle of dip of the rock beds.
A line along the bed surfaces at right angles to the direction of the dip, is called the strike.
The strike is usually measured in so many degrees from north. The angle of dip in the number of degrees from the horizontal.
The rock beds in the area of this Voyage are generally dipping at around 15 degrees towards the east. That makes the strike as north to south, i.e. zero degrees from north.
Towards Task Site 5
The path curves around through a deep valley cutting down through the hill side. You will find a wooden bench here – a good place to take a break. Under Health you can Save and pause the Voyage, to stop your supplies being depleted whilst you take a break.
As you walk into this area (when using the app) you should get an ‘Environment Change’. As always check out this environment under ‘then’ so you are ready to cope with any threat.
Task Site 4 Questions
What can you say about the bed thickness here?
a) all less than 20cm thick
b) all less than 1m thick
c) some more than 1m thick
What is the name given to the rock at this site?
a) Avon Group
b) Black Rock Limestone
c) Gully Oolite
Since the name of the rock includes ‘limestone’ does this rock fizz with acid?