8 Rocks full of pebbles
This rock is a conglomerate
What has formed this imposing feature? This rock is a very good example of a conglomerate, which in this case is very resistant to erosion. The pebbles (many are white quartz) are strongly cemented into the surrounding sand, which is termed the matrix of the conglomerate. The pebbles themselves are termed the clasts of the conglomerate.
Many of the pebbles are more than 5cm across. To move these, the water flow that deposited them, around 365 million years ago, must have been very powerful. Some of the beds here might have been deposited in catastrophic flood events, others by slower moving water (the ones which show cross-bedding). You would not want to be standing here 365 million years ago in the way of these rivers!
Pebbles become rounded by being bounced against one another as they are moved along in the flow of water in a river or the sea. In the case of a river the more rounded they are, the further they are likely to have travelled.
These pebbles could well have set out as lumps of quartz eroded from a vein of quartz in some upland area, perhaps hundreds of kilometres from where they were finally deposited on the bed of this river 365 million years ago.
When rocks are a reddish-brown colour, it often means they were deposited on land (terrestrial deposits) as against deposited under water. The red colour comes from iron minerals, that have been oxidised (made rusty) as a result of exposure to oxygen in the atmosphere.
These sediments were deposited on land, by an extensive network braided rivers – those that split into many criss-crossing streams. The climate was quite hot and dry (arid). The river water came from heavy rainfall over mountains and uplands to the N and NW. See if you can find what plate collisions were happening around 365 million years ago, which might have been responsible for the formation of these mountains.
Can you see layers in the rock? Some layers are sandy and others are full of pebbles. These layers are the beds.
The sandy beds were deposited during times when the river flow was slower, or when the main channels had moved aside. The massively thick beds full of pebbles may represent severe flood events in the river.
The rock here is a sedimentary rock and the layers (beds) would have been horizontal when they were first laid down. Now these rocks are tilted, so at some time after they formed, they must have been affected by some tectonic (earth moving) forces. See Rock Folding next.
Rocks are tilted and folded up by great forces applied over millions of years. When a force is applied over a long time, to seemingly unbendable materials like rock, especially if they are buried at depth, they will ‘flow’ as if they were soft like wet clay.
The dip of the rock beds here is towards the east and is the western arm of a big rock fold that underlies the Forest of Dean. The other side of the fold emerges as steep westward dipping rocks along the eastern side of the Forest of Dean. These are explored in the Wilderness Voyage. The cause of the folding was the forces applied during the time of the Variscan orogeny (mountain-building event), which amongst other things, folded the rocks of the Malvern Hills. The Variscan Orogeny was effectively caused by the African plate pushing against southern Europe.
An orogeny is an event involving major deformation of the crust. The cause of an orogeny will be some form of collision between tectonic plates. An orogeny taking place today is the one still forcing up the Himalays, where the Indian plate is in collision with the Eurasian plate.
The rock beds under the Forest of Dean are down-folded into a shape called a syncline. This is not a symmetrical syncline, as the beds on the west side dip down gently whilst those on the east are almost vertical in places (as seen in Widerness Voyage).
The gentle dip in the west results in the outcrop of the rocks at the surface covering larger areas. In contrast, in the east of the Forest of Dean even the thick formations give only narrow ribbon like outcrops. The diagram of the folding is very simplified as there are numerous other folds and faults superimposed on the big fold. Although not shown, the younger rocks, the limestones and the sandstones of the Carboniferous, are also folded in the same overall way. The folding will also extend downwards into older rocks.
The big picture
This is a simplified geological map of most of the Forest of Dean, including the area of this Voyage and the Wilderness Voyage. The Quartz Conglomerate is picked out in bright red.
The illustration above shows how the Quartz Conglomerate forms very prominent features around the edge of the Forest of Dean and Wye Valley areas.
1 Crags and fallen boulders Coppet Hill
2 Little Doward
3 Huntsham Hill
4 Near Hearkening Rock
5 Puddlebrook (this exposure has lost its striking appearance since highways cut back the rock
and faced it with wire mesh)
6 Crags and fallen boulders The Wilderness
Look carefully how the outcrop of the Quartz Conglomerate is wide on the western side of the area, but a very narrow outcrop on the eastern side. Why might this be? The answer to this was given in the previous section.
Sand, silt and gravel gets carried along in the flow of a river until the energy of the flow of the water decreases and the bigger and heavier particles drop out of suspension and are deposited on the river bed.
Material that is deposited like this is often laid down in layers that slope down to the flat river bed in the direction that the water is flowing. This is called cross-bedding. The inset picture is a good example of cross-bedding.
Can you find cross-bedding in the rock in front of you
Cross bedding close-up
The angle of dip of the cross-bedding can be used to work out the direction of the water current at the time the sediments were deposited.By taking lots of such measurements over a wide area it has been shown the overall direction these rivers were flowing was to the south.
The angle the pebbles come to rest like overlapping roof tiles (called the imbrication) is also used to determine water flow direction. The provenance (where they originate from) of the pebbles can also give clues to ancient river directions. These pebbles are mostly white quartz, obtained from the erosion of veins of quartz in ancient uplands far to the NW. A minority of pebbles are of the mineral jasper (dark red colour) and from rhyolitic lavas (a type of lava rich in silica, produced by explosive volcanoes). Rocks of this type are found in some of the hills of Shropshire and North Wales.
Disaster at 368Ma!
Around 368 million years ago (‘just before’ Quartz Conglomerate started to be deposited) an asteroid smashed into what today is central Sweden. It left a crater 52km in diameter – you can still pick out its shape today. The catastrophic damage such impacts make to the environment can be a cause of mass extinction of many species of life. There was a mass extinction around the end of the Devonian, with 21% of marine families (groups of species) wiped out.
By touching the clock in the app you can bring up information on when asteroid impacts, ice ages, major lava eruptions and mass extinction events have occurred through deep time. There have been many such extreme events through the long history of the Earth and we have not discovered them all.
In search of Site 9
Continue along the track and hunt down the tiny exposure of rock at Task Site 9. You will get an ‘Environment Change’ as soon as leave the Quartz Conglomerate exposure. Check out that environment, a threat is still possible. Most of the visible rock along this section of the track has fallen from the Quartz Conglomerate crags above.
Task Site 8 Questions
What colour are the rock beds here?
When rocks are a reddish-brown colour, it often means that they include material that has come from the land and contains iron which gives the rocks their rusty colour. Which of these minerals contains iron:
The best name for the sandy beds is?
The best name for the pebbly beds is?
c) Boulder beds
Where do you think the pebbles came from?
a) Pebble beach
b) Rock avalanche
c) Man-made concrete
d) Big rivers
What rock or mineral are most of the pebbles made of?
How old is the Quartz Conglomerate Formation?
a) 1000 million years
b) 554 million years
c) 365 million years
d) 28 million years
How far do you think the pebbles travelled in the river before they were deposited on the river bed for the last time?
a) 100 metres
b) A few kilometres
c) Hundreds of kilometres
d) The pebbles did not travel