2 Rocks Beneath

Rocks Beneath – Task Site 2

Good place to stop after steep climb from car park! Look around at the landscape and information for this site in the app. Then answer the questions for site 2 ‘Rocks Beneath’. The flashing green indicator (app only) on the ‘do it’ button just indicates your around the right place for the task site.

The photograph is taken looking due south. If you have a clear day, then looking around to the east you can see the high ground of the Wye Valley, Forest of Dean and May Hill in Gloucestershire. Further still and the Malvern Hills come into view, followed by many others, for example, the Clee Hills and The Wrekin in Shropshire.

 

Rocks Beneath

The solid geology of the hills and valley plays the most prominent role in shaping the landscape. The whole area is underlain by gently southward-dipping sedimentary rocks formed around 400 million years ago in the Devonian period of geological time.

The cut-away view is to illustrate how the hills and valleys in this area are underlain by beds of rock, with an overall slight southerly dip. The colours and beds thicknesses are purely illustrative. The lower parts of the valley are underlain by soft easily eroded Devonian mudstones, with some beds of sandstone. Above the mudstones are beds of harder erosion resistant sandstones, with only occasional thin mudstone layers.

 

Sedimentary rocks

Sedimentary rocks are one of the three main rock types that are found on Earth – these are igneous, sedimentary and metamorphic rocks.

IGNEOUS rock are those that form when hot molten rock (magma) from the Earth’s interior ascends and either erupts from a volcano or cools beneath the ground surface. They are hard rocks made up of interlocking mineral crystals. The crystals are bigger, the more slowly the magma cooled. Granite is an example of an igneous rock that cools slowly underground, whilst basalt has tiny crystals because it cools at the surface.

SEDIMENTARY rocks are those that have formed from the deposition of sediment (such as mud, sand, or pebbles), transported by water (rivers, lakes, oceans), ice (glaciers) or wind. The sediment turns to rock as it is compressed under layer upon layer of sediment above and may be cemented together by dissolved minerals as water seeps through it. Sedimentary rocks often contain fossils – the remains of plants and animals buried with the sediment and turned to stone. Sandstone and mudstone are sedimentary rocks, as is limestone. The latter is mostly made up of the broken or dissolved remains of shelly sea creatures, cemented together by the mineral calcite (the calcium carbonate that the shells are made of).

METAMORPHIC rocks form when any other rocks (igneous or sedimentary) are altered by immense
heat and / or pressure. This usually happens when rocks are caught up in movements of the Earth’s crust, often as tectonic plates collide, burying the rocks deep underground, or by the presence of very hot magma. The minerals in the original rock are changed chemically and the physical structure and appearance of the rock may change. Many metamorphic rocks are very striking in appearance – some have swirling textures or a lustrous appearance.

 

Red Sandstones

Sandstones are the dominant rock in the Black Mts and Brecon Beacons, but why are they red? The red is due to iron oxides, in particular the mineral haematite. The iron oxide minerals act as a cement between the grains of quartz.

Red rocks are indicative of formation under terrestrial (land) conditions. The sandstones of this area vary in colour from an almost pink red through to dark browns with a purple hue, dependent upon which iron oxide minerals are present.

Devonian Rocks

The rocks of the Olchon Valley are sedimentary mudstones, sandstones and limestones. These Devonian rocks tower above the valley on its western side at Black Darren and Red Daren. They can also be seen here on the Cat’s Back ridge on the eastern side of the valley. In the app you could touch the ‘then’ button to see what the landscape here was like in the Devonian Period when these rocks formed.

The spelling of the two Darren’s is based on that used by the Ordnance Survey.
Black Darren with a double ‘r’
Red Daren with a single ‘r’.

 

Mudstones

In all these places, the lower slopes of the valley sides and base are composed mostly of relatively soft mudstones. Mudstones are fine-grained, clayey rocks that do not let water through easily, so most of the modern-day springs in the Olchon valley are to be found emerging at the top of the mudstone, having soaked through more permeable rocks above and seeping out when they can sink no further.

Geologists refer to the average or most common size of the individual bits of sediment that a sedimentary rock is made up of, as its grain size. For example, mudstones are very fine-grained rocks, made up of tiny particles too small to be seen with the naked eye. Sandstones, however, have larger grains which can be seen easily, and the rock may be described as fine- medium- or coarse-grained sandstone, depending on how big the grains are.

Geologists also look at the shape of the grains – whether they are rounded or angular or in between. This can give them an idea of how far the sediment travelled or the extent to which it was worn away (as grains knocked against each other) before it was deposited. So for example, sand grains that have been blown about and abraded by the wind in a desert, or carried many kilometres in a rushing river may be quite rounded, and sand grains that have rolled around for years under the waves on a sea shore may be the most rounded of all.

The extent to which water can seep through a sedimentary rock (i.e. how permeable it is) depends on the grain size and shape, whether it is cemented or fractured, or has other holes in it and how well they are connected. The most permeable rocks are usually those with the most rounded and coarse grains, without cement between.

Rocks that are fractured will also let water through, even if they are otherwise not very permeable. Limestones may also carry water, even though they are cemented by calcite. This is because calcite is soluble in water, especially rain water, which is slightly acidic, explaining why some of the best caves are found in limestone rocks.

 

Why St Maughans?

Geologists name rock beds based on locations where they consider the best example of the rock unit can be found. These mudstone beds in the Olchon Valley were for many years known as the St Maughans Formation, after the village of St Maughans near Monmouth. They have recently been renamed the Freshwater West Formation.

Geologists name rock beds based on locations where they consider the best example of the rock unit can be found. These mudstone beds in the Olchon Valley were for many years known as the St Maughans Formation, after the village of St Maughans near Monmouth. Sometimes, the names are changed when new research identifies a better example. Recently, these beds have been re-named the Freshwater West Formation, after a place on the Pembrokeshire coast where these same rocks (yes, they extend that far!) can be seen in the sea cliffs.

 

Hay Bluff

The Freshwater West Formation also forms the lower part of the impressive scarp face of the Black Mountains, including Hay Bluff on the north side of these hills. The upper slopes are capped with harder rocks that we will encounter later.

 

Hay Bluff from the Malvern Hills

The view of Hay Bluff and the Black Mountains that is visible throughout much of Herefordshire and beyond. Herefordshire is largely underlain by Devonian rocks (Freshwater West Formation) and the uppermost rocks of the older Silurian time. Long ago the rock beds of the Black Mountains would have covered all this area; their removal is a testament to the power of erosion and the vastness of geological time. The Black Mountains and Brecon Beacons are a remnant of these rocks still being ‘eaten away’ by erosion.

 

South of the equator

In the early Devonian period, the land that now forms the British Isles lay about 30 degrees south of the equator. A huge mountain range lay to the northwest – the present-day mountains of Scotland and North Wales are a remnant of this. An ocean lay to the south and Herefordshire was part of the vast coastal plain between the mountains and the sea.

You may wonder how it is that our patch of land was once in the southern hemisphere. It is because the outer rocky surface of the Earth is not one continuous mass – it is formed of about a dozen massive slabs of rocks called tectonic plates. These plates drift slowly on the hotter, more ductile material deeper within the Earth. They move about as slowly as your finger nails grow, but over millions of years, that can take them a very long way.

The land of the continents – the continental crust – is carried on the tectonic plates. The continental crust of England and Wales first formed around 600 million years ago as a line of volcanic islands collided with, and enlarged an enormous land mass called Gondwana, around 60 degrees south, so not far from the south pole.

Later, plate movements caused Gondwana to rift apart to form several new continents, which started to drift northwards. The largest of these was called Laurentia and included the land that is now North America, northern Ireland and Scotland. It was followed by smaller continents called Siberia and Baltica. Another thin strip of land, or micro-continent, rifted away later and was called Avalonia. The east side of Avalonia would ultimately become England, Wales, southern Ireland and near parts of Europe. The new continents drifted northwards and an ocean, called the Iapetus Ocean, opened up between them. Sometimes, particularly during the Silurian period, parts of Avalonia were flooded by the sea, depositing limestones. However, by 420 million years ago, Baltica and Avalonia ‘caught up’ with Laurentia as the Iapetus Ocean closed, drawing the continents inexorably together, whilst a new ocean had developed to the south towards the remainder of Gondwana. The collision of these continents formed the vast ‘Caledonian’ mountain belt.

 

Vast flood plains

The ground you walked on as you climbed the hill to this point is red-brown and often muddy. This is typical of the Freshwater West Formation. On the Devonian coastal plain, rivers running from the mountains towards the ocean, deposited sand in the river channels and mud on their expansive flood plains. These are the mudstones of the lower slopes of the Olchon Valley, occasionally interbedded with sandstones where a channel crossed the plain. Looking at the side of the valley now is like looking through a cross-section of the Devonian flood plain.

At the start of the Devonian, the fast-rising Caledonian Mountains were subject to rapid erosion in a hot, semi-arid climate just south of the equator, similar to the climate of northern Australia today. Vast amounts of reddened debris were supplied to the lower plains, the sedimentary grains being coated with red iron oxide produced as the minerals of the rocks were weathered.

Rivers carried the sediment out across the coastal plain, where their channel deposits formed sandstone, and mudstones accumulated on the flood plain. The river channels migrated across the plain, so the channel deposits moved laterally with time.

Intertidal areas were often waterlogged with brackish (slightly salty) water and periodically the sea would flood across. In the heat, water quickly evaporated at the sun-baked surface and desiccation cracks formed where the flood plain muds and soils dried out.

Groundwater rich in calcite (from sea water or dissolved from Silurian limestones at depth) would be drawn up the cracks, precipitating calcareous nodules just below the soil surface. Sometimes a whole layer of calcareous limestone (a hardpan) would form. These are known as calcretes.

Often within these rocks there are erosion surfaces, where a moving river channel scoured out the earlier sediments and deposited new material with the eroded debris (including mudstone chips and calcrete nodules) forming a pebbly rock or ‘conglomerate’ at the base of the new sandy channel infill.

 

Fish and Scorpions

Plant and fish remains have been found in these rocks in some locations, as this was a time of significant evolutionary change. The earliest (jawless) fish became the dominant fauna in the seas and a range of primitive plants began to colonise the land where some large vertebrates roamed.

 

Plants and burrows

Plant life was establishing on the land. The forests of the time may have been only a metre or two high. The early Devonian plants had shallow roots and no leaves, flowers or fruit.

Skolithus – trace fossils of the former burrows of creatures – are common in parts of the Freshwater West Formation where it was deposited in an inter-tidal setting.

 

Fossil river channels

As you walk towards Task site 3 you will cross this rock exposure. Take a look at the rocks along the left hand (western) side and you can see small river channels from the Devonian time.

The following image shows a small river channel which has cut into the earlier sediments. The channel has then filled with sediments which show current-bedding.

Task Site 2 Questions

 

Use the app to find out what te mean global temperature was around 400 million years ago.

a) 12’C
b) 18’C
c) 25’C
d) 42’C

Do you know the name of the iron oxide mineral that causes te rocks to be red in colour?

a) Calcite
b) Haematite
c) Quartz
d) Feldspar

 

Which image shows the most permeable rock?

a)
b)
c)
d)