Task 1 Site

Take a good look around these caves which have been carved out of the limestone. The caves were probably used by humans from about 20,000 years ago. The bones of a range of animals that once lived in the area around the time of the ice age, have been excavated from the caves. Bones of woolly mammoth, woolly rhinoceros, lion, bison, hyaena and reindeer were found. Human artefacts in the form of flint tools and hearths of Palaeolithic hunters (deposited before 12,000 years ago) are also present [See pictures below]. Man’s use of the cave after the ice age is indicated by other flint tools, hearths, butchered animal bones and pottery fragments from later in the Stone Age and into historic times.

The limestones the caves were cut into formed around 350 million years ago. The caves occupation from around 20,000 years ago is by comparision something that happened ‘yesterday’. Twenty thousand years ago is a long time to us, but is less than one ten thousanth the age of the limestones.

Caves in a limestone cliff

Look at how the cave in the limestone has been carved out by the flow of water.

Then turn around and walk to the nearby fence where you can look out over the field and the valley sloping gently uphill.

King Arthur’s Cave has been excavated by archaeologists and speleologists (latter study cave development). Part of the floor of the cave was dug out vertically to a depth of about 7 metres, revealing a variety of layered cave deposits. The base of these deposits was not reached, showing how much bigger the Cave really is if all the deposits were removed, compared to what we can see today.

Dry valley

The picture below is a reconstruction of how this valley may have looked around 14000 years ago at the end of the last glaciation (called the Devensian), when the snow, thin ice cover and permafrost was melting and in retreat.

This valley has no stream to erode out the hollow shape. It was excavated by flowing water in the past, but there is no clear evidence of where the water came from, or flowed to. Perhaps it was eroded out by meltwaters from an ice sheet to the north west near the end of the glaciation, or maybe by a river system that no longer exists (see Deep Valley of Wye). The valley is less than 1 km long, lies between 110 – 120m above sea level, and at both ends the ground falls away to a much lower level. The rest of the valley has been eroded away, leaving only the part seen today.

In the last glaciation (Devensian) at its maximum approximately 20,000 years ago, ice covered Wales and on its SE side the ice did not extend further than an approximately a line between Hereford – Kentchurch – Usk. This would place the nearest the ice front was to the dry valley around 10 – 15 km away. Most of this area and the Forest of Dean would have been tundra.

Deep valley of Wye

The dry valley (blue arrows) now sits high above the present day level of the Wye. This suggests the River Wye once flowed at a much higher level than today.

A river will erode its way down to a lower level, because sea level falls or because the land it is flowing over is raised. Since the end of the last glacial times sea level has risen (due to glacial meltwater entering the oceans), so that will not be the cause. This suggests the crust around here has risen since the end of the last glacial event (around 14,000 years ago). That would cause not only the river to cut down, but also the water table in the ground to fall leaving dry valleys and dry cave systems.

The vertical movement of the crust is called isostatic rebound. The crust sinks down when loaded with ice or sediment and rises when the burden of ice or overlying rocks/sediment is removed. The crust (lithosphere) sinks into, or rebounds from, the underlying more ‘fluid’ asthenosphere. Much as a ship rises and sinks into the water as it is unloaded or loaded with cargo – just a lot slower!

Isostatic adjustment is occurring in many parts of the Earth today. The movement is typically a few centimetres per century. However, it is unlikely that uplift of the area would fully account for the amount the river has cut down in only 14,000 or so years. As the ice was melting the river water was almost certainly much higher, flooding the already deep gorge (cut over millions of years previously) with huge quantities of glacial meltwater. It must have been a staggering sight!

Caves and water

King Arthur’s Cave was developed over a very long period of time. Veins of iron ore exist in the Cave, within the limestone bedrock. Many caves and cavities containing similar iron ore in the same limestone occur on the Great Doward nearby at Symonds Yat, and in the Forest of Dean. The iron ores were likely deposited in the Triassic period (202 – 252 million years ago), and indicate that the caves they were deposited in are at least of that age. King Arthur’s Cave is likely to have existed in some form in Triassic times.

The shape of the passageways and chambers in King Arthur’s Cave follow a pattern seen in many caves around the world. The upper parts of the Cave are more rounded, and are produced by the limestone being dissolved by flowing water that infills the space completely, so that the limestone is dissolved equally all around the flowing water. Such caves are known worldwide as ‘phreatic’ caves. The lower parts of King Arthur’s Cave indicates sideways and downward solution of the limestone, and were developed where water was flowing with air space above; these are known as ‘vadose’ caves.

The phreatic* phase of the development of King Arthur’s Cave is believed to have formed when the meandering River Wye was flowing across a wide flat plain near sea level; below the plain, the phreatic caves developed below the watertable. The vadose phase was developed as the River Wye incised its gorge. The chamber on the left-hand side of King Arthur’s Cave is thought to be the top of a vertical shaft that drops down through the limestone, and was developed during the vadose phase: an underground pothole. It can be visualised that at this time the Cave was completely underground, with water flowing into the Cave from a source elsewhere, the water then cascading down the pothole into the depths below.

As the River Wye excavated its gorge nearby to King Arthur’s Cave, so the water table began to lower, the flow of water was cut off, and the Cave became inactive. As the gorge developed, weathering and erosion of the sides of the gorge in time exposed the Cave’s underground chambers and passageways. Sediments were laid down in the Cave, and the pothole was filled in.

* Relates to water below the water table, from the Greek phrear, phreat- meaning “well” or “spring”.
  Vadose refers to above the water table from the Latin for ‘shallow’.

Sculptured caves

If you continue up the track northward from King Arthur’s Cave you come to many other caves and hollows in the rocky crags, which have been produced by flowing water in the past.

The crags includes remains of phreatic caves, elongated roughly parallel with the rock face. Some of them have been partially cut away, so only the part lying within the crag remains. Others have their whole cross-sections preserved, at least in part, showing their rounded phreatic form.

It can be visualised that these phreatic caves were once located deep within the limestone bedrock, just as those in King Arthur’s Cave were, and formed when the meandering Wye was located on a wide plain close to sea level. They became inactive once the Wye began to form its gorge and the water table fell, and became exposed when the adjacent (now dry) valley was excavated.

Interestingly, unlike King Arthur’s Cave, many of the caves along the crags have not so far been excavated for any potential animal or human remains / artefacts.

Quarry in oolitic limestone

Carry on walking to just beyond the caves and you come to this quarry, White Rock Quarry. The rock of this quarry is a limestone called Gully Oolite, which is the same limestone the caves are cut into. Today ooids (the tiny round particles making up oolitic limestones) are forming in the warm shallow water around the Bahamas. Some 345 million years ago this place was beneath similar warm sunlit waters.

In shallow sea water under a hot sun, the evaporation of the water concentrates dissolved minerals like calcium carbonate. Calcium carbonate will start to precipitate onto some nucleus, like a grain of sand. The back and forth movement of sea currents will encourage the calcium carbonate to build-up equally around the nucleus, so a spherical nodule forms – an ooid.

The fact that ooids are only forming today in waters that are shallow and in a hot climate, leads geologists’ to infer the kind of environment the Gully Oolite formed in was most likely similar. At the top of the quarry face are the lowest beds of the Llanelly Formation. The rock that you were standing on in the car park. The shallow water Llanelly Formation contains some ‘lenses’ of yellow sand at this location.

Ooids up close

If you can take a magnifying glass to a piece of Gully Oolite you can see something like that shown here. Ooids are the individual round objects that form oolitic (egg stones) limestones. In some locations ooids are completely absent from the Gully Oolite.

If your wondering where the ‘Gully’ is, the name comes from the Gully Quarry on the eastern side of Avon Gorge. The exposure there is termed the type section for this particular rock. A type section is an exposure which shows all the key characteristics of a given rock, so other rock exposures of the same type are named after the type section location. The Llanelly Formation limestone, which just appears at the top of the quarry here, has its type section near the village of Llanelly just off the A465 Heads of the Valley Road, near Abergavenny, South Wales. An area where many of the Carboniferous limestones are very well exposed.

Heading to Task Site 2

From Task Site 1 head down through the wood crossing the dry valley. Check what rock you are over. What was the environment. Possible threats? How warm was the sea? Latitude? How many hours in the day? What would the atmosphere be like to breath?

Upward trough the woods

As you follow the path up through the woods, think about the warm shallow water marine environment that existed here around 350 million years ago. Ahead through the trees you may glimpse a cliff. This is the natural rock wall that helped make the top of the Little Doward such a good fortified location to Iron Age people. The cliff is formed of Black Rock Limestone and Gully Oolite.

Steps to Task SItes 2 and 3

When you reach these steps you will be close to Task Site 2 and 3, plus an ‘Environment Change’ – change in rock beneath.

Task Site 1 Question

Why does the valley near the caves, not hold any water today?

a) Water runs off muddy ground
b) Sandstone below so water soaks through
c) Weather drier now – not enough rain
d) Ground was frozen in Ice Age, but now now