GEOLOGY OF CANNOCK CHASE

A N D GEOLOGICAL GUIDE TO

THE MILFORD-BROCTON A R E A

Paul Green

CONTENTS

1. INTRODUCTION

2. THE GEOLOGICAL STRUCTURE OF CANNOCK CHASE

3. THE PEBBLE BEDS

4. EFFECTS OF THE ICE AGE ON CANNOCK CHASE

5. THE ECONOMIC GEOLOGY OF CANNOCK CHASE

6. CONCLUSIONS

7. A GEOLOGICAL GUIDE TO THE NORTH-WEST CORNER OF CANNOCK CHASE

1. INTRODUCTION

Cannock Chase is a small but highly distinctive region of the West Midlands, occupying 6,500 hectares (65 square kilometres) of mid-Staffordshire. It is a plateau with well-defined, steep margins on all but its southern edge, where it slopes gradually southwards to the South Staffordshire coalfield. Apart from this southern margin the edge of the Chase is defined approximately by the 140 metre contour. The flatter areas of the plateau surface are at about 200 metres but several hills rise above this height, the highest being Castle Ring at 242 metres.

A number of valleys cut out into the plateau, two striking examples being the Sherbrook valley, which dissects the northern edge, and the deep gash which runs north east from Hednesford to Slitting Mill, completely cutting the Chase into two parts (fig. 1). Smaller valleys around the margins of the Chase give the plateau edge a serrated appearance and similarly, the larger valley sides are themselves often highly serrated, as a glance at a contour map of the Sherbrook valley readily shows.

The soil in most parts is a thin, pebbly, acid podsol - i.e. a soil from which nutrients have been leached (washed out), leaving i( very infertile. It has not always been as infertile as it is today, the original natural vegetation was oak forest, but this had largely been removed by the seventeenth century to increase sheep grazing and provide charcoal for local iron furnaces. The disappearance of the oak forest led to rapid deterioration of soil quality and increased soil erosion. Today the soil supports a heathland and birch scrub vegetation, though large tracts have been covered with coniferous plantations which are able to tolerate such soils. Local variations in soil and vegetation occur such as the marshy hollows of some valley floors and the relatively fertile soil of Haywood Warren. At Brocton Coppice are the last remnants of the original oak forest.

What is responsible for this unique landscape? Historical events and modern land management have played their part, but underlying all the human controls is the fundamental influence of geology.

2. THE GEOLOGICAL STRUCTURE OF CANNOCK CHASE

Most of the solid geology of Cannock Chase consists of layers of sand-cemented pebbles interbedded with red sandstones (fig. 2). These are known traditionally as the Bunter Pebble Beds (1) but recent re-naming places them in a group of rocks known as the Sherwood Sandstone Group. Within the group local formations occur and the one in mid-Staffordshire is known appropriately as the Cannock Chase Formation. One of the most characteristic features of the Cannock Chase Formation, making it different from other formations in the Sherwood Sandstone Group, is the abundance of pebbles.

The name given by geologists to a rock composed of pebbles closely cemented together is conglomerate - a word which can be properly applied to pebble beds within the Cannock Chase Formation.

The Cannock Chase Formation was formed approximately 230 - 220 million years ago in the early part of a period of geological time known as the Triassic (fig. 3). On the Chase it is between 100 and 150 metres thick. Directly beneath it are the productive Middle Coal Measures which are approximately 300 million years old and were formed in the upper part of the Carboniferous period. A glance at figure 3 will show that a time gap of perhaps 70 or 80 million years exists between the two rock formations, beginning in the uppermost Carboniferous and lasting the duration of the Permian period. During this time the Coal Measures were uplifted, tilted, folded and faulted (broken) by earth movements and underwent considerable erosion (wearing down), so the land surface on which the pebble beds were then deposited, and which is now the boundary between the two formations, is an irregular one. The name given by geologists to a boundary of this sort is an unconformity and can be recognised in cross section by an angular difference between the rocks above and below it (fig. 4).

(1) Bunter is a German word meaning 'brightly-coloured'

Figure 1 General map of Cannock Chase

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Figure 2 The geology of Cannock Chase, excluding glacial till deposits. (For key see fig. 3 and for extent of till deposits see fig. 16)

Figure 4 A geological cross section through Cannock Chase (see fig. 2 for line of section)

In the southern part of Cannock Chase the cover of Triassic rocks is missing and the Coal Measures outcrop at the surface. Coal Measures is a term covering the sandstones, mudstones and coal seams that were formed in the upper Carboniferous period. The land surface on the Coal Measures outcrop slopes down westwards and looking at figure 2 it is easy to imagine the Coal Measures disappearing beneath the pebble beds. The boundary between the two rock formations is marked by an escarpment in Beaudesert Park (fig. 1), rising to 237 metres in the Rawnsley Hills. A steep scarp slope faces south east and overlooks the forest-covered Coal Measures. The escarpment exists because the pebble beds are harder and more resistant to erosion than the Coal Measures.

The Coal Measures which outcrop on Cannock Chase are the northern part of the South Staffordshire coalfield and like other coalfields in the Midlands it has the structure of a horst. Horst is a German name given to a block of country which has been, upfaulted. Horsts are usually associated with graben, or rift valleys, which are blocks of country that have been downfaulted. Figure 5 illustrates the development of horsts and graben.

and graben

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(c) erosion wears down the horsts

Figure 5(c) illustrates that the effect of eroding a horst is to expose older rocks at the surface side by side with younger rocks, but separated by faults. In South Staffordshire the Coal Measures are the older rocks and younger Triassic rocks sit alongside them. The faults which delimit the South Staffordshire coalfield horst are known as the Eastern and Western Boundary faults and are shown on figures 2 and 4. Alternative names are the Rugeley and Bushbury faults respectively.

The fact that the faults cut through Triassic rocks shows that movement along them has taken place in post-Triassic times. But in fact, the faults could have been initiated in the late Carboniferous or Permian periods and their continued movement after deposition of the Triassic rocks caused the latter to fracture along the same line. This is supported by the fact that vertical displacements in the Triassic rocks are considerably less than in the underlying Coal Measures. The Eastern Boundary (or Rugeley) fault, for instance, has a downthrow to the west of 412 metres in the Coal Measures near Rugeley, but only 61 metres in the Triassic rocks.

Around the margins of Cannock Chase are rocks younger than both the Coal Measures and the pebble beds. The rock which lies on top of the pebble beds is known traditionally as the Lower Keuper Sandstone (1) though recently it has been re-named as the Helsby Sandstone Formation which, like the Cannock Chase Formation, is a sub­division of the Sherwood Sandstone Group. It is a red sandstone with some pebbles, and originally completely covered the Cannock Chase pebble beds. In turn, the sandstone was buried beneath a red mudstone known traditionally as the Keuper marl, but more recently as the Mercia Mudstone Group. The sandstone and mudstone have both been removed from Cannock Chase by erosion and it is quite possible that even younger rocks were deposited over the region and subsequently eroded away.

The Triassic sandstones and mudstones which have been removed from Cannock Chase by erosion are preserved to the north, east and west in the lower ground. This is partly explained by the effects of faulting illustrated in figure 5 and partly by tbe way the rocks have been folded by earth movements. Cannock Chase has been arched up to form a type of fold known as an anticline (fig. 4). The opposite of an anticline is a downfold known as a syncline. The effect of arching rocks upwards into an anticline is to expose them to Increased erosion, while downfolding preserves them in a synclinal basin (fig. 6).

Figure 6 Cross section of an anticline and syncline

The result of eroding an anticline is that older rocks are exposed at the surfce, with younger rocks on either side, and although complicated by faulting, this is the structure of Cannock Chase. The axis of the anticline runs south-east from the Satnall Hills (fig. 2). On either side the rocks dip away from the axis but the angle of dip is never more than a few degrees. On the flanks of the anticline, broken by faulting, are outcrops of the younger sandstone and its outcrop at Etching Hill in Rugeley, in the form of a prominent escarpment, clearly reveals the dip of the rock (fig. 4). Beyond the sandstone are broad outcrops of still younger Triassic mudstones forming the lower ground of the Needwood Basin to the east and west Staffordshire to the west.

(1) Keuper is a German word and is pronounced koyper 6

3. THE PEBBLE BEDS

Few visitors to Cannock Chase can have failed to notice the abundance of round, smooth pebbles loosely strewn over the surface and for young visitors in particular, more accustomed to finding such pebbles on the seashore, the question arises of how they got there. To understand the nature of the pebble beds it is necessary to go back in time and reconstruct the palaeogeography {'ancient' geography) of Britain and the geological events which led to their deposition.

1 At the end of the Carboniferous period, following the deposition of the Coal Measures, earth movements folded and fractured the rocks of south-western Britain lifting them into a mountain chain. These mountains also extended across central Europe and have been variously named as the Hercynian, Armorican or Variscan Mountains after * localities in Europe. Such mountain-building periods, or orogenies, are caused by movements of the earth's crust. These movements involve the jostling and collision of continents and by the Permian period all the world's continents had joined to form a single super-continent called Pangaea. The Midlands region was therefore part of a huge land mass and lay to the north of the foreland of the Hercynian Mountains. Although there was no mountain building in the Midlands the rocks were fractured creating not only the coalfield horsts mentioned in section 2, but also a system of downfaulted graben (rift valleys) extending north from Worcestershire into Staffordshire and Cheshire and then into the northern part of the Irish Sea (then dry) and Northern Ireland. A similar system of rifts existed in the Irish Sea (fig. 7). Throughout the Permian period these valleys widened and filled with sediment (sand and gravel) derived from the surrounding highlands.

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In the Triassic period Pangaea began to separate into the continents we recognise today. This movement created tension in the rocks which caused existing rift valleys to become wider and deeper, and new ones to come into being. In the Midlands the Worcestershire graben openend northwards into low 'troughs' occupying Staffordshire and Cheshire. It was in these sedimentary basins that the Sherwood Sandstone Group (including the Cannock Chase pebble beds) and, subsequent Triassic rocks were deposited. The reason they were able to accumulate to the thickness we see today is that the basins continued to subside throughout the time the sediments were being deposited, so the land surface stayed at approximately the same level (fig. 8).

Figure 8 Palaeogeography of the Midlands at the time of deposition of the pebble beds (after L.J. Wills, Concealed Coalfields)

A further consequence of crustal movements is that continents change their latitude and longtitude. Studies of the way rocks have been magnetised show that in Triassic times Britain lay around 10° north of the equator. The temperature was therefore warm and studies of the rocks prove that the climate was semi-arid. For instance late Triassic evaporite (salt) deposits in Cheshire are evidence of a lake or inland sea which dried up and the red film of iron oxide on rounded and frosted sand grains in some Triassic sandstones is similar to sands found today in arid areas.

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However, the surface was not everywhere and always dry. In the early Triassic period, when the Cannock Chase pebble beds were formed, there appear to have been cyclic changes in the climate, giving alternate wet and dry episodes. Furthermore, some rivers which rose in the wetter highlands could have been large enough to flow through more arid areas for many kilometres before either drying up or reaching a lake or the sea (in the same way that the Nile today flows thorough the Sahara Desert.) Such a river flowed northwards through the Worcestershire-Staffordshire graben in early Triassic times and seems to have divided in the north Midlands before gradually drying up through evaporation and downward percolation (fig. 8). Although an inland sea did exist further to the north east there is no evidence that this fiver ever reached it. The river has been named the Budleighensis river, after deposits which it left behind at Budleigh Salterton in Devon, and it was this river that deposited the pebble beds of Cannock Chase.

The Pebbles

The pebbles of Cannock Chase were deposited as a conglomerate, that is, a rock made of closely packed pebbles held together in a matrix (cement) of red sand. Throughout geological history conglomerates have been formed most typically in high-energy, shallow sea water, but the redness of the sandy matrix on Cannock Chase shows that the pebbles were deposited in a continental environment. The redness is due to oxidation of iron in the sand and is therefore typical of sub-aerial, as opposed to marine, sands. The closeness of packing of the pebbles is shown by the presence of pressure-solution pits on the surface of many of them. These take the form of shallow concave hollows, several millimetres in diameter, and were formed by the pressure of pebbles pressing against each other under the weight of overlying deposits (fig. 9).

Around some pressure solution pits is a low ring of hard-cemented sand which extends the concave depression on the pebble surface and neatly fits round the shape of the adjacent pebble. Such rings are formed when pressure solution releases a silica cement from pebbles which are pressing together. The cement permeates the adjacent sand for a few millimetres and strongly binds it together. This tough, but very localised, cementing contrasts with the considerably weaker binding which more widely holds the pebbles together. Pebbles can sometimes be prised individually by hand from rock faces and frost is easily able to loosen them. Nevertheless, the pebble beds still have sufficient coherence to give stability to the many steep slopes around the margins of the Chase.

Another structure sometimes found on the surface of the pebbles Is curved fracture marks cause by collisions while the pebbles were being transported by the Budleighensis river. These marks tell us that the pebbles were moved under high energy (i.e. fast, turbulent) conditions, a fact which is further supported by the very size of the pebbles. The majority of pebbles are a few centimetres across and about 1 % exceed ten centimetres in size. Pebbles of this size could only have been transported by fast-flowing water §hd we can gain some idea of the velocities involved by reference to a graph first produced by Hjulstrom and shown In figure 10.

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This graph is derived from studies of modern water-transported sediments and confirms that, despite the aridity of the Midlands in the Triassic period, the Budleighensis river must have had a considerable discharge to have deposited pebbles of the size we find on Cannock Chase. However, it would be wrong to assume that its discharge was permanently large. It probably varied in flow both seasonally and over longer-term cyclic climatic changes, though there is no evidence that it ever dried up completely.

The large discharge of any river which flows through a region of low rainfall can only be maintained if its source lies in mountains with a wetter climate. So where was the source of the Budleighensis river? This has always been something of a geological mystery and sources as widely separated as Scotland and northern France have been suggested. The pebbles certainly seem to have been transported over a long distance since a considerable amount of rolling and grinding was "necessary to produce their smooth roundness, especially as the majority are made of tough, resistant quartzite. One way of determining the source is to try and match the pebbles with known outcrops from which they were originally eroded. However, this has not proved easy to do. 80% to 90% of the pebbles land locally over 95%) are a grey-purple quartzite which cannot be matched with any known outcrop. A few quartzite pebbles have been matched with Cambrian outcrops at Nuneaton and Lickey, but only a tiny proportion of the total number of pebbles originated in the Midlands.

One clue is provided by fossils found in the pebbles themselves. These are fossils of animals that lived and died when the pebble rock was first formed not when the Triassic pebble beds were laid down. In other words, the fossils are much older than the pebble beds and were transported into the Midlands (embedded in the pebbles) away from the region where they originally lived. (1) Two examples serve to give a strong indication of the probable source of the river. The first is pebbles of Ordovician rock (fig. 3) containing fossil trilobites which almost certainly lived in the Normandy-Brittany region of northern France. The second is pebbles containing Devonian species which are characterstic of northern France and south west England.

That the Budleighensis river flowed through south-west England is further supported by the presence of heavy minerals in the pebbles on Cannock Chase such as tourmaline, cassiterite, staurolite and garnet. These minerals are all typically associated with the granite outcrops of Devon and Cornwall, and in fact a few granite pebbles are also present, though they tend to be weathered and decomposed. Furthermore, these heavy minerals decrease in number north-westwards and north-eastwards from Cannock Chase, as would be expected if the transporting river flowed in these directions.

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So it seems likely that the Budleighensis river had its sources in the Hercynian mountains of northern France and flowed northwards across what is now the English Channel (but was then dry land). Tributaries from Devon and Cornwall added their volume before the river entered the Worcester - Staffordshire graben. Emerging from the graben, in the region of Cannock Chase, the river then split to the north east and north west before finally drying up. Whether or not the quartzite pebbles which are so common on the Chase came from northern France cannot be determined with certainty. The mountains which were the source of the river have long since disappeared through erosion, and with them presumably the source outcrop of the quartzite.

(1) The environment in which the pebble beds were deposited was not suitable for the preservation of fossils. No doubt some animals did live in the semi-arid river valley, especially reptiles, but the usual requirement for preservation of fossils is rapid burial in relatively quiet, non-abrasive conditions. The area of Cannock Chase was, therefore, quite unsuitable, though elsewhere occasional Triassic fossils have been found, including reptile footprints, impressions left behind by water fleas and even a fish.

Other pebbles to be found in the pebble beds include all grades of clastic sedimentary rocks of pre-Triassic age. A clastic sedimentary rock is one formed by the deposition of eroded fragments in an environment such as the sea floor or a river basin. The main types are conglomerate (cemented round pebbles), breccia (cemented angular pebbles), gritstone, sandstone, mudstone and shale. In each case the fragments are held together by compaction and/or cement. There are, for example, in the conglomerates of Cannock Chase pebbles which are themselves made of conglomerate! Pebbles found within larger pebbles must have undergone at least two cycles of erosion and deposition.

Because sedimentary rocks are usually laid down in horizontal laminae some of their pebbles are disc-shaped, reflecting their flat-bedded structure. Thin laminations can sometimes be seen in the pebbles. Pebbles of rocks which do not possess such laminar structure have a higher index of sphericity, including the homogeneous quartzite which is so common on the Chase.

Many pebbles had a British, rather than French, origin, supplied to the Budleighensis river by tributary streams from the east and west. The position of some of these tributaries is quite well known because they deposited delta fans where they entered the main valley, for example at Polesworth and Bridgnorth (see fig. 8). The pebbles in these fans reflect the source areas frorn which the tributaries flowed. For instance, the Bridgnorth fan contains a large proportion of pebbles made of Carboniferous limestone and chert - rocks which are rare elsewhere in the pebble beds. These pebbles are also large and angular, suggesting'a local origin. Being smaller rivers, with sources in the rain shadow of the Hercynian mountains, these streams probably had irregular discharges by comparison with the main Budleighensis river.

Facies in the pebble beds

The word facies (pronounced 'fashees') comes from the latin word for 'aspect' and is used by geologists who wish to describe a particular characteristic (i.e. aspect) of a sedimentary rock. For instance, a rock sequence charactersied by fossil shellfish may be described as belonging to a shelly facies, regardless of the rock type, or types, involved. It is a useful concept because within a single bed of rock lateral variations may exist which can be described in terms of facies change. In the pebble beds on Cannock Chase four facies can be recognised:

The flat-bedded pebble facies

As its name suggests, this facies consists of horizontal layers of conglomerate. Of the four facies this contains the largest pebbles and the layers may be up to ten metres thick. However, within individual layers smaller graded sequences can often be found. Each sequence starts at the bottom with a mixture of large and small pebbles in a sand matrix and grades upwards through a few tens of centimetres Into smaller, better-sorted pebbles in which there may be little sand (fig. 11).

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These horizontal layers of pebbles have been interpreted by geologists as a layer-by-layer growth of sheets of pebbles on the bed of the fast-flowing Budleighensis river in

times of high flood. Studies of sediment characteristics in present-day rivers reveal that the kind of river which deposits such pebble sheets is one which has a braided channel. A braided river is one which 'chokes' its own channel by the sheer quantity of debris which it transports. The sediment is deposited in accumulations known as bars and at times of low discharge these are left dry while numerous channels wind and interconnect around them (fig. 12). Only in times of flood are the bars submerged and it is then that the coarse sediment is moved. Naturally, sand is more easily moved than pebbles (fig. 10) and unless sand gets trapped between pebbles it tends to be washed out of the shingle bars to be deposited elsewhere in calmer conditions.

During floods not all parts of a braided channel system experience equal discharge and there are side channels and dead-ends (sloughs) in which the flow is not great enough to transport pebbles. It is here that finer sand or mud tends to accumulate. But in such conditions of rapidly moving sediment the position of channels shifts from side to side so that pebble deposits may have considerable lateral extent. Fine sediment deposited in side channels may be buried by coarser fragments and vice versa as the channels shift across the valley floor.

Figure 11 Field sketch taken in the Satnall Hills quarry (the view is to the north west and shows the lower part of the quarry face)

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If we accept this model as being applicable to the Budleighensis river, which deposited the pebble beds of Cannock Chase, then the upward fining of each graded sequence can be interpreted in two possible ways. Firstly, it could represent the waning of a flood.

The reduced flow would have caused finer pebbles to be deposited on top of larger ones, but as this requires no shifting of channels it cannot account for the considerable lateral extent of pebble beds which we see today. This difficulty is overcome by the second interpretation which is that graded sequences represent the lateral shift of channels. In any one place high river discharge (and therefore large pebbles) could be superseded by lower discharge causing smaller pebbles to rest on the larger ones, even while the flood was at its height. The fact that many graded sequences can be seen in vertical sections such as the Satnall Hills quarry reflects the continual downward subsidence of the Budleighensis valley floor in early Triassic times. Upward accumulation of the sediment and downward subsidence of the valley roughly kept pace with each other.

The cross-bedded pebble facies

Cross bedding is a structure common in sediments which have been deposited in a current. In the Budleighensis river the sand and shingle bars were moved by the current in times of flood. The bars had flat tops with more steeply angled fronts sloping down in a downstream direction (fig. 13). Sediment which moved across the flat tops eventually cascaded down the fronts, coming to rest in angled layers which geologists have named cross bedding (also false bedding and current bedding). The layers are usually curved (concave upwards) and always slope down in the direction of the current. Because this structure developed through the downstream migration of flat-bedded layers of pebbles the two facies often pass laterally into each other. They can also bo soon inter-bedded with each other in vertical sequences (fig. 14).

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Figure 13 The development of cross bedding in a bar

The pebbles in the cross bedded facies are similar to those in the flat bedded facies in every way, including the repeated graded rhythms, but they tend to be finer and are surrounded by more sand, reflecting the reduced energy of flow in the sheltered lees of the bars. Because the cross-bedding planes slope downwards in the direction of flow they have proved useful in confirming that the general direction of flow of the Budleighensis river was from south to north.

The pebbly sandstone facies

This occurs as layers or more localised lenses of sandstone in which the number of pebbles varies from place to place. The pebbles are generally smaller than in the two pebble facies reflecting their deposition in more slowly flowing water. On Cannock Chase two kinds of sandstone can be recognised: one is a coarse or medium, red-brown pebbly sandstone present in both thick and thin beds. The sand grains are firmly held together by a silica cement which makes the sandstones more coherent than the conglomerates. This is apparent on disused quarry faces where weathering has left the sandstone layers standing out as narrow ledges. The other kind is a darker red, fine­grained sandstone with few pebbles. It is rich in flakes of a shiny mineral called mica and occurs only in lenses and thin beds. It was clearly deposited in conditions of low-energy flow.

These sandstones often display cross bedding (fig. 11). It is on a smaller scale than that in the pebble facies, but was formed in the same way, i.e. by the movement of sand bars during times of flood. They are found inter-bedded with the pebble facies but may not be very widespread laterally. They have been interpreted as the deposits of relatively slow-flowing water in secondary or side channels of the braided river system (fig. 12), and their interbedding with pebble deposits is the result of lateral channel shifting. Pebbles resting on top of a sandstone layer are a sign of increased energy of flow so it is common to find that the upper surfaces of sandstones are uneven and irregular, having been eroded by the river prior to burial by pebbles (fig. 11).

The mudstone facies

In the high-energy environment of any braided channel system mud is not a common sediment, yet there are places where conditions become calm enough, even if only temporarily, for mud to settle from suspension. These include abandoned channels, backwaters (sloughs) and pools on the rippled tops of bars which are left after a flood subsides. Mud deposits are therefore very localised and of limited vertical and lateral extent. Furthermore, the nature of the environment, with shifting channels and repeated flooding means that mud deposits are no sooner formed than they may be subjected to erosion and removal. Consequently, it is uncommon to find mudstone in the rocks of Cannock Chase.

One way in which mudstone may be found is in the form of pebble or boulder-size inclusions in any of the other three facies. Such inclusions are called intraclasts. They were formed when fairly well-consolidated mud was eroded by increased flow and blocks were broken away and rolled along the river bed, eventually coming to rest on a sand or shingle bar.

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Figure 14 Field sketch of the east face of Brocton quarry

Summary

The available evidence suggests that the rocks of Cannock Chase were formed in early Triassic times by a river system (the Budleighensis river) which flowed from northern France, through southern England and into the Midlands through a subsiding down-faulted valley. Despite the semi-arid climate the river and its tributaries were able to transport pebbles of considerable size over long distances and this was due to increased discharge in times of flood.

A comparison of the Triassic pebble beds with river deposits being formed today elsewhere in the world suggests that the Budleighensis river was a system of braided channels. Within the system bars of sand and shingle accumulated and were moved in times of flood forming both flat and cross bedded layers, with intraclasts of mud.

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This illustrates an important principle of geology known as the law of uniformitarianism. This law states that we can understand the past by studying the processes which are

operating today. The rocks of Cannock Chase provide us with sufficient clues to be able to interpret conditions which existed in early Triassic times. Fortunately, the clues are readily accessible in natural exposures and abandoned quarries making the area ideal for study by the field geologist.

What happened to the Cannock Chase area in subsequent geological periods we cannot say with certainty as there are no rocks of younger age on the Chase. This is not to say that the pebble beds have never been covered by more recent rocks; they almost certainly have. Their absence today is due to removal by erosion. The next event to leave direct and tangible geological evidence was the ice age, so a gap of about 230 million years is unrepresented in the geology of the Chase. It is to the effects of the ice age that we now turn.

4. EFFECTS OF THE ICE AGE ON CANNOCK CHASE

For approximately the last two million years the earth has been in an ice age, known by geologists as the Pleistocene Epoch. During this time climate has fluctuated considerably and on many occasions it became cold enough for ice caps and glaciers to grow in areas which now have a temperate climate, including the British Isles. These occasions are known as glacial episodes and were separated by interglacial episodes during which warming of the climate caused the retreat of glaciers. However, glacial advances and retreats were not straightforward. They were characterised by numerous minor advances, halt phases and retreats.

Throughout the Pleistocene Cannock Chase was affected by many ice advances. However, the effects of one ice advance tend to obliterate or modify the effects of previous ones so any part of today's landscape which is the result of glaciation is largely a product of the last major ice advance. In the Midlands this occurred between 26,000 and 10,000 years ago. It reached its height 18,000 years ago when ice covered the whole of Cannock Chase and extended a few miles beyond to the south east (fig. 15). The ice has been called 'Irish Sea Ice' because it had its origins in southern Scotland and the Lake District and used the Irish Sea basin and Cheshire Plain to make its way southwards into the Midlands. Evidence for this comes in the form of Cumbrian and Scottish stones which were moved into the area by the ice and then deposited when it melted. Such stones are called erratics because they are made of rocks which are quite different from the underlying solid bedrock on which they rest. Erratics are often embedded in a matrix of sand or clay and the name given to such a deposit is till (or boulder clay). Deposits of till are to be found covering the southern margins of Cannock Chase (reaching 240m. above sea level near Castle ring) and on Haywood Warren (200m.) in the north of the Chase (fig. 16). It is the clay content of the till on Haywood Warren that gives the soil its relative fertility, as evidenced by the presence of plants such as the nettle and a small plantation of oak trees.

Erratics in the Cannock Chase region have been matched with rock outcrops in south-west Scotland, the Lake District, northern England and, of course, the local area. However, it cannot be assumed they were all transported into the region during the last glaciation. Previous ice advances must have left erratics which the last ice re-worked and incorporated into its own deposits.

A further difficulty lies in recognising the precise extent of till deposits. On the weathered, gravelly surface of the Chase pebbles would have been picked up easily by the ice and subsequently deposited as till, but to what extent this occurred is hard to tell. In addition, considerable movement of these pebbles took place after the ice had melted but while the climate was still very cold. Such conditions are described as periglacial. At this time the subsoil would have been permanently frozen (permafrost) and therefore, unlike today, impermeable. Seasonal melting of the top soil created a sludge which moved easily down slopes, carrying pebbles with it. This movement is known as solifluction and must have considerably affected the till deposits. It also scooped out hollows on many slopes and is the process responsible for their serrated appearance today. This is particularly noticeable in the contour pattern of Sherbrook

No reference to erratics on Cannock Chase can be made without mention of the 'Glacial Boulder'. It is a rectangular block of granite 1.6 metres long, weighing about 2.5 tonnes, and situated at an altitude of 194m. near the triangulation pillar at grid reference 981182. Its size bears testimony to the erosive power of ice as it was transported from Criffel in south-west Scotland (fig. 15). Today it stands elevated on a man-made pedestal of cemented Triassic pebbles and is a landmark on the Staffordshire Way (fiq. 17).

Figure 17 Field sketch of the glacial boulder

The most dramatic effects which glaciation had on Cannock Chase occurred not when the area was covered by ice, but when the ice began to retreat following its maximum extent 18,000 years ago. Because the ice had advanced originally from the north west it was in this direction that it retreated, but retreat was not a steady, continuous process; it took place in a series of 'jumps' and there were halt phases and even minor re-advance. On one of these halt phases the ice rested around the northern and western margins of the Chase, leaving the plateau surface cold but unglaciated. This was the so-called Pennine halt phase, because the ice also stopped along the western edge of the Pennines at the same time. The ice which rested along the northern margin of Cannock Chase occupied the present Sow-Trent valley (fig. 1) and at one time re-advanced about as far as Yoxall, leaving behind a small moraine (gravel ridge) near Brankley (1521).

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Today the northern margin of the Chase is marked by a series of channels and short valleys which are mostly dry. Some of the channels run approximately east-west parallel with the former ice front. Their floors are irregular, with humps and marshy hollows, and the lack of any continuous downward gradient shows they cannot have been formed by normal stream erosion. The largest of these east-west channels runs west from Seven Springs and has several gaps in its northern side (fig. 18). Two of these gaps are occupied today by streams (the Sher and Old Brooks) which flow north through them towards the river Trent, so cutting across the channel at right angles. Further west this single east-west channel joins with a system of interconnecting channels in the north-west corner of Chase. Between the channels are a number of distinctly separated, steep-sided hills such as Oat Hill and Broc Hill (fig. 18). The channel between these two hills was in fact deepened and straightened in the 1914-18 war to be used as a railway cutting for the military camps on the Chase.

Attempts have been made to explain the existence of these channels in terms of water overflowing from a lake which was trapped between the western edge of Cannock Chase and the margin of the ice. According to this idea the water overflowed eastwards towards the Trent valley, cutting the channels in the process. However, the way the channels interconnect with each other, the irregularity of their floors, and the lack of an overall, single downward gradient show that this cannot have happened.

Studies of glacial margins in parts of the world still affected by ice show that such channels can only be formed beneath the ice (i.e. they are sub-glacial). This happens when meltwater from the ice is unable to flow away from the margin because the slope of the land directs it down beneath the ice. Here, it may flow with great volume and speed through crevasses and tunnels, cutting channels into the under-lying rock, until it finds its way out of the ice and into a normal river valley. In tunnels at the base of the ice the water may be under so much hydrostatic pressure that it can flow up-hill, so unlike a stream at the surface, a single downhill gradient is not essential. This could account for the irregularity of the channel floors on the northern margins of Cannock Chase.

If we accept that the channels were formed sub-glacially then clearly the edge of the ice during this halt phase must have been sufficiently high on the plateau to cover the east-west channels and their intervening hills. This provides a solution to the problem of the gaps in the northern slope of the channel which runs west from Seven Springs. Presumably the gaps represent pre-glacial lines of drainage down into the Trent valley, but water was prevented from using them during glaciation because they were plugged by ice. Meltwater was therefore confined to the east-west channel and unable to take a direct northerly route towards the Trent. Only after the ice had retreated away from the area could the Sher and Old Brooks re-occupy two of the gaps and flow northwards.

In addition to the east-west channels there are a number of smaller channels which generally slope northwards from the high part of the plateau and meet the east-west channels roughly at right angles (fig. 18). These follow the general regional slope of this part of the Chase, down towards the Sow-Trent valley, and also were probably formed beneath the ice. They were sub-glacial chutes which were supplied with meltwater from the edge of the ice and directed it down into the east-west channels. Some of the chutes followed pre-existing valleys (e.g. the Sherbrook valley) while others were smaller and are now completely dry. The location of the smaller ones may have been governed by the development of crevasses in the ice. Indeed, structures such as crevasses or shear planes in the ice could also account for the position of the east-west channels. The relationship between chutes and a channel parallel to the ice margin is shown in figure 19.

The pictures we therefore have of the northern part of Cannock Chase in this glacial halt phase requires an ice margin quite high up on the plateau. Channels at right angles to this margin carried meltwater beneath the ice and fed it into inter-connecting channels which ran parallel to the margin. In the north-west corner of the Chase the outlets of these interconnecting channels face west, but the present contour pattern shows that the subglacial streams emerging from them quickly changed their direction towards the north, flowing eventually into the Sow-Trent valley.

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Figure 19 Diagram to illustrate sub-glacial channels on a surface which slopes down beneath the ice

Further south there is another channel on Cannock Chase which has a different kind of origin, though still connected with the glacial halt phase. It is the deep gash which runs from Cannock to Rugeley and splits the Chase into two halves (figs. 1 and 20). Its time of formation relative to the subglacial channels to the north is uncertain, but it must have occurred after the lobe of ice in the Trent valley had retreated north of Rugeley, because an outlet into the river Trent was necessary. Figure 20 shows that while ice was banked up against the south-western margin of the Chase a meltwater lake formed in the Cannock-Hednesford area. Its south-western shoreline was ice and its other shores were against the slopes of Cannock Chase.

As the lake filled with meltwater it spilled over a col north east of Hednesford and the water flowed down the valley of the present Rising Brook towards Rugeley and the Trent valley. The overflowing water rapidly lowered the col and cut a channel with steep sides and a broad, flat floor (now used as a route for the A460 and a railway). Clearly, some sort of valley must have existed here before this glaciation and the meltwater simply increased its size. The size of the Rising Brook bears no relation to the size of the valley which it now occupies and is therefore described as a misfit stream. The over-deepening of the valley caused tributary valleys to be left 'hanging' higher up on the slopes of the Chase and they now drop steeply into the main valley. Some of these are now dry.

To the north east the valley widens out and the gradient becomes less steep. Here, a fan of gravel was deposited by the overflow stream as its velocity slowed just before entering the Trent. The original village of Rugeley was built on this fan because its permeability made it a drier site than the waterlogged alluvial deposits of the Trent valley.

5. THE ECONOMIC GEOLOGY OF CANNOCK CHASE

The main economic value of Cannock Chase in the past has been the outcrop of productive Coal Measures in the south and east of the area, but in common with many other exposed coalfields, most of the workings are now disused. Mining has now moved on to concealed parts of the coalfield around the margins of the Chase, as at Rugeley and Huntington.

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Figure 20 Position of the glacial lake and overflow channel between Cannock and Rugeley

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The main economic use of the pebble beds is as a source of high-quality gravel for road metal (and to a lesser extent for concrete). Some quarrying still takes place in the Cannock area, but in the north of the Chase many quarries and small pits have been abandoned due to the presence of hard sandstone bands. The quarry at Brocton has since been flooded and is now a nature reserve.

A further use of the pebble beds arises from their high porosity. Groundwater accumulates in the rocks and is held there by the relative impermeability of the underlying Coal Measures. This water is pumped for public use and stored in tanks and reservoirs. Elsewhere, as at Burton-upon-Trent, the properties of the water from the pebble beds are well recognised by the brewing industry.

A more indirect economic aspect of the geology of Cannock Chase is the way in which it influences land use. The present landscape of heathland vegetation and coniferous plantations may be due to medieval deforestation and more recent human management, but a fundamental constraint on land use is the infertility of soils which have developed on the sandstones and conglomerates. Similarly, modern recreational use of the Chase may reflect man-made attractions and wildlife, but it is no coincidence that the most popular locations are those where the ice age had its most dramatic effects. For example, the areas around Seven Springs and Milford Common owe their scenic attraction to the effects of meltwater in sub-glacial channels at the ice margin. Even the road which brings in many of the tourists the (A513) follows one of the sub­glacial channels (fig. 1R).

6. CONCLUSIONS

In most geological studies of south and central Staffordshire emphasis is placed on the Coal Measures. This is understandable in view of their influence on the historical, human and economic geography of the area. Their underground structure is quite well known but surface study is made difficult by the cover of till in the south of the Chase and lack of suitable exposures. The Triassic rocks are of much less economic importance than the coal which they partially conceal and consequently they are given less attention in the more-available literature. One aim of this study has been to show that the Triassic outcrop on Cannock Chase holds a great deal of geological interest both for the specialist and inquisitive day-tripper alike. Exposures are accessible and clearly reveal many of the features referred to in this study.

Cannock Chase exists because it is an upfold of rock which, relative to the rocks around it, is resistant to erosion. The pebble beds hold a wealth of information which enables us to reconstruct the geography of this part of the Midlands during early Triassic times. The area was a semi-arid basin with a powerful, though variable, braided river flowing through it, bringing pebbles from as far away as northern France. What happened between then and the ice age is largely a matter for conjecture since no rocks of more recent age remain on the Chase. Glacial deposits tell us something of the most recent geological events in the area, and it is possible to demonstrate that the final shaping of the Chase, including many of its steep marginal slopes, was due to a combination of ice, glacial meltwater and periglacial processes.

So the two geological episodes which were primarily responsible for the landscape we see today could hardly be more different in terms of environments and processes operating in them. Cannock Chase makes a unique and invaluable contribution to the total understanding of the evolution of the British Isles in both these periods.

Equally, the Chase offers opportunities for the non-specialist to arrive at a better understanding of the environment around him. It is estimated that 3.5 million town-dwelling people live within easy half-day driving range of the Chase and on any warm Sunday afternoon in the summer up to 10,000 visitors from towns in the Midlands converge on the area. But instead of spreading evenly over the Chase they tend to cluster in certain favoured locations which combine scenic and human attractions.

Particularly popular are locations such as Milford Common and Broc Hill in the north­west corner of the Chase. To the geologist this part of Cannock Chase offers the advantages of easy accessibility and good rock exposures. Within an area easily covered on foot or by road most of the geological features mentioned in this study can be seen 'in the field'. 2 3

7. A GEOLOGICAL GUIDE TO THE NORTH WEST CORNER OF CANNOCK CHASE

Figure 21 shows localities of geological interest in the north-west corner of the Chase. All are accessible by car, with only a small amount of walking required at Oat Hill and Brocton quarry. AH roads are shown, with approp rtate parking locations. Alternatively, the localities can be visited on foot and the appropriate paths are also shown on the-map. Tile walk from the Salnall Hills quarry to Brocton quarry is an attractive and instructive one, but is linear rather than circular, so the visitor wishing to take it would have to make transport arrangements? _To complete the walk and take in all the features of interest requires more than just a morning or afternoon. Such is the network of pathways in this part of the Chase that a stranger to tire area would be well advised to use the appropriate Ordnance Survey maps. These are the 1:75000 sheets SJ 81/91 (Cannock) and SJ 82/92 (Stafford),

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Satnal l Hills quarry (983208)

The quarry is reached from the main A513 road by two short tracks, both usable by cars. It is now used as a picnic/recreation spot and there is parking space on the quarry floor.

The quarry is situated just east of the axis of the Cannock Chase anticline and accordingly, in the quarry face, the rocks can be seen dipping at a few degrees to the north east. Figure 22 shows the position of the quarry face in relation to the anticline.

Figure 22 Position of the Satnall Hills quarry on the Cannock Chase anticline

The fold has a gentle plunge (or pitch) to the north. A plunging anticline is one in which the crest of the upfold is at an angle to the horizontal (fig. 23)

In the north-east corner of the quarry (to your right as you look at the main face) the rock face turns to face east, providing a section in which the northerly plunge of the anticline can be seen (fig. 22).

The main quarry face clearly exhibits the characteristics of the flat-bedded and sandstone fades (fig. 11) and the loose gravel on the quarry floor offers opportunity for studying individual pebbles. Features to look for in the quarry include the following:

(a) Repeated, graded sequences are clearly visible in the quarry face. Use the Hjulstrom graph (fig. 10) to work out the velocities required to transport pebbles of different sizes.

(b) Sandstones occur as beds which protrude from the quarry face and also as impersistent lenses. Two sandstones are present. The first is pale brown and has some pebbles and cross bedding. One such bed near the base of the quarry has an uneven upper surface indicating erosion prior to deposition of the overlying conglomerate (fig. 11). The second kind is a deeper red sandstone with no pebbles, containing small, shiny flakes of mica.

(c) The pebbles are well-rounded and smooth, though some may exhibit collision markings. Post-depositional load pressures on the pebbles have caused fracturing and pressure solution pits (fig. 9) with associated hard-cemented sand rims.

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(d) The great majority of pebbles are grey-purple quartzite, and pebbles of white vein quartz are also easy to find. Much less common, but also present, are pebbles of conglomerate, breccia, gritstone, sandstone, rnudstone, limestone and chert. These are sedimentary rocks and very occasionally signs of fossils may be found in them, especially in the finer-grained ones. The shells have been weathered away leaving behind holes or cavities (moulds) that display the imprint on the fossil. Sometimes the solution of a fossil shell leaves behind the mud infill, and internal casts of this sort can also be found. As well as sedimentary rocks, igneous rocks can also be found and can often be recognised as such by the well formed crystals which they contain. However, they occur in various states of decay and some are completely rotten. Some of the most striking pebbles, especially when wet. are the black, green and pink tourmalines, which are often found in association with white quart/. Also adding colour are varieties of chalcedony such as agate, with its characteristic banding, and jasper which occurs in reds, greens and browns.

(e) Crumbly fragments of red mud (the mudstones fades) may be found as inclusions in the quarry face.

The sub-glacial channels

From the Satnall Hills quarry, by car, drive back to Milford Common and turn left opposite the Barley Mow Inn towards Brocton. Just to the east of this road lies tire Interconnecting system of sub-glacial channels and intervening hills. There are several parking spaces lying just off the road and from any of these a short walk to the summit of Oat Hill offers opportunities to view not only the channels, but to the west the Sow valley where the meltwater emerging from the channels ended up.

On foot from the Satnall Hills quarry part of the channel system can be followed, and the lack of a single direction of gradient experienced at first hand. From the quarry, cross the A513 to the picnic site 0pp08lte (the Punch Howl). From here a broad track leads south-westwards. At first tho track flnnks the Shorbrook valley hut then enters the channel between Oat Hill and Harts Hill Follow the track in this same direction to the point near Mere Pool where several tracks meet (979201). Here, the route of the first-world-war railway track can bo observed. It follows a cutting in the channel between Oat Hill and Broc Hill and crosses the Mere valley on a man made embankment. Mere Pool is the result of excavating gravel foi the embankmenl -rod was part of the drainage system for the military camp. From here it is possible to ascend Oat Hill and observe more of the channel system.

The glacial boulder (980182)

By car from Oat Hill follow the road into Brocton and at the small green turn left into Chase Road. The steep hill leads up to the plateau surface I his is a private, but accessible, road with 'sleeping policemen' to ensure slow driving Before reaching the boulder the road flanks a large quarry cut into the side of Coppice Hill (977192). The quarry itself is fenced off but from the road good views of the pebble beds can be obtained. The boulder is a further kilometre along the road, by the trlangulatlon pillar and there is a car-parking space by the boulder, on the opposite side <>f the road,

If continuing the walk from More Pool to the boulder follow the old railway embankment and the track along the southern side o! Mere valley. 1 his track offers a fine opportunity for studying the characteristics of a sub glacial channel. The track turns to follow Flollywood Slade and emerges at a 'crossroads' above the Coppice Hill quarry. Turn right here and walk down to the road, turn left and the boulder is a further 800 metres along the road.

At the boulder study the granite rock and look for the pink veining which runs diagonally through it (fig. 17). Notice also the pebbles in the pedestal. The large ones just beneath the boulder show what kind of size the Budleighensis river was capable of transporting and reference could be made to the Hjulstrom graph (fig. 10) to work out the velocities required.

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Brocton quarry {968186}

To reach Brocton quarry by car from the glacial boulder drive back into Brocton. At the end of Chase Road turn left into Oldacre Lane. At the end of this lane there is a small car-parking area and from there is it a short walk to the quarry.

On foot from the glacial boulder follow the path shown on figure 21 which leads to a road. Follow the road down for 300 metres and turn right into the track opposite Brocton Gate Farm. The quarry can then be reached by scrambling up the bank to the right or by following the track round to the end of Oldacre Lane and then into the quarry by its main entrance.

Because the quarry is flooded access to the rock face is impossible, but good views can be obtained across the pool (fig. 14). The two pebble facies, as well as sandstones can be seen in the quarry face and the cross-bedding can be observed sloping down to the north, confirming the direction of f low of the Budleighensis river. Binoculars are an advantage here because they allow graded sequences to be discerned in the sloping cross beds (and can also abe used for studying the bird wildlife in this nature reserve).

A short ' code ' for geological f i e ldwork on Cannock Chase

1. Wear strong footwear if walking and suitable clothing for the weather conditions. Jeans are unsuitable in wet weather, unless protected by waterproof over-trousers.

2. Walkers should beware of traffic, especially on the busy A513, and drivers should take care to park off roads.

3. If walking the route, do not overestimate your own capability. As a linear route it is about 4 kilometres (2 Vt miles) long, mainly along undulating, rough tracks.

4. Do not attempt to climb on the Satnall Hills quarry face. The pebble beds are too loosely cemented to support weight. The top of the quarry is accessible by footpath but there is no fencing, so take care near the edge. Remember, there could be people below, so do not dislodge pebbles.

5. The quarry face is stable enough to make the quarry safe as a picnic site, so safety helmets are not essential, but do not undermine the face by hammering. Geological hammers are not required for this fieldwork as there are plenty of loose pebbles lying around.

6. Follow the country code. Keep to the tracks; do not disturb living plants and animals; do not leave litter; do not play a radio and generally, disturb the environment as little as possible.

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