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The hills are shadows, and they flow

From form to form, and nothing stands;

They melt like mist, the solid lands,

Like clouds they shape themselves and go.

ALFRED TENNYSON: In Memoriam

A WANDERER returning to his native village in the Peak District finds his favourite haunts unchanged. The dales and streams, cliffs, hills and moors are all just the same as they were in his youth. If, however, he has studied the rocks they tell him that this peaceful, enchanting scene is but an episode in a long and eventful story which moves so slowly that for the brief period of his lifetime it seems to have been at a standstill. His going and coming have been no more than the flicks of a fly’s wings. That story is recorded for him in the rocks of the district; in the limestone of the uplands on the south; in the grits capping the moorlands of the north and forming the ridges which girt the uplands on either side; and in the shales which underlie the fertile vales that lie between the areas occupied by these two types of rock (Fig. 2, see here).

THE FOUNDATION ROCK

The limestone teems with fossils which may be seen and collected wherever the surface of the rock has been washed by the rain for a long time or etched by weak acids seeping down from the covering of soil. These fossils are the remains of creatures that lived in an ancient sea which 280 millions of years ago occupied the whole district. How different was the outlook then! Blue sea extended to the horizon in all directions except the south where, in the offing, stood the miniature mountainous island of Charnwood which lay across the area now known as the Midlands. Its rivers were small and carried very little sand and silt into the sea, the waters of which were in consequence clean and clear. The sea was of no great depth and the scenery of its sunlit floor varied from place to place. Here and there were forests of stone lilies, animals that were allied to the starfishes. Each one grew upon a tall stalk built up of rings of lime piled one upon another to a height of eight or ten feet. The main body of the creature was at the top and carried five branching arms spread out like the fronds of a palm tree, to catch both the sunshine and the small organisms upon which it fed. When the stone lily died its flesh decayed and the fairy bead-like rings of the stalk and the limy framework of the arms and body fell to the floor of the forest and in the course of many generations built up deep deposits of calcareous debris.

Out in the open, beyond the bounds of the stone lily forests, lay coral reefs. These were produced by the combined activities of myriads of polyps. Superficially they resembled modern reefs but the structural details of the individual corals were strikingly different. Surrounding the forests and reefs were spacious wastes of mud formed from the shells and bodies of minute organisms which fell in a perpetual drizzle from the waters overhead. Burrowing in the mud or crawling over its surface were many worms and other creatures that fed upon the mud or caught the drizzle as it fell. These latter included lamp-shells, a type of animal that is scarce today but was then varied and numerous and played a much more important part in the economy of the sea floor. Like cockles and mussels their bodies were also enclosed in shells, often prettily shaped and ornamented. They were usually small shells but some were giants a foot or more in diameter.

Of special interest were certain curious molluscs belonging to the far-off ancestral stock of the Pearly Nautilus which lives today in the waters of the Pacific Ocean. Like this, they had shells which were divided into a succession of chambers separated by thin partitions. One large type is known as Orthoceras because its shell was straight and not closely coiled like that of the nautilus. Provided with a battery of tentacles round its mouth and with an apparatus for jet propulsion, it preyed upon fishes and other more peaceful creatures. The Goniatites were much smaller and their shells were closely coiled. In them the partitions between the chambers were folded, sometimes in sharp angles (gonia=angle) which suggested the name. These beautiful little creatures may be pictured as spending their days flitting or crawling over the coral reefs and browsing upon the coral polyps.

Fig. 2. Geology of the Peak District. A–B is the line of the section shown in Fig. 4 (see here)

The shells of all these animals added their quota to the deposits that were being laid down upon the sea floor. Nevertheless, though the sea was shallow, it did not become filled, for its foundations were subsiding at about the same rate as the deposits were accumulating. Thus it came about that they ultimately attained a thickness of nearly 2,000 feet.

A temporary but fascinating feature in this submarine scenery was the occasional presence of small volcanoes. The ashes which they shot forth into the waters above settled down and became mixed with the mud beneath. Sheets of lava, full of steam bubbles, were poured out and flowed far and wide over the sea floor (Fig. 3, see here). The dark-coloured rock into which these lavas solidified is known as Toadstone and has a striking appearance due to the fact that the bubbles have been filled with a white mineral.

All the deposits described above consolidated and became the limestone which forms the Derbyshire upland. It is sometimes called the Mountain Limestone but to geologists it is known as the Carboniferous Limestone.

THE LATER ROCKS

The northern shores of the Carboniferous Limestone sea lay 200 miles away and stretched across the centre of the Scottish region. Scotland, at that time, was part of a great North Atlantic continent drained by large rivers flowing southwards. The general geographical picture thus presented was not unlike that of the United States with the Mississippi flowing into the Gulf of Mexico. These rivers carried the debris formed by the destruction of the uplands and by the rain washing the plains into the sea. Deltas of grit and sand were formed and banks built out along the shoreline. The fine muds were, however, carried farther afield and eventually reached the Peak District. Then for some time the sea-water was alternately clear and turbid, but eventually the latter condition prevailed. The mud accumulated and in course of time became those rocks known as the Edale shales which underlie the peaceful meadows of Edale and Darley Dale.

The fauna in the waters underwent a corresponding change. The stone lilies, corals and many of the brachiopods and molluscs departed from the area. A few of the last remained and were joined by other kinds of goniatites and bivalves. Meanwhile the deltas and sandbanks extended and began to invade the district from time to time. The quality of the water also changed from being saline through brackish to fresh. Marine animals disappeared and were succeeded by a less varied and sparse population. At first this included Lingula, a curious tongue-shaped brachiopod which had already existed for 250 millions of years from Cambrian times onwards, and was destined to continue in the world for a similar stretch of time until the present day. As the waters freshened still more, Lingula and its associates migrated elsewhere and were replaced by crowds of bivalves such as Carbonicola which resembled the mussel of the present-day rivers and canals. Towards the close of this phase in the story of the Peak District the occasional influxes of coarse sediments became more copious. Banks of grit and gravel were formed and ultimately became that massive hard rock known as Millstone Grit.

Fig. 3. Volcanic rocks occurring in part of the limestone of the Peak District. (Based on H. H. Bemrose)

Owing to slight oscillatory movements in the level of the region these deposits were sometimes raised above water level and produced a low-lying landscape of sandbank and water channels. Spores wafted by the breezes, seeds carried by the streams from the continent enabled plants of the northern continent to settle on this new land surface. In the warm moist atmosphere they quickly germinated and produced a jungle growth of fern-like plants and strange-looking trees. Of the latter the smaller ones resembled the Tree Ferns which now grow in the tropical forests of the East Indies. Others, known as Calamites, were closely related to the horse-tails, those tall weeds which look like miniature Christmas trees and today grow profusely in wet waste places. Some of the trees towered to a height of 60 or 80 feet and had trunks as much as five feet thick. The bark was often decorated with scale-like markings which suggested the name Lepidodendron (lepido=scale) for these trees. Their branches and twigs had a furry covering of small lancet-shaped leaves. The modern relatives of these trees are not to be sought for in luxuriant forests but on bleak moorlands where the Stags Horn Moss (Lycopodium) is to be found straggling through the grass. These plants do not grow from seeds but, like ferns, they reproduce by means of minute pollen-like spores.

Among the undergrowth of ferns and in the pools and sluggish streams were lowly types of four-legged animals, represented today by newts or salamanders, creatures which resemble fishes in that they lay their eggs in water and their young must spend at least the early part of life breathing by means of gills.

These conditions, so different from the present, lasted long enough for deep deposits of vegetable debris to accumulate and become peat. A gentle down-sinking of the whole region then ensued. The sandbanks with their cloak of peat were submerged and were gradually buried under thick beds of mud and clay. The peat, squeezed by the pressure of the increasing load and changed by complex chemical reactions ultimately turned into coal. The mud also was compacted into shale.

The sequence of events last portrayed was repeated a number of times and thus was built up a series of massive grit layers interspersed with shales and occasional seams of coal. These rocks were all destined, in the fullness of time, to play a large part in the scenery and other amenities of the Peak District. The layers of grit and shale, which had attained a thickness of a thousand feet, extended far beyond the bounds of the district and covered a vast area including the north of England and southern Scotland.

The transport of so much sediment from the North Atlantic continent implies a corresponding destruction of the rocks in its uplands and a general wearing down of its whole surface to lower levels. As one outcome of all this the formation of coarse grit ceased and henceforth only fine sand, silt and mud were transported into the southern waters. Deposition and sinking went on in unison continuously, except for an occasional upward oscillation which converted much of the region, including our district, into an extensive fenland of mud-flats, upon which forests grew once more and thick peats accumulated. When the sinking movement was renewed, sea-water sometimes flowed in and spread everywhere, bringing with it marine animals, especially goniatites. Such marine conditions lasted only for a short time and gave place to a long period of fresh-water conditions when mud accumulated and buried the peat, which was in turn converted into coal.

In this way 4,000 feet of clays, shales and fine sandstone with occasional coal-seams and marine bands accumulated. These are spoken of collectively as the Coal Measures. The economic importance of the coal suggested the word Carboniferous as the most suitable name for the whole sequence of rocks hitherto dealt with, including Mountain Limestone, Edale Shales, Millstone Grits and Coal Measures; and for the long period of time, amounting to 60 millions of years, which they represent.

The countryside now occupied by these Coal Measures is comparatively low-lying. Apart from tip heaps it has a scenic beauty of its own but makes no contribution to the amenities of the Peak District. It does however provide the basis for the livelihood of dense populations. From its cities at week-ends and holiday times streams of hikers and pleasure-seekers pour forth to find renewal of energy and refreshment of mind and spirit in the dales and on the moors.

THE ARRANGEMENT OF THE ROCKS

The process of forming those rocks which make up the Peak District was completed millions of years ago. Two sets of agencies then came into action and recorded their activities in quite different characters. One, which was concerned with rearranging the rocks, completed its work in a relatively short period of time. The other, concerned with destroying and carving the rock of the district, experienced a long interruption of its activities during the Mesozoic Era.

The hiker on Kinderscout or Bleaklow rejoices in the fresh breezes and in the fact that he is 2,000 feet or more above the sea. Though he takes that fact for granted it is nevertheless full of significance. Kinderscout is a little plateau with a relatively flat surface defined along its margins by rough, often precipitous slopes of Millstone Grit, for that is the rock which immediately underlies the plateau. When it was formed, that slab of grit was part of an extensive sandbank lying close to sea-level. As already seen, it was subsequently buried under Coal Measures down to a depth of some 5,000 feet, that is to say 7,000 feet below its present level. How then did it come to be at its present height? This was brought about by the joint action of two types of movement of the earth’s crust, of which the first must now be discussed.

Just as the process of burial described above was being completed, great revolutionary events began to take place across the south of the British area and the north of France. Mighty pressures in the earth’s crust acting from north and south crumpled the rocks along a belt of country about 200 miles wide and heaved them up into a mountain range of Alpine and even of Himalayan proportions. A small backwash of these great events was felt in the region of the Peak District. That threw the Carboniferous rocks into a number of folds which have exerted an important influence upon the physical features as seen today. As one result, the western margin of the district was crumpled into a series of narrow folds which merged into the main axis of the Pennines. Chief among these was a broad fold, elongated from south to north, which is known as the Derbyshire Dome. In cross-section it is asymmetrical and has the form of a wave that is just about to break; that is to say its eastern side or limb rises slowly to a crest from which the western limb drops down more rapidly and is broken by a series of rock fractures or lines of faulting. The slowly rising eastern limb is itself crossed by a succession of minor folds which run at right angles to the crest and which curve southwards beyond the east margin of the dome.

It is, of course, impossible to see a complete section across this and the other folds at any one point, but the visitor wandering from place to place, along footpath, road or rail, will see the rock layers exposed to view in valley sides, in railway cuttings and quarries, or in cliff faces. Sometimes they are seen to lie flat and horizontal. At other times they dip gently or even steeply. Geologists have carefully measured these angles and recorded them upon maps from which they have been able to piece together the structure of the region as a whole (Fig. 4, see here).

The contemplation of such folded rocks creates the impression of a time of turmoil and grievous upset, a time when nature took the rocks in her hands and squeezed them as a child squeezes its plasticine. That impression is, however, quite false. The moulding of the rocks into these varied and complex forms was spread over a period of several millions of years and took place so slowly that except for an occasional earthquake the placidity of the scene was never disturbed.

It must not be supposed that any part of the district ever attained such lofty heights as 7,000 feet or more, for as the crests of the folds rose above the general level of the surrounding country, they became the targets for the destructive action of such agencies as frost, rain and running water. In this way the Coal Measures, while they were being slowly uplifted, were simultaneously skimmed off the upfolds and the debris was transported to the lowlands which during the succeeding or Permian period, were to some extent covered by sea. At the base of the Permian rocks today there is a layer of this debris, a rock called Breccia made up of angular fragments of Carboniferous rocks. This lies unconformably upon the upturned and bevelled edges of Coal Measure shales, sandstones and coal-seams. This unconformity is a silent witness to the fact that when this Breccia was being laid down the Coal Measures covering the upfolds in the Peak District had been removed. The Millstone Grits thus exposed were then attacked by the same destructive agencies. Being more resistant they were not so completely destroyed except over the south of the dome where the limestone core was exposed to view. Along its margins the limestone layers tilted downwards and disappeared under the surrounding shales and grits.

In this way the general surface plan of the district was established no less than 220 million years ago. In the southern half it exhibits a large roughly oval area of limestone encircled by Millstone Grit and shales with grit layers, which are arranged in a pericline; that is to say they dip outwards from the centre of the limestone dome. In the northern half of the area the grit covering remains. There the rocks are horizontal or nearly so along a north-south axis, but on either side of this they dip outwards beneath the east and west flanks of the dome.

Meanwhile, the mighty folding movements along the axes of the Armorican mountains had ceased but the destructive agencies went on working. As long as any part of this intensely folded zone stood up above the level of the lowlands the rivers flowed down the slopes and spread their burden of gravel, sand and mud across the plains. Thus as the mountains were being laid low the plains were being levelled up. The Peak District with its folds thus became only a small feature in the vast expanse and was therefore gradually covered up and buried under the waste from the distant mountains and remained out of sight throughout the whole of the Mesozoic Era.

A PROLONGED BURIAL

Subsequent to this burial, a cavalcade of great events swept across the Peak District without leaving any trace of its passage upon the present landscape. Nevertheless the story would be incomplete without some reference to those events.

The uprising of the Armorican mountains athwart the path of the dominant rain-bearing winds greatly influenced the climate of the British area. The warm moist climate which encouraged the growth of the Coal Measure forests and swamps passed away, a dry arid climate set in and barren deserts replaced the luxuriant forests and tangled swamps. The Peak District area lay hidden away under the centre of a vast plain swept by hot winds and dust-storms.

The section is drawn along the line A-B as shown on the geological map in Fig. 2 (see here). The individual grits of the Millstone Grit series, being lenticular in torm cannot be traced continuously across the area. Those outcropping in the east are indicated by inital letters as follows: K-Kinderscout Grit, C-Chatsworth Grit, RR-Rough Rock. On the west side Black Edge is formed by the Chatsworth Grit. (Prepared by Prof. W. B. R. King, Sc.D., F.R.S., based by permission on maps of the Geological Survey and other sources)

Fig. 4. Geological section across the central Peak District

Far away on the distant mountains, during rainy seasons, the rivers became swollen into flood. Their waters spread on the plains as shallow transitory lakes and deposited their sand and mud in thin sheets everywhere. Sometimes the flood-waters found their way into and replenished the more permanent lakes. When the dry season set in and the floods ceased the temporary lakes were dried up and the permanent were reduced by intense evaporation. In both cases the salts that were in solution were left behind. The lakes consequently became more and more salty until such minerals as calcium sulphate and sodium chloride were precipitated. In modern times these chemical deposits have been exploited for gypsum and alabaster in south Derbyshire and for salt in Cheshire.

At last this arid period known as the Triassic drew to a close. Its plains began to subside, were carried down below sea-level so that these too became submerged. This event ushered in the second division of the Mesozoic Era, known as the Jurassic Period. At first this new sea tended to be muddy but later its waters were often clear during long stretches of time and then limy deposits were laid down. The latter contained numerous small round pellets resembling the roe of fishes. For this reason the rocks formed from these deposits are known as Oolitic Limestones (oos=egg).

The animals that came along with these marine waters were many of them similar to but not identical with those that lived in the Carboniferous sea. There were some near relatives of the Pearly Nautilus but they were few in number and all closely coiled. Descendants of the goniatites abounded but their shells were more prettily ornamented and the partitions between their chambers were complexly folded and even frilled. They are therefore distinguished under the name Ammonites. When the waters were clean and clear, stone lilies and corals became common but differed from their Carboniferous allies in many details. Sea-urchins, sea-snails and bivalves also contributed their quota to the building up of the Jurassic deposits and rocks.

Shoals of fishes swarmed in the open waters. Their bodies were clothed in an armour-like mosaic of thick bony scales covered with shiny enamel. They were preyed upon by a new type of animal unknown in Carboniferous waters. These were large reptiles shaped like fishes and almost as perfectly adapted for a life spent wholly in the sea. Conspicuous among these was Ichthyosaurus, a fearsome creature with a large mouth as well-equipped with sharp teeth as that of the crocodile today.

During Jurassic times the floor of the sea was subject to occasional undulatory movements. The crests of some of these undulations rose above sea-level and formed long stretches of land on which grew strange-looking trees allied to the Monkey Puzzle (Araucaria) and to tree ferns. As yet there were no herbs with coloured flowers, no grasses, no grassy swards. The landscape must have been a drab expanse, varied here and there only by the dark green foliage of the trees. The scene was, however, enlivened by the presence of many kinds of reptiles of all sizes, crawling, running on all fours or leaping like the kangaroo. Some small forms fed on insects. The larger ones fed on foliage or preyed upon their fellows. There were also some with such large bulky bodies that only by wading up to their necks in water could they support their great weight. Other lightly-built reptiles had large flimsy wings like those of the bats. These pursued and fed upon dragonflies and other flying insects which were at home in the air. As yet there were no birds. From all this it is evident that this period was one in which reptiles dominated air, land and sea. There were a few furred animals or mammals, but they were no larger than rats and mice and survived largely because they led a furtive existence in the crevices of rocks or hiding in the foliage of the trees.

The Jurassic Period was brought to a close by a slow, nearly uniform, lowering of the earth’s crust in the British area. The sea flowed in and eventually submerged the whole with the exception of the highlands of Scotland and Wales. The waters of this sea were clear and warm and teemed with myriads of minute organisms that made tiny shells no larger than a pin’s head. These lived their lives floating in the sunlit waters near the surface. When they died their shells drizzled down to the floor and helped to form deep deposits of white mud which ultimately solidified into the white chalk which makes up the cliffs of Kent and the Downs. The period during which this chalk was formed is known as the Cretaceous (creta=chalk). It was the last of the great divisions of the Mesozoic Era. The thickness of the chalk deposits grew to be 1,000 feet or more and the laying down of the last of those deposits marked the ending of that period and era. The long process of burying the Peak District now ceased.

RE-EMERGENCE

The Mesozoic Era lasted about 120 million years, during which the Peak District lay hidden under an ever-increasing cover of limestones, sandstones, clays and chalk. The time of its eclipse was now, however, drawing to a close, for the British area, after its prolonged period of sinking, began to rise once more. The floor of the chalk sea was uplifted and converted into dry land, an event which marked the opening of the Cainozoic Era.

This uprising did not take place uniformly over the whole area but was most marked along a line running northwards through the area of the Peak District and the Pennines and resulted in the appearance of a low elongated dome. This formed an island which had the scenic features of chalk downs. The crest of this dome served as a natural water divide and many streams flowed down its gentle slopes. The sea floor continued to rise and the boundaries of the land were extended outwards in all directions. The streams lengthened out across the broadening plains as these rose out of the water. Uniting with one another they merged into larger rivers flowing to the sea which by this time was far away.

Wherever streams and rivers flowed they excavated channels which as they deepened were widened into valleys. All this involved the gradual removal of large quantities of rock. The chalk was the first to be attacked. After that the more deeply seated layers of the Jurassic and Triassic rocks were penetrated, a piece of work which was begun and carried farthest upon the original dome. There at last the Carboniferous rocks began to peep out along the beds of the streams. The latter continued to carry away the waste formed by the gradual destruction of the younger rocks on either side of the valleys. But still the streams went on cutting their channels more and more deeply into the Carboniferous rocks. These channels were also widened into valleys and these old rocks came to occupy a correspondingly larger portion of the valley sides. The Mesozoic rocks were steadily reduced to mere remnants capping the hills on either side. Eventually even these disappeared completely and left the Carboniferous rocks in sole possession of the Peak District.

The long eclipse had ended. The precise date for that event is unknown for the complete removal of all the waste from the destruction of so much rock left no tangible evidence behind. It is, however, probable that it happened shortly before or at the opening of the Pliocene period, which was the last of the four major periods into which the last or Cainozoic Era has been divided.

The removal of the Mesozoic rocks was not the only great event that happened during this long period of time. The living garment that clothed the landscape also changed. True flowering plants that produced seeds in closed caskets had come into being in other parts of the world during Cretaceous times. These invaded the new lands as they rose into being over the British area. Among them were many kinds of woodland trees and numerous flowering herbs and grasses which covered the ground as with a multi-coloured carpet. Over this beetles crawled, butterflies flitted and bees hummed. Winged reptiles had passed into oblivion and their place was taken by birds. The long reign of reptiles was over and only a few insignificant representatives remained. On the other hand, mammals were rising rapidly to a position of dominance. A wonderful variety of insect-eaters, vegetable and flesh feeders haunted the woodlands and sported on the grassy plains. Many of these began to show recognisable resemblances to modern and familiar types, to horses, deer, wolves, cats and monkeys. Usually they showed some features that would seem to us to be peculiar.

The Peak District now entered upon the last phase of its long and eventful history. The carving of the scenery as seen now had begun. Even during the time of its eclipse the blue-print for the general arrangement of its hills, valleys and moorlands was being drawn. When in early Cainozoic times the streams and rivers first began to score the surface of the chalky slopes, their direction of the flow was the same as that in which the Mesozoic rocks were dipping. Just as a saw when it has begun cutting through a log of wood must keep to the same line in which it started, so likewise a stream, having once made a track, must keep to it. Having made its bed it must lie on it.

Thus it came to pass that the arrangement of streams laid down upon the surface of the chalk was eventually incised into the underlying Carboniferous rocks. These, however, had been folded long ago and the layers had been tilted at varying angles in differing directions. Henceforth the streams flowed on regardless of the directions in which the rocks dipped or of the way in which they were folded. Such a drainage system is said to be superimposed. One outcome of this is that the scenery along the valleys is more varied than alongside normal streams. Thus, for example, in the main valley of the area, that of the Derwent, grit scars, limestone gorges and wide fertile dales alternate with one another.

THE ICE AGE

One last event remains to be mentioned. The period popularly known as the Great Ice Age was on the whole a time of mild and even warm climate, interrupted now and then by polar conditions when icefields formed over the mountainous regions and, flowing thence, covered the lowlands and filled up the adjoining sea basins. During one of the earliest of these interruptions, or glaciations, all but the extreme south of England was covered with ice. Nevertheless the presence on the South Pennine moorlands of rocking stones and other stones fantastically shaped by prolonged weathering in pre-glacial times indicates that some of the highest parts of the Peak District were never subjected to the scouring action of overflowing ice.

During a later glaciation ice from the Irish Sea invaded the lowlands of Lancashire and Cheshire and, impinging against the flanks of the West Moors, attained an altitude of 1,250 feet. At that time a trickle of ice finding its way across a gap at the Dove Holes, northwest of Buxton, entered the tributary valleys of the Derwent and even reached the vicinity of Matlock. This ice did not, however, leave any appreciable impression upon the scenery of the district. Nevertheless, whenever such arctic conditions prevailed they intensified the action of more normal agents. The snow which accumulated in the winter melted in the early summer. On the shale areas the ground became sodden in the daytime and frozen at night with the result that soil creeping and land sliding took place on a massive scale and greatly accentuated the concavities of the valley slopes.

Elsewhere in England and north-west Europe, Old Stone Age Man appeared upon the scene and gained a livelihood by hunting. Outside the Peak District, though quite near, the caves at Creswell have yielded a very full and unique record of his sojourn in this part of England. In striking contrast is the solitary discovery of one flint implement referable to that time within The Peak in the vicinity of Wirksworth. It is sufficient, however, to indicate that Palaeolithic Man was an occasional visitor. As will be seen later, evidence for the presence of prehistoric man is much more abundant in post-glacial times.

REFERENCES

MARSHALL, C. E. Guide to the Geology of the East Midlands. University of Nottingham (1948)

Memoirs of the Geological Survey

WRAY, D. A., EDWARDS, W., and TROTTER, F. M. The Pennines and Adjacent Areas. Third edition. British Regional Geology. H.M.S.O. (1954)

WATERS, R. S. and JOHNSON, R. H. The Terraces of the Derbyshire Derwent. East Midland Geographer: 9 (June, 1958)