Chap19

[302]

CHAPTER XIX.

RECAPITULATION OF THE GENERAL ARRANGEMENT OF THE STRATIFIED FORMATIONS OF ENGLAND.

THE geology of England and Wales is much more comprehensive than that of Scotland, in so far that it contains many more formations, and its features therefore are more various. England is the very Paradise of geologists, for it may be said to be in itself an epitome of the geology of almost the whole of Europe, and much of Asia and America. Very few European geological formations are altogether absent in England. On the Continent, however, some have a larger importance than in England, being more truly oceanic deposits in some cases, and more thoroughly developed lacustrine or terrestrial deposits in others. In some countries larger than England the whole surface is occupied by one or two formations, but in England nearly all the formations shown in the column (p. 30) are more or less developed. Those of Silurian age lie chiefly in England, in Cumberland and Westmoreland, and in the west, in Wales (fig. 57, p. 304). Above them lie the Old Red Sandstone and Devonian rocks, occupying large areas in Herefordshire, Worcestershire, South Wales, and in Devonshire and Cornwall. Above the Old Red Sandstone come the Carboniferous strata, which form large tracts of Devonshire, Somerset, and part of Gloucestershire,

[English Formalions. 303]

and in South Wales skirt the Bristol Channel, and stretch into the interior in Pembrokeshire, Glamorganshire, and Monmouthshire; while in the north they border North Wales, and form a broad backbone of country that reaches from the borders of Scotland down to North Staffordshire and Derbyshire. Other patches, here and there, rise from below the Secondary strata into the heart of England. (See Map.)

The general physical structure of England, from the coast of Wales to the Thames, will be easily understood by a reference to fig. 57, p. 304, and to the following descriptions; and this structure is eminently typical, explaining, as it does, the physical geology of the greater part of England south of the Staffordshire and Derbyshire hills.

The Lower Silurian rocks of Wales (No. 1) consist chiefly of slaty and solid gritty strata, accompanied by, and interbedded with, numerous feispathic lavas and beds of volcanic ashes, marked + ; and mingled with these there are numerous bosses and dykes of felstone, quartz-porphyry, greenstone (diorite), and the like. These last, by their superior hardness, give a mountainous character to the whole of North Wales, from Merionethshire to the Menai Straits. In part of north Pembrokeshire also, in a less degree, igneous rocks are largely intermingled with the Lower Silurian strata, and these, by help of denudation, now form a very hilly country.

Without again entering into details, it is here sufficient to state that the Cambrian arid Lower Silurian epoch was ended in the British area by disturbance and contortion of the strata, and their upheaval into land. This disturbance necessarily gave rise to long-continued denudations of this early English land, both by ordinary

[303]

FIG. 57

Diagrammatic Section from the Menai Straits across Wales, the Malvern Hills, and the Escarpments of the Oolitic Rocks and the Chalk See Map, line 6.

Nos. 1 to 3, 5, and g represent the disturbed Cambrian, Lower and Upper Silurian, and Old Red Sandstone mountainous country of North Wales, the adjacent countries on the east, and the Malvern Hills g.

6 to 8 the plains and slightly undulating grounds of the New Red Sandstone, Red Marl, and Lias.

9 and 10 the great Oolitic escarpment of the Cotswold Hills, forming the first tableland.

11 the great escarpment of the Chalk, forming a second tableland, above which lie the Eocene strata, 12.

The Upper Oolites, close below the Chalk escarpment 11, are in places of less height relatively to the sea than the edge of the Oolitic escarpment at 9.

[English Formations. 305]

atmospheric agencies, and also by the action of the waves of the sea of a younger Silurian period, the evidence of which is seen in the conglomerates of the Upper Llandovery beds, which, mingled with marine shells, lie unconformably on the denuded edges of the Cambrian and Lower Silurian strata of the Longmynd in Shropshire, like a consolidated sea beach. Slow submergence then took place beneath the Upper Silurian sea, in which the Upper Silurian rocks were gradually accumulated unconforinably till, perhaps, they entirely buried the Lower Silurian strata (2, fig. 57), for in places they attained a thickness of from three to six thousand feet.

As shown in Chapter VIII. the uppermost Upper Silurian beds of Wales pass insensibly into a newer series, known as the Old Red Sandstone (3, fig. 57), formed, if we include the entire formation, of beds of red marl, sandstone, and conglomerate, which in all the British areas by the absence of marine shells, and the occasional presence of crocodilians, land reptiles, and of fish (whose nearest allies live in the rivers and lakes of America and Africa, or in the brackish pools of Australia), seem to have been deposited in lakes. In Wales, these strata again pass upwards into the Carboniferous Limestone, which is overlaid in Wales, Derbyshire, and, Lancashire, by the Millstone Grit and the Coalmeasures.¹

In Yorkshire, Durham, Northumberland, and Scotland, the Carboniferous Limestone has no pretension to be ranked as a special formation, for it is broken up into a number of bands interstratified with masses of

¹ This is not shown in fig. 57, but the Carboniferous Limestone No. 4 is shown in fig. 67, p. 330, lying, as it does in North Wales, unconformably on Silurian rocks.

[306 English Formations.]

shales and sandstones bearing coals. In fact, viewed as a whole, the Carboniferous series consists only of one great formation, possessing different lithological characters in different areas, these having been ruled by circumstances dependent on whether the strata were formed in deep, clear, open seas, or near land; or actually, as in the case of the vegetable matter that forms the coals, on the land itself.

The English Carboniferous rocks differ from the Scottish beds in this, that in general they have not been mixed with igneous matter, except in Northumberland and Derbyshire, where, in the last-named county, the Carboniferous Limestone is interbedded with ashes and lava, locally in Derbyshire called 'toadstones.' In South Staffordshire, Colebrook Dale, the Clee Hills, and Warwickshire, there is a little basalt and greenstone, which may possibly be of Permian age, intruded into, and perhaps also partly overflowing, the Carboniferous rocks in Permian times; but in Glamorganshire, Monmouthshire, North Staffordshire, Lancashire, and Yorkshire, where the Coal-measures are thickest, no igneous rock of any kind occurs. There and elsewhere in England the Coal-measures as usual consist of alternations of sandstone, shale, coal, and ironstone.

Next in the series come the Permian rocks (2, 3, 4, fig. 30, p. 141), which, however, rarely occupy so great a space in England, as materially to affect the larger features of the scenery of the country. They form a narrow and marked strip on the east of the Coalmeasures from Northumberland to Nottinghamshire, where they chiefly consist of a long, low, flat-topped terrace of Magnesian limestone (see Map), interstratified

[English Formations. 307]

with two or three thin beds of red marl sometimes containing gypsum. The scarped edge of this limestone, which is sparsely fossiliferous, faces west, and overlooks the lower undulations of the Coal-measure area.

There are other patches of Permian sandstones, mans, breccias, and conglomerates, in the South of Scotland, the Vale of Eden, and the West of Cumberland, and they are also here and there present on the borders of the Lancashire, North Wales, Shropshire, and all the Midland coal-fields, and on the Silurian rocks o the Abberley and Malvern Hills. Throughout all the districts enumerated above, these Permian strata chiefly consist of red sandstones, conglomerates, and mans, and part of them, in the districts of the Malvern and Abberley Hills, near Enville, and at Bromsgrove, consist of consolidated true Permian glacial boulder-clays.

The Permian beds form the uppermost members of the so-called Palæzoic or old-life period—a term somewhat unphilosophical, in so far that it partly conveys a false impression of a life essentially distinct from that of later times. But it is at present convenient, for all geologists know when the word palozoic is used what formations are meant, embracing all the strata from those of Permian date down to the lower Laurentian. During the time they were forming, this and other parts of the world suffered many oscillations of level, accompanied by denudations, as shown in previous chapters.

Before the end of this Palaeozoic epoch, the Permian beds were deposited in great inland salt lakes, analogous to the Caspian Sea and other salt lakes in Central Asia, at the present day. That area gives the best modern idea of the state of much of the world during Permian times.

[308 English Formations.]

In the same continental area, and partly on the Permian rocks, partly on older subjacent strata, the New Red Sandstone and Marl of our region were then deposited in lakes perhaps occasionally fresh, but as regards the marl certainly salt. These formations fill the Vale of Clwyd in North Wales, and in the centre of England range from the mouth of the Mersey round the borders of Wales to the estuary of the Severn, eastwards into Warwickshire, and thence northwards into Yorkshire, along the eastern border of the Magnesian limestone (see Map). They are absent in Scotland. In the centre of England the unequal hardness of its subdivisions sometimes gives rise to minor escarpments (Nos. 4 and 6, fig. 32, p. 154), most of them looking west over plains and undulating ground formed of soft red sandstone. Such escarpments are especially remarkable in the case of the Keuper sandstone, which lies at the base of the New Red Marl. These strata frequently form a good building stone, often white, and because of their hardness having better resisted denudation than the red sandstones below, they stand out as bold cliffy scarps facing west, with long gentle slopes to the east. Such are Hilsby lull, that looks out upon the Mersey, near Frodsham; the beautiful terraced scarps of Delamere Forest, the grand castle-crowned cliff of Beeston by the North Western Railway, near Tarporley, and the beautiful heights, often well wooded, that stretch from thence to the south, and form the Peckforton Hills. There, among spots that haunt the memory, in the ancient park of Carden, scarped by nature and cut into terraced walks and caverns, among the red and white cliffs grow great rhododendrons, which sow themselves in every mossy cleft of the rocks; luxuriant brackens, male ferns, lady ferns,

[English Formations. 309]

Lastræas, and others of smaller growth, while all forest trees attain a goodly growth, and low down in the flat, deer are grazing up to the gates of the old broadfronted timbered Hall. It is indeed a splendid sight to stand on the edges of these scarped hills and look across the great rolling plains of New Red Sandstone below, bounded by Moel Famau and all the mountains of North Wales that surround the beautiful Vale of Clwyd; or twenty miles further south, from the abrupt cliff of Grinshill, to see the tall spires of Shrewsbury backed by the renowned Caer Caradoc, the Wrekin, the high line of the flat-topped Lougmynd, and the craggy Stiper Stones.

The New Red Marl passes insensibly into the Rhætic beds, which again pass insensibly into the Lower Lias. In England there is therefore a gradation between the New Red Marl and the Lower Lias.

The Lias series, Nos. 3, 4, 5, fig. 5, consists of three belts of strata, running from Lyme Regis on the south-west, through the whole of England, to Yorkshire on the north-east: viz. the Lower Lias clay and Limestone, the Middle Lias or Marlstone strata, and the Upper Lias clay. The unequal hardness of the clays and limestones of the Liassic strata causes some of its members to stand out in distinct minor escarpments, often facing west and north-west. The Marlstone No. 4, forms the most prominent of these, and overlooks the broad meadow-land of Lower Lias clay that form much of the centre of England.

Conformable to and resting upon the Lias are the various members of the Oolitic series (6 to 11, fig. 5).¹ That portion termed the Inferior Oolite occupies the base, being succeeded by the Great or Bath Oolite,

¹ See also the 'Column of Formations,' p. 30.

[310 English Formations.]

Cornbrash, Oxford Clay, Coral Rag, Kimeridge Clay, and Portland beds. These, and the underlying formations, down to the base of the New Red Sandstone, constitute what geologists term the Older Mesozoic or Secondary formations, and all of them, from their approximate conformability one to the other, occupy a set of belts of variable breadth, extending from Devon and Dorsetshire northwards, through Somersetshire, Gloucestershire, and Leicestershire, to the north of Yorkshire, where they disappear beneath the German Ocean.

FIG. 58.

1. Portland Oolite.
2. Purbeck Limestones and Marls.
3. Wealden Sands and Clays.
4. Cretaceous strata.

When the Portland beds had been deposited (see figs. 39 and 58), the entire Oolitic series, in what is now the south and centre of England, and much more besides in other regions, was raised above the sea-level and became land. Because of this elevation, there is evidence in the Isles of Purbeck, Portland, and the Isle of Wight, and in the district known as the Weald, of a state of affairs which must have been common in all times of the world's history. We have there a series of beds, consisting of clays, loose sands, sandstones, and shelly limestone, indicating, by their fossils, that they were accumulated as a delta and in lagoons in an estuary, where fresh water and occasionally brackish water and marine conditions prevailed at the mouth of a great continental river. The position of these beds, with respect to the Cretaceous strata, will be seen in

[English Formations. 311]

fig. 72, p. 339, marked w, h, proving that they are intermediate in date to the Oolites and Cretaceous rocks, for in the Isle of Purbeck, near Swanage, they are seen lying between the two (fig. 75, p. 348).

This episode at last came to an end, by the complete submergence of the Wealden area, and of the greater part of England besides; and upon these fresh-water strata, and the Oolitic and other formations that partly formed their margins, a set of marine sands and clays were deposited in the south of England, consisting of the Atherfield Clay and the Lower Greensand s, d (fig. 72, p. 339) is now often classed with the Upper Neocomian beds of the Continent, but in England they have till lately generally been known as the Lower Cretaceous strata. The distinction is not important to my present purpose. Then comes the clay of the Gault, above which lies the Upper Greensand. Resting upon the Upper Greensand comes the Chalk (No. ii, fig. 57, and c. fig. 72), the upper portion of which contains numerous bands of interstratified flints, which originally were partly marine sponges, since silicified. The Chalk, where thickest, is from one thousand to twelve hundred feet in thickness. The Liassic and Oolitic formations were sediments spread in warm seas sirrounding an archipelago of which Dartmoor, Wales, Cumberland, and the Highlands of Scotland formed some of the islands. But the Chalk was a deep sea deposit, formed to a great extent of microscopic foraminifer, and while it was forming in the wide ocean, it seems probable that the old islands of the Oolitic seas subsided so completely, that it is doubtful whether or not even Wales and the other older mountains of Britain were almost entirely submerged.

During the period that the Oolitic formations formed

[312 English Formations.]

part of the land through which the river flowed that deposited the Wealden and Purbeck beds, they were undergoing constant waste, so that in the course of time, having been previously tilted upwards to the west with an eastern dip (fig. 59), they were worn into what I. have elsewhere termed a plain of marine denudation (see p. 497). The submergence of the Wealden area was followed by the progressive sinking of the Oolitic and older strata further west, so that, as the successive members of the Cretaceous formations were deposited, it happened that by slow sinking of the land, the Upper Cretaceous strata gradually overlapped the edges of the outcropping Oolitic and Liassic formations, till at length they were intruded on the New Red series, and even on the Palæozoic strata of Devonshire itse1f, as shown in fig. 59.

The upheaval of the Chalk into land brought this epoch to an end, and those conditions that contributed to its formation ceased in our area. As the uppermost member of the Upper Secondary rocks, it closes the record of Mesozoic times in England.

This brings us to the last divisions of the British strata which I shall now name. These were deposited on the Chalk, and are termed Eocene formations (No. 12, fig. 57, p. 304). At the base they consist of marine and estuary deposits, known as the Thanet Sand, and Woolwich and Reading beds, and which are of comparatively small thickness, say from 50 to 150 feet. These lie below the London Clay and form the outer border of the London basin. The Woolwich and Reading beds are found in the Isle of Wight, and in part constitute the Hampshire and London basins. In these we have in places the same kind of alternations of fresh

[313 English Formations.]

FIG. 59

Overlap of the Oolitic and other Strata by Cretaceous Formations.

1. Represents the Paleozoic strata. 2. The New Red beds. 3. The Lias. 4, 5, 6. Various members of the Oolites, and 7. The upper Cretaceous strata. The dotted line represents the original continuation of the Chalk westward, the slope marked x the present escarpment of the Chalk, and the hills marked with asterisks (*) outlying patches of the same, the relics left by old denudations, and which help to prove the original far westward extension of the Upper Cretaceous strata.

[314 English Formations.]

water and marine shells that I mentioned as occurring in the Wealden and Purbeck strata; but with this difference, that though the shells belong mostly to the same genera, they are of different species—the old freshwater life is replaced by new.

Upon the London Clay, which is a marine formation, varying from 200 to 500 feet thick, the Bracklesham and Bagshot beds were deposited. These consist of marine unconsolidated sands and clays, occurring as outliers—isolated patches left by denudation around Bagshot, and elsewhere on the London Clay, and overlying the same formation in the Isle of Wight, where they are well seen in Alum Bay. In both these places they are only sparingly fossiliferous, but at Bracklesham and Barton, on the Hampshire coast, they contain a rich marine molluscan fauna of a tropical or subtropical character. Upon these were formed various newer fresh-water strata, occasionally interbedded with thin marine bands, the whole evidently accumulated at the mouth of a river. For the names of these minor formations, I refer the reader to the column, p. 30.

I have in this chapter given a brief recapitulation of the geological and stratigraphical positions of the series of the larger and more solid geological formations that are concerned in producing the physical structure of England (see Map), and I will in the following chapters endeavour to show by the help of fig. 57, and other diagrams, the part that these formations play in producing the scenery of the country.