7. Geology and Soil.

By Geology we mean the study of the rocks, and we must at the outset explain that the term rock is used by the geologist without any reference to the hardness or compactness of the material to which the name is applied; thus he speaks of loose sand as a rock equally with a hard substance like granite.

If all the soil were stripped from the surface of the county there would be found underlying it several kinds of rocks. These occupy the areas shown by the colours on the geological map. To study these rocks, however, it is only necessary to examine them in pits, quarries, and railway cuttings. Their extent can be discovered by observing changes in the nature of the soil and the shape of the ground.

The Permian area is occupied mainly by Magnesian Limestone, so-called because it contains magnesium carbonate. Along its eastern border a narrow strip of deep red clay, Permian marl, exists but it is not separately indicated on the map.

In the next area the rock is a soft sandstone. It is often bright red, variegated with greenish bands; hence the name Bunter (Germ. bunt = variegated). In many places it contains an abundance of pebbles, usually of quartzite. It is a very porous rock, and rain-water therefore soaks into it easily instead of running off. This is the reason why so few streams have their source in this part of the county. An exception occurs in the case of the tributaries of the Erewash and Leen. The hills in which they rise are capped with this sandstone resting on a bed of Permian marl. The water cannot sink from the stone into the marl and so it runs out at the margin and forms the sources of these streams.

Waterstones: Sherwood (freshly cut surface showing alternating bands of sandstone and clay).
Waterstones: Sherwood (freshly cut surface showing alternating bands of sandstone and clay).

In the Waterstones area there are alternating bands of sandstone and clay.

The Keuper marl presents a marked contrast to the Bunter. It consists of a bright red clay with here and there an insignificant layer of sandstone called “skerry.” When rain falls, a little soaks into the skerry but the greater portion flows off at once and forms the streams which are so characteristic of this part of the county. Near its eastern margin and between Laxton and Askham this clay contains layers of gypsum.

In the Lias area, blue-grey limestone is found bordering the Keuper; the remainder is occupied by clay.

In 1876 a boring was made near Collingham in the extreme east of the county. It penetrated the ground to a depth of over 2000 feet. Starting in the Lias it passed successively through rocks identical with the Keuper marl, Waterstones, Bunter, and Permian. This shows that the rocks seen at the surface slope one under another towards the east. In fact they form extensive layers lying one on the top of another. The area over which one of these layers comes to the surface is called the outcrop. The coloured areas on the map show the position and extent of the outcrop of each layer.

The Permian layer, unlike those above it, is thickest in the north. As it passes southwards, it gradually becomes thinner until it eventually disappears entirely near Radford.

These layers of course extend beyond into the surrounding counties, where many other layers occur. The presence in these rocks of such things as fossils and ripplemarks shows that nearly all have been laid down in water. Now suppose that a trench or cutting, about 1000 feet deep, were made across England, the edges of these layers would appear in the sides of this trench, and the layers, or strata as they are called, would be seen to overlap one another like pennies in a pile which has fallen down. If the coins were carefully slipped back into place once more the pile thus formed would illustrate the way in which the table of systems here given has been compiled. If such a pile were made out of the rocks themselves it would be between 10,000 and 20,000 feet high. The rocks represented at the bottom of the table are the oldest, those at the top are the youngest. For convenience of reference they are divided into three main groups; Primary or Palaeozoic, Secondary or Mesozoic, Tertiary or Cainozoic. They rest upon a foundation of still older rocks which may be spoken of as the Precambrian. These main divisions are further subdivided into systems.

The names of these systems are arranged in order in the table. On the right hand side, the general characters of the rocks of each system are stated.

When these rocks were first laid down in water they were flat and horizontal. Afterwards some of them became tilted, as for example the rocks already described above. In other counties, as for instance Derbyshire, the layers have been folded into arches and troughs. When this folding started in Derbyshire the Permian, Bunter, and later rocks had not been formed, and the Carboniferous Limestone, which may be seen to-day in the beautiful dales of that county, was buried under a thick covering of grey clays, dark shales, and light-coloured sandstones with occasional coal seams. This covering, which is now spoken of as the coal-measures, then extended continuously from Lincolnshire to North Wales. When the folding began the coal-measures were swept off the tops of the arches by the action of the rain, frost, rivers, and streams. Thus the underlying limestone was exposed and raised to form the Derbyshire Pennines. Meanwhile the coal-measures were left in the saucer-like troughs on either side and were afterwards buried under the Permian and later rocks. The eastern saucer is still largely buried out of sight, but its western margin outcrops in Yorkshire, Derbyshire, and West Nottinghamshire and assumes a south-easterly trend at Wollaton, where it disappears under the Bunter. This outcrop is the visible coalfield. The buried portion is the concealed coalfield.

In other counties, as for example in our neighbour Leicestershire, another kind of rock is found which has not been formed in water. At one time it was situated deep down in the earth and was so hot that it was molten. In this condition it was forced towards the surface. Sometimes it cooled before it reached the surface, at other times it found an outlet and then it produced great volcanoes and was poured forth as lava. Rocks formed in these ways are called igneous rocks. Though they do not occur naturally in Nottinghamshire they may often be seen used as setts, curbs, and pillars.

Keuper Clay in Mapperley brickyards.
Keuper Clay in Mapperley brickyards.

All these rocks together are the block out of which rain, frost, and streams have sculptured the surface features of the county. The Magnesian Limestone has resisted the carving action of these agents. Coming up from beneath the Bunter it rises steadily towards the west. With a capping of sandstone here and there it forms the watershed of the Western Hills. The Keuper marls have not resisted to the same extent. The surface has been planed flat and has formed the gently-sloping plateau which extends from the Central Hills to the plain of the Trent and Vale of Belvoir. The western faces of these hills are formed by the edge of the more resistant Waterstones.

The Bunter produces much of both the highest and the lowest ground in the county. Where streams exist they have carved it easily. ‘Where they do not exist, the rain though assisted by frost has had little chance, for it has been absorbed.

We may now turn to the soil which these varying strata afford, for it is the debris formed by the action of air, rain, frost, and plants upon these latter that composes it. For this reason the soil varies with the underlying rock. Where this is clay, the soil is clayey. Where it is sandstone with pebbles the soil is sandy and stony, or even gravelly. Where it is sandstone alternating with clay, a sandy or clayey loam exists. The limestone gives rise to a calcareous loam.

Most of the soils in the county lie where they were formed and therefore have the same character as the underlying rock. Some, however, differ greatly from this. These have been washed by the rain from soils of widely-separated districts, carried as sand, silt, or mud by the streams, and deposited to form alluvium by the rivers in time of flood.

The largest area of alluvium in our county is in the Trent Vale. Here it usually lies upon a coarse gravel, which was deposited before the river had dwindled to its present size. In some places the gravel is still not covered even by the highest floods. These gravel patches stand up like islands in the alluvial plain, as is shown in the illustration on p. 145.

In bygone ages glaciers and ice-sheets also transported soils, but only slight traces of their action are found in Nottinghamshire.

In places which, until modern times, were swamps and marshes the soil usually contains so much vegetable material as to be black and peaty. Occasionally shells are so abundant that a shell marl has been produced, as for example at Gotham.