A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

But there are other deep hollows of a cylindrical form found in England, France, and elsewhere, penetrating the white chalk, and filled with sand and gravel, which are not so readily explained. They are sometimes called "sand-pipes," or "sand-galls," and "puits naturels," in France. Those represented in the annexed cut were observed by me in 1839, laid open in a large chalk-pit near Norwich. They were of very symmetrical form, the largest more than 12 feet in diameter, and some of them had been traced, by boring, to the depth of more than 60 feet. The smaller ones varied from a few inches to a foot in diameter, and seldom descended more than 12 feet below the surface. Even where three of them occurred, as at a, [fig. 101.], very close together, the parting walls of soft white chalk were not broken through. They all taper downwards and end in a point. As a general rule, sand and pebbles occupy the central parts of each pipe, while the sides and bottom are lined with clay.

Mr. Trimmer, in speaking of appearances of the same kind in the Kentish chalk, attributes the origin of such "sand-galls" to the action of the sea on a beach or shoal, where the waves, charged with shingle and sand, not only wear out longitudinal furrows, such as may be observed on the surface of the chalk near Norwich when the incumbent gravel is removed, but also drill deep circular hollows by the rotatory motion imparted to sand and pebbles. Such furrows, as well as vertical cavities, are now formed, he observes, on the coast where the shores are composed of chalk.[82-A]

That the commencement of many of the tubular cavities now under consideration has been due to the cause here assigned, I have little doubt. But such mechanical action could not have hollowed out the whole of the sand-pipes c and d, [fig. 101.], because several large chalk-flints seen protruding from the walls of the pipes have not been eroded, while sand and gravel have penetrated many feet below them. In other cases, as at b b, similar unrounded nodules of flint, still preserving their irregular form and white coating, are found at various depths in the midst of the loose materials filling the pipe. These have evidently been detached from regular layers of flints occurring above. It is also to be remarked that the course of the same sand-pipe, b b, is traceable above the level of the chalk for some distance upwards, through the incumbent gravel and sand, by the obliteration of all signs of stratification. Occasionally, also, as in the pipe d, the overlying beds of gravel bend downwards into the mouth of the pipe, so as to become in part vertical, as would happen if horizontal layers had sunk gradually in consequence of a failure of support. All these phenomena may be accounted for by attributing the enlargement and deepening of the sand-pipes to the chemical action of water charged with carbonic acid, derived from the vegetable soil and the decaying roots of trees. Such acid might corrode the chalk, and deepen indefinitely any previously existing hollow, but could not dissolve the flints. The water, after it had become saturated with carbonate of lime, might freely percolate the surrounding porous walls of chalk, and escape through them and from the bottom of the tube, so as to carry away in the course of time large masses of dissolved calcareous rock[83-A], and leave behind it on the edges of each tubular hollow a coating of fine clay, which the white chalk contains.

I have seen tubes precisely similar and from 1 to 5 feet in diameter traversing vertically the upper half of the soft calcareous building stone, or chalk without flints, constituting St. Peter's Mount, Maestricht. These hollows are filled with pebbles and clay, derived from overlying beds of gravel, and all terminate downwards like those of Norfolk. I was informed that, 6 miles from Maestricht, one of these pipes, 2 feet in diameter, was traced downwards to a bed of flattened flints, forming an almost continuous layer in the chalk. Here it terminated abruptly, but a few small root-like prolongations of it were detected immediately below, probably where the dissolving substance had penetrated at some points through openings in the siliceous mass.

It is not so easy as may at first appear to draw a clear line of distinction between the fixed rocks, or regular strata (rocks in situ or in place), and alluvium. If the bed of a torrent or river be dried up, we call the gravel, sand, and mud left in their channels, or whatever, during floods, they may have scattered over the neighbouring plains, alluvium. The very same materials carried into a lake, where they become sorted by water and arranged in more distinct layers, especially if they inclose the remains of plants, shells, or other fossils, are termed regular strata.

In like manner we may sometimes compare the gravel, sand, and broken shells, strewed along the path of a rapid marine current, with a deposit formed contemporaneously by the discharge of similar materials, year after year, into a deeper and more tranquil part of the sea. In such cases, when we detect marine shells or other organic remains entombed in the strata, which enable us to determine their age and mode of origin, we regard them as part of the regular series of fossiliferous formations, whereas, if there are no fossils, we have frequently no power of separating them from the general mass of superficial alluvium.

The usual rarity of organic remains in beds of loose gravel and sand is partly owing to the rapid and turbid water in which they were formed having been in a condition unfavourable to the habitation of aquatic beings, and partly to their porous nature, which, by allowing the free percolation of rain-water, has promoted the decomposition and removal of organic matter.

It has long been a matter of common observation that most rivers are now cutting their channels through alluvial deposits of greater depth and extent than could ever have been formed by the present streams. From this fact a rash inference has sometimes been drawn, that rivers in general have grown smaller, or become less liable to be flooded than formerly. But such phenomena would be a natural result of any considerable oscillations in the level of the land experienced since the existing valleys originated.

Suppose part of a continent, comprising within it a large hydrographical basin like that of the Mississippi, to subside several inches or feet in a century, as the west coast of Greenland, extending 600 miles north and south, has been sinking for three or four centuries, between the latitudes 60° and 69° N.[84-A] There might be no encroachment of the sea at the river's mouth in consequence of this change of level, but the fall of the waters flowing from the interior being lessened, the main river and its tributaries would have less power to carry down to its delta, and to discharge into the ocean, the sedimentary matter with which they are annually loaded. They would all begin to raise their channels and alluvial plains by depositing in them the heavier sand and pebbles washed down from the upland country, and this operation would take place most effectively if the amount of subsidence in the interior was unequal, and especially if, on the whole, it exceeded that of the region near the sea. If then the same area of land be again upheaved to its former height, the fall, and consequently the velocity, of every river would begin to augment. Each of them would be less given to overflow its alluvial plain; and their power of carrying earthy matter seaward, and of scouring out and deepening their channels, would continue till, after a lapse of many thousand years, each of them would have eroded a new channel or valley through a fluviatile formation of modern date. The surface of what was once the river-plain at the period of greatest depression, would remain fringing the valley sides in the form of a terrace apparently flat, but in reality sloping down with the general inclination of the river. Everywhere this terrace would present cliffs of gravel and sand, facing the river. That such a series of movements has actually taken place in the main valley of the Mississippi and in its tributary valleys during oscillations of level, I have endeavoured to show in my description of that country[85-A]; and the freshwater shells of existing species and bones of land quadrupeds, partly of extinct races preserved in the terraces of fluviatile origin, attest the exclusion of the sea during the whole process of filling up and partial re-excavation.

In many cases, the alluvium in which rivers are now cutting their channels, originated when the land first rose out of the sea. If, for example, the emergence was caused by a gradual and uniform motion, every bay and estuary, or the straits between islands, would dry up slowly, and during their conversion into valleys, every part of the upheaved area would in its turn be a sea-shore, and might be strewed over with littoral sand and pebbles, or each spot might be the point where a delta accumulated during the retreat and exclusion of the sea. Materials so accumulated would conform to the general slope of a valley from its head to the sea-coast.