Rain-Wash.
Not long age I was walking over Middleton Moor in Derbyshire, an elevated exposed tact of land lying a mile or two north-east of Dovedale. The most elevated parts of the Moor—Arbor Low, Lean Low, &c.—rise from 1,200 to 1,300 feet above the sea, and lie near its margins, whilst the centre is depressed. After walking over the limestone rock, barely covered with short grass, which constitutes the greater part of the district, I found in a hollow near a farm house a considerable spread of a light red loamy deposit, which had evidently been largely dug, probably to spread over and improve the neighbouring land. It appeared to be from six to twelve or more feet in thickness, and contained no stones or fragments of foreign rocks. I had little more than a passing glance at this red clay, but it struck me that it was an interesting instance of the chemical action of rain water on the typical rock of this district—the Carboniferous or Mountain Limestone.
The Carboniferous Limestone contains about ninety per cent. of carbonate of lime, and, in addition, some silicate of alumina, silica, oxide of iron, &c. Now, the rain water in falling through the atmosphere dissolves out of the latter a little carbonic acid gas, and this combines with the carbonate of lime, forming a bi-carbonate, which is soluble in water, and which is consequently carried off in solution by the latter, thus causing the "hardness" of the water of limestone regions. The other ingredients of the limestone, however, are left behind. Slowly and gradually do they descend the hill sides. On the very hill-tops the bare rock peeps forth, but the slopes are covered with a few inches of debris, over and through which the rain water courses, gradually abstracting the soluble part, the carbonate of lime, and mechanically "moving on" the other ingredients, until they arrive of the lowest point. I note that all the slopes of a given inclination had a terraced or step-like appearance, the lines running regularly, as if ruled at intervals of a few feet. If it be a valley through which a river runs, the insoluble substances on reaching the bottom are then carried off by the stream, whose waters they make muddy and turbid; but on the limestone moors most of the water percolates through the rock, or disappears down swallow-holes, leaving the residue in the surface hollows. Here it accumulates. Most of it is clay, but there is some sand, and the whole is tinged red by the oxide of iron, which, by exposure to the air and water has been raised, if it were not previously in that condition, to the state of the hydrated peroxide. If we could measure the cubical contents of such a deposit, and also by analysis determine accurately the composition of the rock from which it was derived, we might obtain some interesting results as to the amount of denudation of the district in recent times. We might, perhaps, also obtain some idea of the rate of denudation, and the time which it had taken to form such deposits. In connection with this, the average rainfall might be made to yield results in aid. In the south of England the clay-with-tints, which covers part of the chalk downs has had a similar origin[1] to our Derbyshire rain-wash; the red cave-earth of limestone caverns (Kent's Cave for example) is a similar residue. All the soil of our fields, of course, is in large measure due to the disintegrating and chemical action of rain, but I should prefer to retain the term rain-wash, geologically, for any residuum which is wholly due to its action. I should be glad to hear from any correspondents whether they have observed any instances of deposits similar to that noted above, and under what conditions. I believe there are sore thick clay beds at the foot of the Weaver Hills, which have been mistaken for boulder clay, but are really nothing but rain-wash, but these I have not yet examined; will anyone describe them for us?
This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.
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- ↑ Might not the well-known bed of red clay in Tideswell Dale have had a similar origin.