| 1. Vegetable soil. | ||
| 2. Boulder clay, thirty to forty feet. | ||
| Stratified beds | { | 3. Yellowish gravelly sand. 4. Peaty silt and clay. 5. Fine ferruginous sand. 6. Coarse shingle, two to three feet. |
| 7. Coarse, stiff boulder clay, fifteen to twenty feet. | ||
A few more cases of intercalation of stratified materials in the true till were also found in the same valley.
In a cliff of stiff brown boulder clay, about 20 feet high, on the banks of the Carmichael Water, Lanarkshire, Professor Geikie observed a stratified bed of clay about 3 or 4 inches in thickness. About a mile higher up the stream, he found a series of beds of gravel, sand, and clay in the true till. “A thin seam of peaty matter,” he says, “was observed to run for a few inches along the bottom of a bed of clay and then disappear, while in a band of fine laminated clay with thin sandy partings occasional fragments of mouldering wood were found.”[112]
At Chapelhall, near Airdrie, a sand-bed has been extensively mined under about 114 feet of till. This bed of finely stratified sand is about 20 feet thick. In it were found lenticular beds of fine pale-coloured clay containing layers of peat and decaying twigs and branches. Professor Geikie found the vegetable fibres, though much decayed, still distinct, and the substance when put into the fire burned with a dull lambent flame. Underlying these stratified beds, and forming the floor of the mine, is a deposit of the true till about 24 feet in thickness. In another pit adjoining, the till forming the floor is 30 feet thick, but it is sometimes absent altogether, so as to leave the sand beds resting directly on the sandstone and shale of the coal-measures. At some distance from this sand-pit an old buried river channel was met with in one of the pit workings. This channel was found to contain a coating of boulder clay, on which the laminated sands and clays reposed, showing, as Professor Geikie has pointed out, that this old channel had been filled with boulder clay, and then re-excavated to allow of the deposition of the stratified deposits. Over all lay a thick mantle of boulder clay which buried the whole.
A case somewhat similar was found by Professor Nicol in a cutting on the Edinburgh and Leith Railway. In many places the till had been worn into hollows as if part of it had been removed by the action of running water.[113] One of these hollows, about 5 or 6 feet wide by 3 or 4 feet deep, closely resembled the channel of a small stream. It was also filled with gravel and sand, in all respects like that found in such a stream at the present day. It was seen to exhibit the same characters on both sides of the cutting, but Professor Nicol was unable to determine how far it may have extended beyond; but he had no doubt whatever that it had been formed by a stream of water. Over this old watercourse was a thick deposit of true till.
In reference to the foregoing cases, Professor Geikie makes the following pertinent remarks:—“Here it is evident that the scooping out of this channel belongs to the era of the boulder clay. It must have been effected during a pause in the deposition of the clay, when a run of water could find its way along the inequalities of the surface of the clay. This pause must have been of sufficient duration to enable the runnel to excavate a capacious channel for itself, and leave in it a quantity of sand and shingle. We can scarcely doubt that when this process was going on the ground must have been a land surface, and could not have been under the sea. And lastly, we see from the upper boulder clay that the old conditions returned, the watercourse was choked up, and another mass of chaotic boulder clay was tumbled down upon the face of the country. This indicates that the boulder clay is not the result of one great catastrophe, but of slow and silent, yet mighty, forces acting sometimes with long pauses throughout a vast cycle of time.”[114]
At Craiglockhart Hill, about a mile south of Edinburgh, an extensive bed of fine sand of from one to three feet in thickness was found between two distinct masses of true boulder clay or till. The sand was extensively used for building purposes during the erection of the city poorhouse a few years ago. In this sand-bed I found a great many tree roots in the position in which they had grown. During the time of the excavations I visited the place almost daily, and had every opportunity of satisfying myself that this sand-bed, prior to the time of the formation of the upper boulder clay, must have been a land surface on which the roots had grown. In no case did I find them penetrating into the upper boulder clay, and in several places I found stones of the upper clay resting directly on the broken ends of the roots. These roots were examined by Professor Balfour, but they were so decayed that he was unable to determine their character.
In digging a foundation for a building in Leith Walk, Edinburgh, a few years ago, two distinct beds of sand were passed through, the upper, about 10 feet in thickness, rested upon what appeared to be a denuded surface of the lower bed. In this lower bed, which evidently had been a land surface, numbers of tree roots were found. I had the pleasure of examining them along with my friend Mr. C. W. Peach, who first directed my attention to them. In no instance were the roots found in the upper bed. That these roots did not belong to trees which had grown on the present surface and penetrated to that depth, was further evident from the fact that in one or two cases we found the roots broken off at the place where they had been joined to the trunk, and there the upper sand-bed over them was more than 10 feet in thickness. If we assume that the roots belonged to trees which had grown on the present surface, then we must also assume, what no one would be willing to admit, that the trunks of the trees had grown downwards into the earth to a depth of upwards of ten feet. I have shown these roots to several botanists, but none of them could determine to what trees they belonged. The surface of the ground at the spot in question is 45 feet above sea-level. Mr. Peach and I have found similar roots in the under sand-bed at several other places in the same neighbourhood. That they belong to an inter-glacial period appears probable for the following reasons:—(1.) This upper sand-bed is overlaid by a tough clay, which in all respects appears to be the same as the Portobello clay, which we know belongs to the glacial series. In company with Mr. Bennie, I found the clay in some places to be contorted in a similar manner to the Portobello clays. (2.) In a sand-pit about one or two hundred yards to the west of where the roots were found, the sand-bed was found contorted in the most extraordinary manner to a depth of about 15 feet. In fact, for a space of more than 30 feet, the bedding had been completely turned up on end without the fine layers being in the least degree broken or disarranged, showing that they had been upturned by some enormous powers acting on a large mass of the sand.
One of the best examples of true till to be met with in the neighbourhood of Edinburgh is at Redhall Quarry, about three miles to the south-west of the city. In recently opening up a new quarry near the old one a bed of peat was found intercalated in the thick mass of till overlying the rock. The clay overlying and underlying the peat-bed was carefully examined by Mr. John Henderson,[115] and found to be true till.
In a quarry at Overtown, near Beith, Ayrshire, a sedimentary bed of clay, intercalated between two boulder clays, was some years ago observed by Mr. Robert Craig, of the Glasgow Geological Society. This bed filled an elliptical basin about 130 yards long, and about 30 yards broad. Its thickness averaged from one to two feet. This sedimentary bed rested on the till on the north-east end of the basin, and was itself overlaid on the south-west end by the upper bed of till. The clay bed was found to be full of roots and stems of the common hazel. That these roots had grown in the position in which they were found was evident from the fact that they were in many places found to pass into the “cutters” or fissures of the limestone, and were here found in a flattened form, having in growing accommodated themselves to the size and shape of the fissures. Nuts of the hazel were plentifully found.[116]