In sinking a pit at Orchardhead, about a mile to the north of Grangemouth, the workmen came upon the boulder clay after passing through about 110 feet of sand, clay, and gravel. On the upper surface of the boulder clay they found cut out what Mr. Stirling believes to have been an old watercourse. It was 17 feet deep, and not much broader. The sides of the channel appear to have been smooth and water-worn, and the whole was filled with a fine sharp sand beautifully stratified. As this channel lay about 100 feet below the present sea-level, it shows that if it actually be an old watercourse, it must have been scooped out at a time when the land in relation to the sea stood at least 100 feet higher than at present.
Buried River Channel from Kilsyth to the Clyde.—In all probability the western half of this great hollow, extending from the watershed at Kilsyth to the Clyde, is also an old river channel, probably the ancient bed of the Kelvin. This point cannot, however, be satisfactorily settled until a sufficient number of bores have been made along the direct line of the hollow, so as to determine with certainty its width and general form and extent. That the western channel is as narrow as the eastern is very probable. It has been found that its sides at some places, as, for example, at Garscadden, are very steep. At one place the north side is actually an overhanging buried precipice, the bottom of which is about 200 feet below the sea-level. We know also that the coal and ironstone in that quarter are cut through by the trough, and the miners there have to exercise great caution in driving their workings, in case they might cut into it. The trough along this district is filled with sand, and is known to the miners of the locality as the “sand-dyke.” To cut into running sand at a depth of 40 or 50 fathoms is a very dangerous proceeding, as will be seen from the details given in Mr. Bennie’s paper[283] of a disaster which occurred about twenty years ago to a pit near Duntocher, where this trough was cut into at a depth of 51 fathoms from the surface.
The depth of this hollow, below the present sea-level at Drumry, as ascertained by a bore put down, is 230 feet. For several miles to the east the depth is nearly as great. Consequently, if this hollow be an old river-bed, the ancient river that flowed in it must have entered the Clyde at a depth of more than 200 feet below the present sea-level; and if so, then it follows that the rocky bed of the ancient Clyde must lie buried under more than 200 feet of surface deposits from Bowling downwards to the sea. Whether this is the case or not we have no means at present of determining. The manager to the Clyde Trustees informs me, however, that in none of the borings or excavations which have been made has the rock ever been reached from Bowling downwards. The probability is, that this deep hollow passes downwards continuously to the sea on the western side of the island as on the eastern.[284]
The following journals of a few of the borings will give the reader an idea of the character of the deposits filling the channels. The beds which are believed to be boulder clay are printed in italics:—
Borings made through the Deposits filling the Western Channel.
Bore, Drumry Farm, on Lands of Garscadden.
ft. | ins. | |
|---|---|---|
| Surface soil | 2 | 6 |
| Sand and gravel | 3 | 6 |
| Dry sand | 11 | 0 |
| Blue mud | 8 | 6 |
| Light mud and sand beds | 13 | 0 |
| Sand | 31 | 6 |
| Sand and mud | 8 | 0 |
| Sand and gravel | 19 | 6 |
| Sand | 8 | 6 |
| Gravel | 24 | 4 |
| Sand | 5 | 0 |
| Gravel | 9 | 6 |
| Sand | 71 | 6 |
| Sand (coaly) | 1 | 0 |
| Sand | 9 | 0 |
| Sand (coaly) | 1 | 0 |
| Sand | 10 | 3 |
| Red clay and gravel | 4 | 8 |
| Sand | 1 | 5 |
| Gravel | 2 | 0 |
| Sand | 2 | 8 |
| Gravel | 10 | 6 |
| Sand | 1 | 6 |
| Gravel | 8 | 10 |
| Clay stones and gravel | 33 | 3 |
——————— | ||
297 | 10 | |
Bore on Mains of Garscadden, one mile north-east of Drumry.
ft. | ins. | |
|---|---|---|
| Surface soil | 1 | 0 |
| Blue clay and stones | 60 | 1 |
| Red clay and stones | 18 | 0 |
| Soft clay and sand beds | 7 | 0 |
| Gravel | 6 | 0 |
| Large gravel | 9 | 0 |
| Sand and gravel | 7 | 0 |
| Hard gravel | 1 | 6 |
| Sand and gravel | 16 | 6 |
| Dry sand | 30 | 0 |
| Black sand | 2 | 0 |
| Dry sand | 33 | 0 |
| Wet sand | 8 | 0 |
| Light mud | 5 | 0 |
| Sand | 3 | 0 |
| Gravel | 5 | 6 |
| Sandstone, black | 0 | 6 |
| Blue clay and stones | 1 | 4 |
| Whin block | 0 | 10 |
| Sandy clay | 4 | 6 |
——————— | ||
219 | 8 | |
Bore nearly half a mile south-west of Millichen.