3. LIDDESDALE
A remarkable development of puys lies in that little-visited tract of country which stretches from the valleys of the Teviot and Rule Water south-westwards across the high moorland watershed, and down Liddesdale. Through this district a zone of bedded olivine-basalts and associated tuffs runs in a broken band which, owing to numerous faults and extensive denudation, covers now only a few scattered patches of the site over which it once spread. The geological horizon of this zone lies in the Calciferous Sandstones, many hundred feet above the position of the top of the plateau-lavas ([Map IV.]).
So great an amount of material has been here removed by denudation that not only has the volcanic zone been bared away, but the vents which supplied its materials have been revealed in the most remarkable manner over an area some twenty miles long and eight miles broad. Upwards of forty necks of agglomerate may be seen in this district, rising through the Silurian, Old Red Sandstone, and lowest Carboniferous rocks. It fills the geologist with wonder to meet with those stumps of old volcanoes far to the west among the Silurian lowlands, sometimes fully ten miles away from the nearest relic of the bedded lavas connected with them.[479] That these vents, though they rose through ground which at the time of their activity was covered with the volcanic series of the plateaux, do not belong to that series, but are of younger date, has been proved in several cases by Mr. Peach. He has found that among the blocks composing their agglomerates, pieces of the sandstones, fossiliferous limestones and shales of the Cement-stone group, overlying the plateau-lavas, are to be recognized. These vents were therefore drilled through some part at least of the Calciferous Sandstones, which are thus shown to have extended across the tract dotted with vents. After the volcanic activity ceased, fragments of these strata tumbled down from the sides into the funnels. Denudation has since stripped off the Calciferous Sandstones, but the pieces detached from them, and sealed up at a lower level in the agglomerates, still remain. Mr. Peach's observations indicate to how considerable an extent sagging of the walls of these orifices took place, with the precipitation not merely of blocks, but of enormous masses of rock, into the volcanic chimneys. In one instance, between Tudhope Hill and Anton Heights, a long neck, or perhaps group of necks, which crosses the watershed, shows a mass of the red sandstone many acres in extent, and large enough to be inserted on the one-inch map, which has fallen into the vent ([Fig. 175]).
[479] They have been recognized and mapped by Mr. B. N. Peach for the Geological Survey. See Sheets 11 and 17, Geol. Surv. Scotland.
Fig. 174.—Section showing the connection of the two volcanic bands in Liddesdale.
1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The lavas of the Solway plateau; 4. Agglomerate neck with lava plug, belonging to the plateau system; 5. Calciferous Sandstone series; 6. Thick Carboniferous Limestones; 7. Tuff, and 8. Lavas, of the upper volcanic band, connected with the puys; 9. Agglomerate neck with lava plug belonging to the puy-system; 10. Basic sill.
Fig. 175.—Diagram to show the position of a mass of Upper Old Red Sandstone which has fallen into the great vent near Tudhope Hill, east of Mosspaul.
1. Upper Silurian strata; 2. Outlier of Upper Old Red Sandstone; 2´. Large mass of this formation in the vent; 3. Agglomerate of the neck with andesite intrusion (4).
The materials ejected from the Liddesdale vents include both basaltic lavas and tuffs. The former are sometimes highly vesicular, especially along the upper parts of the flows. They are thickest towards the north, and in Windburgh Hill attain a depth of at least 300 or 400 feet. In that part of the district they form the lower and main part of the volcanic series, being there covered by a group of tuffs. But a few miles southwards, not far to the west of Kershopefoot, they die out. The tuffs then form the whole of the volcanic band which, intercalated in a well-marked group of limestones, can be followed across the moors for some six miles into the valley of the Esk, where an interesting section of them and of the associated limestone and shales is exposed ([Fig. 174]). At Kershopefoot, a higher band of basic lava overlies the Kershopefoot limestone, and can be traced in scattered patches both on the Scottish and English side of the Border.
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Transcriber Note
Minor typos corrected. Text rearranged to avoid split paragraphs at full page images.