Fig. 242.—Section along the course of the Cleveland Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the Memoirs of the Geol. Surrey, Geology of Cleveland, p. 61).
a, Liassic shales, sandstones and ironstones; b, the dyke.
Fig. 243.—Section across the extreme upper limit of Cleveland Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow).
a a, Jurassic shales, etc.; b, Dyke.
Another feature connected with the upward termination of the dyke is well seen in some parts of the ground through which the two foregoing sections are taken. Mr. Barrow informs me that at Ayton a level course has been driven into the hill for mining operations, at a height of 400 feet above sea-level, and the dyke has there been ascertained to be 80 feet broad. Higher on the hill, close to the 750 feet contour—line, its breadth is only 20 feet, so that it narrows upward as much as 60 feet in a vertical height of 350 feet. Its contraction in width during the last twenty feet is still more rapid, and in the last few yards it diminishes to two or three feet, and has a rounded top over which the strata are bent upward. The accompanying section ([Fig. 243]) across the upper part of the dyke will make these features clear.
Fig. 244.—Upper limit of Cleveland Dyke in quarry near Cockfield (after Mr. Teall).
a a, Carboniferous shales; b, dyke.
Further to the west an exposure of the upper limit of the dyke has been described and figured by Mr. Teall. In 1882, at one of the Cockfield quarries ([Fig. 244]), the dyke was "seen to terminate upwards very abruptly in the form of a low and somewhat irregular dome, over which the Coal-measure shales passed without any fracture, and only with a slight upward arching."[189]
[189] Quart. Jour. Geol. Soc. xl. p. 210.
Near the other or north-western termination of this great dyke, similar evidence is found of an uneven upper limit. After an interrupted course through the Alston moors, the dyke reaches the ground that slopes eastward from the edge of the Cross Fell escarpment. Its highest visible outcrop is at a height of 1700 feet. But westwards from that point the dyke disappears under the Carboniferous rocks, and does not emerge along the front of the great escarpment that descends upon the valley of the Eden, where among the naked scarps of rock it would unquestionably be visible if it reached the surface. Its upper edge must rapidly descend somewhere behind the face of the escarpment, for the igneous rock crops out a little to the west of the foot of the cliff, about 1000 feet below the point where it is last seen on the hills above. Here the top of the dyke has a vertical drop of not less than 1000 feet, in a horizontal distance of five miles, as shown in [Fig. 245], which has been drawn for me by Mr. J. G. Goodchild.