CLYDE SEA AREA. 643 



The Clyde Sea Area belongs geologically to two distinct regions separated by the 

 line of the Great Fault, and the change in the nature of the rocks coincides with 

 a change in the character of the channels as well as of the scenery of the coast. 

 North-west of the fault, amongst the slates and schists of the Highlands, the typical 

 fjords with their deep rock basins occur : south-east of the fault the Old Eed Sandstone 

 and Carboniferous formations edge the wider and shallower reaches of the " firth." 



The Great Plateau is a barrier ridge separating the deep water of the North 

 Channel from the deep water of the enclosed basins. The average depth over it is 

 about 24 fathoms, and its area 313 square miles. The central ridge, as limited by 

 the 25-fathom contour lines, is wide both on the Cantyre and the Ayrshire coasts, 

 but narrows toward the centre, and from its narrowest part the great basaltic mass 

 of Ailsa Craig (two-thirds of a mile in diameter), rises to a height of 1097 feet 

 above the surface. The bottom of the plateau is apparently composed of fine mud 

 and sand. The coast is comparatively steep on both sides. On the Cantyre shores 

 only a few small burns enter, and the drainage area of streams entering between 

 Davaar Island and Cove Point only measures 3 square miles ; but on the Ayrshire 

 side, the Stinchar and Girvan, each having a course of about 20 miles, flow down the 

 Silurian strata of the western slopes of the southern uplands, and enter the sea, 

 carrying the drainage of 280 square miles. The most remarkable feature of the 

 Great Plateau is its extreme flatness, and the absence of marked banks or hollows 

 (see sections 1, 2, Plate VII. ; sections 9, 10, Plate VIII. ). It is important to remember 

 that the exchange of water between the ocean and the Area must take place across this 

 great natural counter, and must be passed to and fro in sheets having a thickness not 

 greater than 24 fathoms, or 144 feet, at the utmost. The entire volume of water 

 covering the Plateau at low tide is 5 "8 cubic sea miles, and at high tide, 0*3 

 additional. The contour line of 50 fathoms runs almost straight across from the 

 Mull of Cantyre to the Mull of Galloway. 



The whole Sea Area might be supposed to be an extension of the Great Plateau 

 in which various basins have been hollowed out, and it is at least possible that the 

 formation of these basins was due to some such action. 



The Arran Basin is the largest depression inside the Barrier Plateau, having a 

 total area of 685 square miles, one half of which has a depth greater than 30 

 fathoms. The outline of this basin resembles the Greek letter X, the long stroke 

 representing the eastern part of the basin, which lies east of Arran, the shorter 

 stroke, the western branch of the basin occupying Kilbrennan Sound. The slope of 

 the basin is extremely steep in the angle between the two limbs off the rugged north 

 coast of Arran, but along the Ayrshire coast the slope is very gentle, corresponding 

 to the wide shallows of Ayr and Irvine Bays, and the low-lying ground which borders 

 them inland. The basin surrounds the south of Bute, and runs up " Lower Loch 

 Fyne" to the Otter Spit. Axial sections along the west, east, and central divisions 

 (sections 1, 2, 4, Plate VII.), show its relation to the Barrier Plateau and to 



