410 Mr Maclaren on Ancient Beaches near Stirling. 



than 40 feet above the Carse, and the tides would play freely through 

 the hollow hero, as well as through the great channel between S and A, 

 long after the passage at /, by the north side of Abbey Crag, was 

 closed. 



There is another small elevation at o, one mile south-west of Stir- 

 ling behind the hill L, which looks like a terrace, but is ill defined 

 and equivocal. 



In speculating on the changes which the alluvial deposits have 

 undergone from the action of the sea, at the different levels alluded 

 to, it may be proper to advert to the state of things which preceded 

 those changes. When the Forth and Clyde canal was projected, 

 Smeaton carried a survey over the low grounds east of Loch Lomond, 

 which separate the Carse from the basin of the Clyde. He found 

 the summit-level at the Bog of BoUat, in the parish of Drymen, to 

 be 222 feet above high water in the Clyde. When the sea therefore 

 stood at a higher level than this — say 250 feet — two long narrow 

 sounds (like the Sound of Mull) would extend across Scotland, 

 uniting the Friths of Forth and Clyde, one by the line of Kelvin 

 Water or the present canal (whose summit-level is 147 feet above 

 the sea), the other along the valleys of the Rivers Endrick and Forth ; 

 the Campsie and Gargunnock hills forming an island between them. 

 Our concern at present is only with the northern sound. While the 

 sea had a free passage here, strong currents like those of the Pent- 

 land Frith, would set through it, and not only prevent any new de- 

 posit of matter, but sweep away much of the old alluvium. When the 

 sea subsided below the level of the Pass of Bollat, the currents would 

 cease, and the destruction of the ancient deposits would be arrested. 

 The mud, sand, and gravel, poured in by the Forth, the Teith, the 

 Allan, and other streams, would no longer be swept away, but distri- 

 buted by the alternate motions of the tide, first along the shore, and 

 ultimately over the bottom of the valley, replacing those portions of the 

 older alluvium which had been carried off. Even when the currents held 

 their free course across this part of Scotland, portions of the older allu- 

 vium would escape their action, and remnants of terraces formed of it, 

 no doubt exist ; but, if my argument is good, continuous well-marked 

 terraces should not be looked for in the district alluded to, at an eleva- 

 tion much exceeding that of the Pass, which is 222 feet. To such causes 

 I think the extreme rarity of ancient marine terraces, except at low 

 elevations and in sheltered localities, may be ascribed. With regard 

 to the ridge k in section 3, therefore, neither its height (155 feet), 

 nor the absence of terraced deposits at corresponding elevations else- 

 where, form any valid objection to its being considered as part of the 

 bottom of the ancient sea. From its shape, and its evident trunca- 

 tion on the south side, I infer that it was once many feet higher, and 

 that it probably formed a shoal behind the barrier of Abbey Crag. 

 We find a similar shoal in the present Frith on both sides of Inch 

 Colm, where the water deepens suddenly from 3 fathoms to 15.— 



