488 T. F. Jamieson — Changes of Level in Glacial Period, 



having saved them from being swept away by the ice. Mr. Jack 

 thinks that these beds were deposited during what he terms ' the 

 great submergence,' but he makes out this submergence to have 

 taken place at a period after the shelly till on the Endrick was 

 formed. This is a point, however, in which I am unable to agree 

 with him. Some of the sand, gravel, and stony mud was probably 

 heaped up by the flank of the glacier which pi'oduced the shelly till, 

 and about the same time, but the finer stratified beds of clay and 

 sand near Balfron, I think, are just those portions of the glacial 

 marine beds which escaped destruction owing to their sheltered 

 position behind the ridge separating the two ice streams. The 

 Forth glacier must have gone far eastward, but during its final 

 retreat it has left at the Loch of Menteith a fine group of moraines 

 which show no sign of having ever been submerged by any 

 subsequent change of level, although their base is only about 50 feet 

 above reach of the tide. I am, therefore, decidedly of opinion that 

 no great submergence has taken place since the advance of the 

 glaciers which produced the shelly boulder-clay on the Endrick at 

 the south-east corner of Loch Lomond. But after these glaciers 

 vanished, Loch Lomond (which is only 23 feet above the present 

 sea-level) appears to have again become a salt-water creek inhabited 

 by the later group of northern mollusca, whose shells, not crushed 

 and broken, but entire and undemolished by any subsequent glacier, 

 are still to be found in the present bed of the lake at the island of 

 Inchlonaig and other places, where they had no shelter and where 

 they could not have escaped destruction by such an agency.^ 



Here, then, we have evidence of two distinct deposits of marine 

 beds separated by an interval of land-ice, and it is to this second and 

 lesser submergence that I would assign what we may for convenience 

 call the Clyde beds, although they are by no means confined to the 

 Clyde basin, but are met with in a multitude of places along the 

 west coast and its numerous creeks. The lowest height of the 

 watershed between the Forth and Clyde is 160 feet near Kilsyth, 

 and 220 near Balfron on the line of the Stirling and Balloch 

 Eailwa}'. The submej"gence which led to the formation of the later 

 Arctic shell-beds would therefore not extend across the watershed 

 nor reach so far up, 50 or 60 feet being about the upper limit of 

 these beds in the Clyde district, and the group of mollusca whose 

 remains are found in them does not indicate much depth of water. 



When I first made my acquaintance with the Clyde beds more 

 than 40 years ago, I was struck with the abundance of the shells, 

 their fine preservation, and the multitude of places in which they 

 occur, all so unlike what I had experienced on the east side of the 

 Kingdom. But I felt at a loss to understand how there should be 

 such a difference between the two sides of the country, and it was 

 not until long after that I came to perceive what I now take to be 

 the true meaning of it all. 



' See J. Adamson, in "Wernerian Soc. Memoirs, vol. iv, p. 334 ; also Paloeonto- 

 graphical Soc, vol, xxviii, p. 42. 



