Fig. 35
We also seem to have a test of the comparatively modern origin of the mounds of till which surround the above mentioned chain of lakes (of which that of Forfar is one), in the species of organic remains contained in the shell-marl deposited at their bottom. All the mammalia as well as shells are of recent species. Unfortunately, we have no information as to the fauna which inhabited the country at the time when the till itself was formed. There seem to be only three or four instances as yet known in all Scotland of mammalia having been discovered in boulder clay.
Mr. R. Bald has recorded the circumstances under which a single elephant's tusk was found in the unstratified drift of the Valley of the Forth, with the minuteness which such a discovery from its rarity well deserved. He distinguishes the boulder clay, under the name of 'the old alluvial cover,' from that more modern alluvium, in which the whales of Airthrie, described at p. 53, were found. This cover he says is sometimes one hundred and sixty feet thick. Having never observed any organic remains in it, he watched with curiosity and care the digging of the Union Canal between Edinburgh and Falkirk, which passed for no less than twenty-eight miles almost continuously through it. Mr. Baird the engineer, who superintended the works, assisted in the inquiry, and at one place only in this long section did they meet with a fossil, namely, at Cliftonhall, in the valley of the Almond. It lay at a depth of between fifteen and twenty feet from the surface, in very stiff clay, and consisted of an elephant's tusk, thirty-nine inches long and thirteen in circumference, in so fresh a state that an ivory turner purchased it and turned part of it into chessmen before it was rescued from destruction.
