The old alluvia (3) are obviously of fluviatile origin, and show us that after the deposition of the clays, etc., of the 100-feet beach the sea retreated, and allowed the Tay and its tributaries to plough their way down through the marine and estuarine deposits of the "third" glacial epoch. These deposits would appear to have extended at first as a broad and approximately level plain over all the lower reaches of the valleys. Through this plain the Tay and the Earn cut their way to a depth of more than 100 feet, and gradually removed all the material over a course which can hardly be less than 2 miles in breadth below the Bridge of Earn, and considerably exceeds that in the Carse of Gowrie. No organic remains occur in the "old alluvia," but the deposits consist principally of gravel and sand, and show not a trace of ice-action. Immediately overlying them comes the well-known peat-bed (4). This is a mass of vegetable matter, varying in thickness from a few inches up to 3 or 4 feet. In some places it seems to be made up chiefly of reed-like plants and sedges and occasional mosses, commingled with which are abundant fragments of birch, alder, willow, hazel, and pine. In other places it contains trunks and stools of oak and hazel, with hazel-nuts--the trees being rooted in the subjacent deposits. It is generally highly compressed and readily splits into laminæ, upon the surface of which many small reeds, and now and again wing-cases of beetles, may be detected. A large proportion of the woody débris--twigs, branches, and trunks--appears to have been drifted. A "dug-out" canoe of pine was found, along with trunks of the same tree, in the peat at Perth. The Carse-deposits (5), consisting principally of clay and silt, rest upon the peat-bed. The occurrence in these deposits of _Scrobicularia piperata_ and oyster-shells leaves us in no doubt as to their marine origin. They vary in thickness from 10 up to fully 40 feet.[CS] [CS] For a particular account of the Tay-valley Succession, see _Prehistoric Europe_, p. 385.