The river is only a small one and its catchment area is very limited; it has therefore a rapid fall, amounting to 30 feet in the mile between St Austell and the sea. With this fall the valley is still silting up and its alluvium rising, principally through the abnormal amount of sediment and granitic sand sent down by the china-clay works. If we take the fall of the buried channel, this amounts to about 45 feet to the mile, for the rock-floor at Pentuan lies about 60 feet below the sea-level. This rock-floor is composed of hard slates. The successive deposits met with above the slate were as follows, commencing with the lowest:— (_a_) _The tin ground_, or stratum in which the whole of the stream-tin is found. It lies on the solid rock and is generally from three to six feet thick, sometimes even ten feet. It extends across the valley, except where turned by a projecting hill or rock, when it is found to take the supposed ancient course of the river, which is generally under the steep bank opposite. This last observation (often made by tinners) is important, for it suggests that the heavy tin-ore was brought down by exceptional floods, such as would swing violently to the outer side of the curve, and there cut a steep bluff, under which would be left the heaviest gravel. This observation and the noteworthy absence of any contemporaneous animal remains in the tin-ground, suggest that the bottom layer may date back to Pleistocene times, when the climate was colder and floods more violent. It is not clear how far seaward the valley may then have extended; probably not more than half a mile at most. The tin ground was worked near Pentuan for 1400 yards along the valley, and averaged about 52 yards in breadth. So here again we meet with a fairly wide flat-bottomed valley, not a narrow =V=-shaped gorge; we may therefore take it that the base-level had been reached and that this base-level was identical with that met with in the rivers which open into Plymouth Sound. (_b_) On the tin ground were rooted numerous oaks, which had grown and fallen on the spot. Their timber was so sound that Colenso applied one of the trees to make the axle of a water-wheel, and his comment on this is excellent. “It appears to me likely that at this period, the rising of the sea had so far checked the current of the river as to prevent its discharging the mud and sand brought down with it; thus the roots were buried [submerged?] to a considerable depth, and the trees killed, before the timber underwent its natural process of decay.” At one spot he records finding oysters still remaining fastened to some of the larger stones at the top of the tin ground and to the stumps of the oaks.