It is drawn to scale from the engineer's section, and shows at a glance the three channels. The deepest and widest channel is that occupied by glacial deposits; an intermediate channel (shown in black) is occupied by silt and submerged forests; and a shallower channel is occupied by the present Humber and its alluvium. One interesting point, however, this section does not happen to illustrate. Somewhat lower down the Humber we come to gravels and silts full of sub-arctic marine mollusca and Corbicula fuminalis, exactly as in regions further south, and presumably of the same age as the deposits we have already mentioned as found at March in the Fenland and at Grays in Essex. The exact relation of these Corbicula-beds to the deep channel filled with glacial drift, below the marshes of the present Humber, is still somewhat uncertain, but the marine beds clearly rest on boulder clay, and seem also to be overlain by another glacial deposit.
The section leaves no doubt that in post-glacial times the Humber cut a channel about 60 feet below its present bed, or to just the same depth as did the Thames. This may possibly be an accidental coincidence; but it is very suggestive that both these rivers should have cut their beds to the same depth. Such coincidences suggest that we are dealing with a period when each of our great rivers was able to cut to a definite base-level, below which it could not go. This
