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36 and to a series of trial borings for a tunnel beneath the Humber, the structure of the valley has been clearly laid open. It is much the same as that of the Thames; but as we are in a glaciated area we find, as in the Fenland, that much of the erosion had taken place before or during the Glacial Epoch, for boulder clay occupies part of the valley.

Boulder clay or till not only occupies part of the valley, it descends far below the present river bottom and even below the lowest submerged forest. This we find always to be the case in the glaciated parts of Britain; but whether the deep trenching is due to the ploughing out of a trough by a tongue of the ice-sheet, to sub-glacial streams below sea-level, or to erosion by a true sub-aerial river is still a doubtful point. However, this question must not detain us; we are not now dealing with elevations and depressions of so ancient a date, and must confine our attention to post-glacial movements.

The section shown in fig. 3 will explain better than any words the structure of the Humber Valley.