Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/150

112 112 S. V. WOOD, JTTN., AND F. W. HARMER ON THE these altitudes and down to the sea-level, would at first sight make it doubtful how far this limit to the submergence is a sufficient ex- planation of the mode in which the deposit is distributed. One circumstance, however, seems to corroborate it, which is that the Middle Glacial gravel at Finchley is largely made up of pebbles derived either from the pebble-beds associated with the contiguous Lower Bagshot outliers of Essex or from those beneath the London Clay ; and we must suppose that those sources were above water at the time, in order to supply the pebbles, as these do not appear to have been introduced by the action of glacier ice. The highest of any of these sources does not attain an elevation exceeding 440 feet ; and most of them only reach elevations nearly 100 feet less than this, and not much exceeding that of the Middle Glacial itself at Finchley. If, however, this limit to the submergence during the accumulation of the Middle Glacial was the case, we should regard it as an indica- tion that, so far from the glacial submergence having been an unin- terrupted one, an emergence must have succeeded the Contorted Drift, because, the entire thickness of the Lower Glacial deposit around Cromer having itself exceeded 200 feet, there must have been far more than 200 feet of water over it in order to trans- port the bergs which, in grounding, have deposited such great masses of Marl in that Drift, contorting it in the process. Some of these masses are so enormous that it is hardly possible to suppose that a berg capable of transporting one could have grounded in much less than 1000 feet of water*; so that it seems to us that when these masses were thus introduced, Northern Norfolk had undergone a submergence far beyond what is thus supposed to have prevailed during the accumulation of the Middle Glacial. One of these masses forms the entire cliff a short distance west of Woman -Hy the Gap ; and this it was our fortune once to see nearly free from the usually obscuring talus ; and it seemed to be about 300 yards in length by 60 feet in height. Its breadth, of course, was concealed. It is clear that these introductions took place before the formation of the gravel overlying the Contorted Drift in the Norfolk cliff — because the line of denudation dividing this drift from the overlying gravel is clearly defined, and cuts across the Drift and its included masses indiscriminately. If, therefore, the land rose after the formation of the Contorted Drift to such an extent that during the"; deposit of the Middle Glacial the depth of water did not exceed 400 feet, we need not hesitate to suppose that it emerged altogether, and that the Lower- Glacial sea-bottom was converted into land, if other features, such as that indicated possibly by section XVIII. at Starston, and the excavation of valleys through this sea-bottom, with the formation in them of the interglacial bed of the Yare valley, point in this direction. therefore spread out under water with a wider base, float in a much less depth of water than those of tabular form ; but, before they had time to acquire that shape, they would probably have parted with the freight of marl carried at their bottoms.
 * Bergs of great dimensions which have acquired a pinnacled form, and