The widening of the valleys, where they were cut in soft strata, led to the development of small lateral valleys to the right and left, leaving only narrow divides between their head waters and those of the next valley. When the land sank these divides were flooded, and so were developed the shallow cross connexions, much as we now see them.
In order that it may not be imagined that this reconstruction is merely hypothetical, it will be as well to give some evidence that such an elevation and submergence did take place in this district as in others. We cannot in this little book deal with the whole of the evidence, so we will take the Southampton Dock excavations as sufficient to prove our point, condensing the account from the Geology of Southampton, published by the Geological Survey.
The general section at Southampton Docks is as follows, though the thickness varies considerably at different points, and the greatest depth of the old valley has not yet been proved:—
| Feet | |
| Estuarine silt | 20 |
| Peat, old vegetable soil, or tufaceous marl; ox, pig, horse, pine, beech, birch, oak, and hazel | variable up to 17 |
| Gravel, with reindeer | 10 or more |
The bottom gravel is apparently of Pleistocene date, though it may include also a basement bed belonging to the newer deposits. T. W. Shore recorded from the peat above the gravel a fine stone hammer-head of Neolithic date and worked articles of bone, but no instruments of metal were found. The associated marl was full of freshwater shells.
Poole Harbour tells a similar story, and evidence of this submergence is seen in the various submerged forests found along the Dorset and Devon coasts, opposite the mouths of the valleys. These rocky coasts are, however, so different from those we have just been describing, that they will more conveniently be treated of in a separate chapter.
CHAPTER VII
CORNWALL AND THE ATLANTIC COAST
On travelling westward into Cornwall we enter a region which is extremely critical in any enquiry as to the amount of change that the sea-level has undergone. As long as we were dealing with ancient river-channels opening into enclosed seas, like the North Sea or Irish Sea, it might be said that the depth to which the channel was cut was not necessarily governed by the sea-level. It might be governed by the level of an alluvial plain, which then extended for hundreds of miles further, and had its upper edge high above the sea-level.
This cannot be said in Cornwall, for there the sea-bed shelves rapidly into deep water, and the coast would not be far away, even were the land raised 200 feet or 300 feet. The rivers then as now must have flowed almost directly into the Atlantic Ocean, and their channels must then as now have cut nearly to the sea-level of the period.
The Cornish rivers yield most valuable information. It so happens that many of them bring down large quantities of tin ore from the granitic regions, and this ore being very heavy tends to find its way to the bottom of the alluvial deposits, out of which it is obtained in the same way as alluvial gold in other countries. On following this detrital tin ore downwards towards the estuaries the “tinners” or alluvial miners found in many cases that a rich layer descended lower and lower till it passed well below the sea-level in some of the ancient silted-up valleys. Some of these tin deposits were so rich that it paid even to divert the rivers, dam out the sea, and remove the alluvium to considerable depths in search of the ore.