GEOLOGY 



in thickness from place to place, the thickest portions often within a few 

 yards of bare rock or of quite thin Drift. The six volumes of Borings 

 and Sinkings, published by the North of England Institute of Mining 

 and Mechanical Engineers, are full of valuable details bearing upon 

 the distribution of this oldest of the Glacial deposits. 



All pre-Glacial valleys were necessarily choked up with this clay 

 and most of them are so still, the post-Glacial rivers not having by any 

 means always chosen to follow the ancient channels, and having often 

 preferred to wear down new valleys through virgin rock to digging 

 along their old courses through the stiff intractable material under which 

 they were buried. These concealed pre-Glacial valleys and there are 

 many of them are known as * washes,' and frequently present formid- 

 able barriers of barren ground to the miner between the denuded edges 

 of coal-seams. The best known of these washes or washouts is the 

 long one which, first recognizable high up the Wear valley near Witton- 

 le-Wear, follows more or less parallel to the present river (but rarely 

 coinciding with the actual tbaliveg now existing) to Durham city, half 

 of the market place in which is situated upon it ; thence to near 

 Chester-le-Street. Here instead of approximately following the present 

 river and its valley it turns abruptly to the north, actually crosses (as the 

 railway also does) the watershed between the Wear and the Tyne, and, 

 following the Team valley, reaches that of the Tyne 150 feet beneath 

 its bottom level. This pre-Glacial wash is filled with boulder clay and, 

 above that, with later clays, gravels, and sands which, in places, attain a 

 thickness of more than 300 feet. Similar ancient river courses similarly 

 hidden from view by Glacial infillings are numerous, and a number in 

 the north-eastern portion of the county have quite recently been care- 

 fully and successfully worked out with much skill and patience by 

 Dr. David Woolacott. 



Above the Boulder Clay are vast thicknesses of sand and gravel, 

 as well as limited patches of laminated (locally, * leafy ') clays, which 

 are largely the result of the reasserting of the material of the older 

 clay and of silty accumulations in ice-dammed or moraine-dammed 

 lakes at the melting of the ice and after. There is no evidence 

 in Durham of any true Interglacial Period, these gravels and sands, 

 which are usually called the Upper Glacial gravels and sands, being 

 the final set of accumulations due to any phase of the reign of cold. 

 They can be excellently studied along the banks of the Derwent and 

 Wear, where numerous cuttings, both artificial and natural, expose 

 sections of great height and length. Exactly the same kinds of stones 

 are found in these loose deposits as in the Boulder Clay, but the 

 polished and scratched faces which they exhibit in the latter are as a 

 rule effaced by the rolling to which the blocks were subjected during 

 the dtbdcles of the later or melting stage. 



It is clear from a study of the Drift of Durham that one great glacier- 

 sheet came from the Tyne valley and from north-west Northumberland 

 and swept due south across lower (or eastern) Durham towards the York- 

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