A HISTORY OF DURHAM 



THE PLEISTOCENE OR DRIFT DEPOSITS 



From Upper Triassic times no geological period has left traces of 

 its deposits in Durham until the Pliocene or latest Tertiary ages had 

 passed away and the arctic cold of the great Ice Age had covered the 

 greater part of Britain with snow and ice, and had brought it to the 

 condition now prevailing in Greenland. To that Glacial time is due 

 the irregular but often thick cloak of Drift deposits that at the 

 present day conceals beneath it so many of the valleys and other 

 features which denudation had sculptured and eroded on the outcrops 

 of all the older formations so far enumerated and described in these 

 pages. 



In this cold Pleistocene epoch all but some of the very highest 

 portions of the county in the west was, as we cannot but believe, 

 entirely smothered under an ice sheet which probably began as small 

 glaciers gliding down the upper dales, and gradually increased in size 

 until these merged into larger glaciers running from north to south 

 across the lower and eastern half of the region. At its maximum the 

 heights bare of ice formed but a small nunatak or rocky island in the 

 Yad Moss area. Then, as the severity of the climate was relaxed, the 

 great complex sheet of ice melted away in its lower parts, and waned 

 until the original hill-land glaciers had returned to their original beds 

 and to their original insignificance. Finally, the last of the glaciers 

 dwindled and died out, leaving the country much as we see it now. 



Traces of these successive changes are year by year being recognized 

 with the certainty due to constantly increasing knowledge, but it must 

 be admitted that a great deal more work is required in Durham before 

 anything like a final verdict can be given respecting the history of 

 all the difficult deposits grouped under the term ' Glacial.' 



Concerning the lowest of these, the stiff clay studded with boulders 

 — of which many are obviously foreigners that have reached their 

 present abiding place after much travel — the clay known par excellence 

 as the Boulder Clay, there is not now much doubt. Few geologists see 

 in it, now, the material dropped into the sea from floating icebergs. It 

 is recognized by almost all as the equivalent of the Moraine profonde of 

 Swiss glaciers, i.e. as the ground-down mud interspersed with fallen 

 blocks which underlies moving ice on land. That this Boulder Clay 

 or ' Till' sometimes attains a thickness of 200 feet or even more is 

 evidence enough of the enormous thickness of ice beneath which it was 

 accumulated. The polishing and grooving of the rock surface on which 

 the clay lies is also evidence enough of the movement by which the 

 vast muddy mass was urged over the subjacent floor, and the determina- 

 tion of the place of origin of the travelled stones within the clay yields 

 information as to the directions followed by the ice-currents in their 

 flow over the region. The innumerable pit-sections and boring-records 

 which arc available as to the superficial deposits of the entire county, 

 whether in the coalfield or the leadficld, show how widespread is this 

 great Boulder Clay formation ; but they also show how rapidly it varies 



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