A HISTORY OF DURHAM 



Oxenhall near Darlington, known as the ' Hell Kettles.' These sink- 

 holes vary from 75 to 114 feet in diameter. 



It is sufficiently clear that during the period of geological time 

 represented by these red beds the area now occupied by south Durham 

 was much in the conditions observable in the Salt Lake regions of Asia, 

 north-eastern Africa, or north-western America conditions of dwindling 

 inland sheets of water in an arid climate of evaporation, and of salt and 

 gypsum deposition such as the late Sir Andrew Ramsay showed many 

 years ago have so constantly accompanied the accumulation of red-hued 

 sandy strata. 



THE IGNEOUS ROCKS 



Most remarkable and, in all probability, with the exception of the 

 Minettes, oldest of the igneous rocks of Durham, is the famous Great 

 Whin Sill, which, though exposed within the county boundaries only in 

 the inlier between Middleton in Teesdale and Caldron Snout, is yet the 

 cause of perhaps the finest scenery in the county. This sill (sill means 

 a stratum simply in north country dialect) is a huge sheet of intrusive 

 basaltic rock strictly speaking, ' diabase ' -which is known from a few 

 miles south of Berwick to as far south as Lunedale in Yorkshire, a 

 distance of over 80 miles, and which crops out to the west of this 

 Durham inlier along many miles of the Pennine escarpment and more 

 especially at Highcup Nick. It possibly underlies the whole of the 

 county of Durham, though this will probably never be proved. So vast 

 an intrusive sheet is very exceptional unique indeed as regards Britain 

 in times later than those during which the much more ancient Dalradian 

 sills of Scotland were injected. In the Middleton inlier it lies very near 

 to the Ordovician and Silurian floor, upon which the Lower Carboni- 

 ferous rocks were laid down as has already been mentioned (see p. 3) ; 

 but it is well within the last named series and, although in many places 

 where its position has been ascertained with accuracy (as in mine shafts, 

 etc.) beyond the inlier, it is found to shift its horizon as much as even 

 1,000 feet in some cases (a sufficient proof of its intrusive character were 

 other convincing evidence lacking), yet it is always within the Carboniferous 

 Limestone Series. This important fact is not, however, enough to enable 

 one to say more as to the age of the Whin Sill than that it is younger than 

 the highest horizon to which it has risen. It is post-Carboniferous Lime- 

 stone probably (all but certainly so) ; it is possibly of Permian or even of 

 much later date. The thickness of the sill, considering its enormous area 

 of at least 400 square miles, is extraordinarily uniform, continuing for 

 long distances from 80 to 100 feet, though to the west sometimes much 

 thinner, and sometimes 150 feet or even more. It sometimes splits up 

 into two or even three sheets. In the Middleton tract it is a single sheet 

 and very thick, forming the magnificent columnar scars of Cronkley and 

 the waterfalls of High Force and Caldron Snout. At Stanhope in Wear- 

 dale, in which neighbourhood the main sill is met with in many lead 

 mines, an upper ' split ' or branch known as the Little Whin Sill crops 



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