THE GLACIAL GEOLOGY OF NORTHUMBERLAND 9 1 



The boulder clay passes upwards into less stony, prismatic 

 clays of various colours and sometimes different boulder- 

 content. Occasionally there are evidences of local uncon- 

 formities between the lower and upper clays, as at Hadston 

 Carrs [21], near Sea Houses [13], at Brunton and Blyth; 

 again, the junction is frequently marked by a course of large 

 boulders laid flat, as at Hadston Carrs and Horsebridge Head. 

 The sections at the places last named show the clay-beds 

 dipping towards the pre-glacial valleys south of the exposure, 

 the dip being about the same as the slope of the pre-glacial 

 surface. In the larger drift-filled valleys some of the upper 

 clays are black and leafy, and have evidently been deposited 

 under water ; from some clay-pits near Newcastle animal 

 tracks in these clays have been described [19]. 



Composition of the Boulder Clay. — Though much field work 

 remains to be done before any particular boulder-clay can be 

 traced with anything like certainty to its source (or sources, 

 for the contents are often greatly mixed), yet certain broad 

 lines of transport can be recognised without great difficulty. 

 Of the rocks exposed in Northumberland, those of the 

 Cheviots are the most important for determinative purposes. 

 Curiously enough, the characteristic augite-granites of that 

 region are of comparative rarity in the drift deposits of the 

 county ; what specimens have been transported by ice occur 

 to the east of Cheviot, and usually in the form of fairly large 

 erratics. The porphyrites, however, have spread out to all 

 points east and south of their source. They dwindle in 

 number and size in the Tweed valley and on the North-East 

 Coast. They occur in great numbers on the high fells between 

 the Rede and the Coquet, on the Simonside hills up to 

 1,100 feet, and on Darden Fell at 1,250 feet; they abound in 

 the Pont valley, are scarce in the Hart and absent from the 

 Upper Wansbeck. The western limit passes down the Rede 

 water, over the Ottercaps, then south of the Hart Burn to the 

 Wansbeck at Bolam ; thence it runs almost due south to the 

 neighbourhood of Heddon. 



