TERRACED SLOPES IN NORTH TYNEDALE. 41 



Vv^ould be originally many traces of the successive levels at 

 which these inland lakes had stood at different times, excava- 

 ted on the slopes of the valley and its transverse openings on 

 similar sites to those whereon we now find the parallel ter- 

 race-lines. But what vestiges, if any, remained subsequent to 

 the far more powerful action of the descending ice-bergs and ice- 

 floes of the "drift period," is the difficult question to decide. 

 There are evidences yet to be discerned of the passage of gla- 

 ciers down North Tynedale, when it was either a firth or arm of 

 the sea, or a chain of inland lakes. What are these proofs of 

 glacial action, then, to which with good reason the formation of 

 the Grien Roy terraces, and to some extent our own parallel 

 ridges might be ascribed ? They are found in the general phy- 

 sical characteristics of the valley ; in the fact that glacial cur- 

 rents from the colder north to the warmer south, in the short 

 summer of that pleistocene, or boulder drift period, have worn 

 and denuded the crags and hills on its eastern bank, whilst the 

 rugged strata of the opposite slopes are marked with thick ac- 

 cumulations of clay, sand, and gravel. The direction of the 

 stream is the ordinary one of that period, nearly from north- 

 west to south-west ; and by its agency have been laid bare such 

 bold head-lands as the Mill Knock, near Birtley, and the basaltic 

 range between Grunnerton and Barrasford, which confront the 

 west or north-west. Then, again, the same tremendous trans- 

 porting power could alone account for the moraines of sand, and 

 gravel, and boulders, found, for example, along the great es- 

 carpment above the Steel Burn, and on the platform above the 

 Birtley terraces — the debris of huge water-worn blocks, espe- 

 cially, brought down and detached from the under surface of the 

 brief summer avalanches or slowly melting ice-bergs,''' As these 

 gradually forced their ponderous masses along the valley, they 

 have further left theii' unmistakeable traces here and there in the 

 smoothing and grooving, and scratching of the rock surfaces, 

 which were of too enduring a natui-e to be displaced ; and this, 



* Near to the top of the Wark Mote Hill, on the north side, Mr. Hodgson noticed on his 

 visit to Mounces in 1814 "an alluvial block of granite, about a ton in weight. — Memoir, 

 Vol. I., p. 140. 



