DESCRIPTION OF THE FORMATIONS 371 



present fault boundaries; so that Lower Orcadian beds may correspond 

 to Upper Caledonian. 



The basal beds are preserved in certain small areas in Aberdeen and 

 Banff. These show in some cases clearly the form of old filled river 

 valleys. A. Geikie describes that which is now dissected by the River 

 Avon, latitude 57° 20' north, longitude 3° 20' west. 



"A marked structure in some parts of the conglomerate is a kind of false- 

 bedding of the stones. Between the gently inclined lines which mark the true 

 dip, the stones of each bed of conglomerate are arranged in a rude stratifica- 

 tion obliquely across the line of the valley at angles of 17° to 20°. They thus 

 appear to dip toward the hills (representing the old valley walls), while the 

 true inclination of the conglomerate beds is away from them." 2S 



This is a highly significant feature. It seems clearly to represent the 

 stream-shingling of gravels, by which the longer axes of the pebbles slope 

 upstream. The situation and character does not suggest beach sorting. 



Old Red of the Moray Firth, — Passing to the old Eed Sandstones 

 around the Moray Firth, Geikie states : 



"We have found them throughout that extended tract mainly conglomeratic 

 and arenaceous — the conglomerates in thick masses coming in again and again 

 on successive platforms, while interstratified with them lie bands of grey clay 

 and shale full of calcareous nodules containing fish remains. These fossilifer- 

 ous bands retain their distinctive characters, lithological and palseontograph- 

 ical, throughout the whole district'" (page 446). 



These strata stand in great contrast to the vast continuous flagstone 

 series of Caithness to the north, though they are considered by Geikie to 

 be equivalent to the upper portion of the Caithness section. 



Formations of the Orcadie basin in Caithness. — A tabulation of the 

 formations which comprise the whole terrane, with an abstract of such 

 features as bear most strongly on the problem of origin, is based on 

 Geikie's complete report in the Transactions of the Eoyal Society of 

 Edinburgh. 29 



Nearly the whole of Caithness consists of Old Eed Sandstone. The 

 interior is almost entirely covered with peat and glacial drift, but the 

 powerful action of the ocean on these rather friable rocks has exposed 

 admirable sections often several hundred feet in height around the entire 

 coastline. These have been compiled by Geikie into a type section of the 

 whole series. 



28 A. Geikie : On the Old Red Sandstone of western Europe. Trans. Roy. Soc. of 

 Edinburgh, vol. xxviii, pt. ii, 1878, p. 427. 



29 Vol. xxxviii, pt. ii, 1877-1878, pp. 355-421. 



