THE OLD RED SANDSTONE OF THE ORKNEYS. 401 



clinal axis passes almost through Maeshowe down Summersdale into the Kirbuster 

 district of Orphir. In the Heddle Hills of Firth, to the east of this line, the dips are 

 mostly east and north-east, very gentle in the flag quarries, now disused, which crown 

 the hills on both sides of Finstown. From the latter village to Kirkwall we have a 

 rolling succession of gentle anticlines and synclines with axes striking north and south ; 

 seen well in the shores of Firth and Kirkwall Bays, where the same beds crop out again 

 and again. There are no steep dips and no traces of any important dislocation, but 

 from Summersdale to Kirkwall, on the whole, the dip is eastward, and we are ascending 

 very gradually in the series. In the quarries to the west of Kirkwall there is a very 

 slight north-west dip, while along the shore to the east of Kirkwall Bay the dips are 

 strongly east. The change is marked by a line of crushed rock which runs under 

 Kirkwall in a N.N.E. direction, and emerges on the shore at Cromwell's Fort. This 

 seems to be the northward continuation of the fault described by Peach and Horne as 

 running from Howquoy Head in Holm, northwards along the shore, and forming the 

 eastern boundary of the sandstones of Scapa Flow. # This may be possibly a continua- 

 tion of that already described as passing through Egilshay into Westray. At any rate it 

 is an important feature in the structure of this part of the Mainland of Orkney, for to the 

 west of it lie the gently rolling beds described, while to the east the dips are steep as 

 a rule, and the rocks thrown into very pronounced folds. In other words, it forms a 

 natural geological boundary to the East Mainland of Orkney. 



The East Mainland District. 



The second area in which it has been proved that the Rousay group of fossils occurs 

 in Orkney is that around the town of Kirkwall, in which Hugh Miller remarked their 

 presence more than forty years ago. The structure of the East Mainland has not 

 that simplicity which characterises the West Mainland. To the south-west it is 

 bounded by the fault described by Peach and Horne, which brings down the sandstones 

 of Scapa against the flags. The flags along this fault are probably the lowest rocks 

 exposed, for through the whole area there is a constant tendency to a northerly dip, 

 varied, of course, by the subsidiary folds, and the highest rocks occur only in the 

 northern half of the district. Two series of rocks occur — the Eday sandstones in two 

 areas, Berstane Bay and Deerness, the Rousay beds elsewhere. 



The structure is clearly defined, an anticlinal axis occupied by the flags passing up the 

 centre of the district in a north-east direction, and forming the Ness of Tankerness, while 

 on each side a syncline brings in the overlying rocks, the sandstones (sect. 3). The section 

 along the public road from Kirkwall to Dingieshowie, Deerness, affords a very good index 

 to the general structure. For a mile or more after we leave Kirkwall, the rocks are 

 steeply inclined to the east and north-east, disturbed, no doubt, by the great fault whose 



* Peach and Horne, op. cit., p. 11. 



