378 J. BARRELL FLTJVIATTLE ORIGIN OP OLD RED SANDSTONE 



the flatness of the surface and unequal upbuilding. Over the delta of the 

 Nile, for instance, similar deposits are at present forming. Examples of 

 somewhat more permanent lakes due to the damming of a main drainage 

 axis by tributary fans may be cited in Tulare Lake, in the Great Valley 

 of California, or Lake Wulur, in the Vale of Cashmere. 



Calcium carbonate deposits, as previously noted, originate also over the 

 surface of floodplains in arid or subarid climates by continued evapora- 

 tion of subsurface waters without the presence of any permanent super- 

 ficial water body. The calcareous sandstones found in these Devonian 

 deposits are suggestive of such relations, but the presence of limestones 

 of considerable extent and some degree of purity, associated particularly 

 with the gray flags and sometimes holding fish fossils, seem to show, how- 

 ever, that the purer limestone strata were laid down in shallow temporary 

 lakes, rather than developed by evaporation under arid conditions over 

 the periodically desiccated floodplain. 



LOWEST UPPER OLD RED OF THE SHETLAND ISLANDS 



The lowest horizon of the Upper Old Red occurs in the Shetland 

 Islands, and the rocks are described as follows by Plett : 



"A little west of Lerwick, coarse conglomerates are faulted against the 

 inetamorphic series. They dip towards the east, and are succeeded at the 

 town of Lerwick by reddish and grey sandstones, often current-bedded, and 

 sometimes containing large rounded pebbles of qnartzite, granite, etc. At the 

 point southeast of Lerwick known as the Nabb, grey micaceous sandstones 

 occur, full of plant-remains, and containing also the small crustacean Estheria 

 membranacea. On the opposite shore of Bressay Island the first beds met 

 with are brownish and grey sandstones, often conglomeratic, and sometimes 

 brecciform, with occasional grey and reddish shales. ... In crossing Bres- 

 say the dip of the rocks is consistently east or southeast, varying from ten to 

 thirty degrees. The commonest rocks are grey, micaceous, thin-bedded sand- 

 stones, with coarser, less micaceous, gritty seams, often current-bedded. The 

 sandstones contain rounded clay galls, and their surfaces are often covered 

 with blackened fragments of plants and shreds of fine shale. . . . 



"In view of the persistent easterly dip, often at fairly high angles, the whole 

 thickness of this series must be several thousand feet ; but the evidence of 

 faulting along the shores of the Sounds is sufficient to render exact estimates 

 impossible. The fish beds in Cullinsburgh Voe are rather above the middle of 

 the Bressay Sandstones. The fossils occur in a thin-bedded, flaggy, grey mica- 

 ceous sandstone, and the plates are black in color and well preserved. With 

 them thin black impressions of plants are exceedingly common. The strata 

 were evidently laid down in shallow water, close to land ; and the general 

 facies of the rocks and of the fauna is in harmony with the supposition that 

 they were fresh-water deposits." S3 



33 Plett : On the age of the Old Red Sandstone of Shetland. Trans. Roy. Soc. Edin- 

 burgh, vol. xlvi, pt. ii, 1908, pp. 315, 316. 



