INTERPRETATION OF THE ORCADIAN DEPOSITS 377 



shallow lakes which are dried out only at occasional times. Such flood- 

 plain or playa lakes may exist readily under climates with marked dry 

 seasons, but shift in area broadly under such conditions and leave wide 

 mud-cracked zones. 



The writer has elsewhere compared the stratigraphic characters of the 

 Orcadie basin to those of the Seistan basin in eastern Persia. 31 



Significance of mud-cracks and rain-prints. — The Wick and Thurso 

 flagstone groups, possessing a total thickness of some 10,000 feet, com- 

 prise the middle two-thirds of the Caithness section. One of the most 

 striking features noted by Geikie is the presence throughout of mud- 

 cracks. He also mentions that the red shales and sandstones of Brae- 

 more are sometimes pitted as if by raindrops. It has been stated further 

 that the flagstone groups exhibit the same lithologic characters in the 

 Orkney and Shetland islands. 



Eeasons have been given in full elsewhere for holding that mud-cracks 

 occurring regularly through argillaceous strata of great depth and hoii- 

 zontal extension can only be produced by fluviatile deposition over broad 

 floodplains under climates with periodic rainfall, or in the central playa 

 basins toward which such rivers drain. 32 The alternating sandy and 

 shaly character of the strata of these mud-cracked formations finds also 

 in this view a natural explanation. 



It has been noted further that on passing downward toward the base of 

 the system, or on the same stratum toward the margins of the basin, the 

 yellowish, grayish, or greenish flagstone shale strata pass into reddish and 

 arenaceous beds. Since the flagstone portions are interpreted to belong- 

 to the lower portions of the floodplains, the reddish arenaceous beds into 

 which they pass laterally must be from their position also terrestrial and 

 more nearly marginal to the basin. The explanation of the contrast in 

 color is that, from being deposited on the better-drained borders of the 

 basin and possessing naturally a more porous nature, oxidation of the 

 beds during deposition has taken place more freely. 



Significance of intercalated limestones. — Especially in the flagstone 

 groups, but also to some extent in the other groups, the presence of highly 

 calcareous shales was noted, with occasional strata of thin-bedded impure 

 limestones. Such fresh-water calcareous deposits originate on the bot- 

 toms of lakes protected to some extent from the inwash of sediment and 

 are especially apt to occur where evaporation is considerable. Such water 

 bodies, of a temporary and shifting nature, are a usual accompaniment 

 of fluviatile aggradation in broad basins or over terminal deltas, due to 



31 Bull. Geol. Soc. Am., vol. 23, 1912, pp. 418, 419. 



32 Journal of Geology, vol. xvi, 1906, pp. 524-568. 



