356 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



one subject to dry seasons, the most important criterion consists of marks 

 of subaerial exposure, developed both broadly and vertically through 

 mechanical sediments. These consist chiefly of mud-cracks, rain-prints, 

 and impressions of roots in situ. Where these occur in such relations 

 they seem to show conclusively a terrestrial origin, and not to represent 

 the littoral facies of a marine or lacustrine formation. Not only, as 

 stated previously, does the shore form at any one time but a narrow border 

 to the accumulating mantle, but, except on the front of an advancing 

 delta, it is commonly a region of erosion rather than of accumulation- 

 Tidal flats are furthermore flooded and drained twice per day, with the 

 result that they are always limited in width and are cut by deep channels. 

 These conditions are quite distinctive from those which are found inland. 

 For over deltas and playa basins, on the contrary, every part is alternately 

 covered by water and by air for considerable periods of time. Where the 

 climate is suitable mud-cracking is developed habitually and on a broad 

 scale. 



Mud-cracking in chemical sediments — that is, in limestones^ most 

 typically the impure or "water-limes" — must, however, be distinguished 

 in significance from the cracking in claystones. Lime carbonate is car- 

 ried in solution and its deposition as limestones requires a comparative 

 absence of sand and clay, the mechanical deposits carried by rivers and 

 by waves. The solutions, to have sufficient concentration, may come from 

 permanent water bodies, either lakes or seas. The deposit, therefore, 

 comes not from the direction of the land, but from the direction of the 

 water. The cracking goes on between the extreme levels of high and low 

 water, and the slight shifting of level is not a tidal, but at least a seasonal, 

 phenomenon. Such mud-cracking of limestones is a playa phenomenon, 

 and especially in certain earlier ages, when the lands were baseleveled and 

 lay awash with the sea, broad areas seem to have been at times marine 

 playas. Marine fossils, often of depauperated kinds, occur sometimes in 

 mud-cracked limestones. The nearest approach in the modern world is 

 found, doubtless, in the Bann of Cutch, an area of 10,000 square miles 

 south of the Indus, flooded by the sea for a part of the year, during the 

 period of onshore monsoon winds. 



In the detection of mucl-eracks in ancient formations reasonable care 

 must be used to avoid mistaking for them a polygonal cracking of the 

 rock arising after its solidification. The two, however, are readily dis- 

 tinguished- True mud-cracks always have a filling; the polygons are 

 irregular, but do not show irregularity constant in one direction. True 

 mud-cracks, although easily separated from simulated features, are, how- 

 ever, often very difficult to detect, as the filling may be identical in nature 



