Desert Conditions in the Trias. - 561 
itself to the outlines of the substance, but produces a sharp cut edge 
where it leaves the object. Similarly, sand moving with great velocity 
keeps its initial path, while similar particles moved slowly by a current 
of water over a curved surface would roll over the leeward slopes and 
abrade during their descent. It must not be concluded that a plane 
surface can only be produced by the action of wind-driven sand. In 
Switzerland I have seen plane surfaces and sharp cut edges, produced 
in the beds of torrents carrying sand in suspension, quite indis- 
tinguishable from those produced by wind action. The plane surface 
then is largely a result of high velocity, and while this is of rare 
occurrence in currents of water it is common in currents of air. 
When driven sand strikes a stone in the open desert it is deflected 
upwards and round the sides so that plane surfaces are formed on 
three planes. This is the typical form to which the name ‘ dreikanter’ 
has been applied, and the finding of these in any deposit is regarded 
as one of the surest indications of wind action. 
Sand blowing over a flat surface of rock tends to widen joints and 
open them out into a funnel shape, with the wide end facing the 
direction of the wind. The sides of the opened joints, too, are almost 
invariably undercut. 
It has been shown by Professor Watts, Mr. Walcot Gibson, and 
others that the surface of the country had been carved by denuding 
forces into valleys and hills before the Trias was deposited. The 
newer rocks, by filling in the low grounds, smoothed down the land- 
scapes, and in some places hills were completely covered. In no part 
of Britain do Triassic rocks attain a greater altitude than 800 to 900 
feet above sea-level at the present day, and we have no proof that they 
ever extend much above this level. They are essentially deposits of 
the lowlands. Their distribution has been admirably summarised by 
Professor Bonney,' and I can add nothing to his lucid description of 
their occurrence. He shows that there exist two foci of coarse frag- 
mental rocks, one in the western and northern Midlands and the other 
in Devonshire. No one now doubts that Professor Bonney is right in 
claiming these coarse deposits as of fluviatile origin. At the places 
mentioned, rivers reached the plains (whence they came does not 
matter at present) bringing down pebbles, well worn in transit, and 
no doubt a large amount of finer material as well. 
Taken as a whole the deposits get finer as we go away from the 
foci, and on the extreme borders of the areas covered by Trias they 
consist of exceedingly minute fragments. At a subsequent stage in 
the history of the Trias, pebbles occur again, indicating a return to the 
conditions under which the lower beds containing pebbles were formed. 
Assuming that desert conditions prevailed during Triassic time— 
interrupted, perhaps, by pluvial periods, when rapid streams brought 
down the pebbly constituents—let us see how far they show evidence 
of the action of wind. Except in the finer deposits such as the Keuper 
Marl true bedding is almost entirely absent. Current bedding there is 
in plenty, and frequently very steep—too steep, indeed, for the angle 
of rest of sand laid down in water. In extreme cases of steep false 
1 “On the Origin of the Trias’’?: Proc. Yorks. Geol. Soc., vol. xvi (1906), p. 1. 
DECADE VY.—VOL. IV.—NO. XII. 36 
