MARKINGS IN PENNSYLVANIAN SANDSTONES 79 
has shown that when soft beds are overlain by unequally disturbed 
heavier and firmer beds there is a marked deformation of the former 
caused by differential weighing of the firmer upper beds. In the 
Osage the overlying heavy sandstones are equally distributed over 
large areas, but were the strands inclined, slipping could conceivably 
take place in sediments of the right composition producing modified 
slickensides on the bedding planes. Again, were there readjust- 
ments within the subsiding geosynclinal basin during deposition, 
such as faulting, which Mr. A. W. McCoy tells the writer took place 
in the Osage basin, slipping would be expected along bedding 
planes, just as slipping is known to have been caused by earth- 
quakes. 
Difficulties not overcome by a slipping hypothesis, even assum- 
ing that sandy clays and sandstones would slide, are the explana- 
tions of systems of cross-markings, and the absence of buckling 
of the strata or of intraformational conglomerate. Slickensiding 
after consolidation is not a possibility because the markings are 
clearly molded, not gouged. 
5. Tidal action.—Current and tidal action produce a variety 
of markings on the strand, a number of which have been figured 
by Dr. Clarke and which can be reproduced in the Osage sandstone 
and in the Tesnus formation sandstone of the Marathon region, 
Texas; especially the ‘““mud flows” which are interpreted as plunge 
and undertow markings of retreating waves on the beach. The 
larger ones, which are 2 to 5 inches in width and 1 inch in depth 
may have been formed, as suggested to the writer by Mr. C. A. 
Hartnagel, of the New York State Museum, in tiny puddles of 
water on a gently sloping beach swirled by sudden gusts of wind. 
Professor Hall’s theory of the origin of the parallel markings is by 
- current action: 
The only assignable cause for these ridges is the action of a current flowing 
over the surface of the strata, sometimes transporting sand and at other 
times coarser material which furrowed the surface upon which the subsequent 
deposits were made.? 
«Dr. J. A. Udden has kindly called the attention of the writer to photographs 
of sandstone slabs showing minute faults and wavy subparallel folds, the origin of 
which he believes to have been creep of settling muds before consolidation, but these 
folds do not resemble the strand markings (Univ. of Texas, Bull..246 (1912, Pl. 25). 
2 OP. cit., pp. 234-35. 
