154 B. Smith—Ball or Pillow-form Rocks. 
the crests of the small anticlines. Had the folding been due to 
packing caused by cleavage-stresses the whole bed would have folded, 
instead of only the middle portion. The most likely explanation to 
account for the facts is that the composite bed was deposited on 
a slight slope, and a creeping movement was set up, whilst the layers 
were still soft, causing the bed to become undulose internally, and 
to thicken at the same time. The top and bottom of the bed would 
remain relatively undisturbed, but each individual layer of sandstone, 
between the little layers of mud in the middle of the group, would 
move slightly in the direction of the creep. 
Signs of current-action and uneven deposition are common in these 
beds. In the quarry south-south-west of Nant Gaer, near Bryn 
Eglwys, there is a 6in. band of slightly calcareous and fossiliferous 
laminated sandstone with uneven upper and lower surfaces. The 
upper has flowing curves which correspond roughly with the surfaces 
of the ripple-marks, whilst the lower is much more irregular, for the 
sandstone fills up little pockets that either have been torn out of 
the mud by current-action, or have been formed by the lowest sand- 
stone lamin buckling downwards whilst in a pasty state. ‘The 
higher rippled lamine of sandstone truncate this lower pocket-filling 
set (Fig. 8). 
Fig. 8.—Sandstone in shale, Nant Gaer, near Bryn, Kglwys. 
Most of this effect, as in the case of the Keuper ‘skerries’ of 
Nottinghamshire, I should attribute to current-action,’ for the wave- 
length and amplitude of the ripples agree very closely with those 
proper for sandstones of this type, as worked out by Sorby.? The 
occurrence is altogether different from that shown in Fig. 7. 
Conclusion. 
Ball or pillow-form structures in sandstones, and certain bucklings 
and foldings, seem to be most satisfactorily explained on the 
assumption that they are primarily due to internal readjustments 
of freshly and unevenly deposited sediments, acting mainly under 
gravitation. These readjustments may be aided, or started, by the 
action of strong currents. 
The above-mentioned structures and their attendant phenomena are 
not confined to Paleozoic sandstones,* but must occur frequently in 
1 “The Upper Keuper Sandstones of Hast Nottinghamshire ’’: GEOL. MAG., 
1910, pp. 306-7. 
2 “On the application of Quantitative Methods to the Study of Rocks’’: 
Q.J.G.S., vol. lxiv, pp. 171-233, 1908. 
* I have recently seen ball structures in the sandstone of St. Bees Head, 
Cumberland, and Dr. R. lL. Sherlock tells me that pillow-form masses of 
Magnesian Limestone overlie puckered Marl Slate in a cutting at the north end 
of Annesley Tunnel in Nottinghamshire. 
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