Vol. 67.] PERMIAN TO THE TRIAS IN NOTTINGHAMSHIRE. 89 



Such a mass of sand, surrounded by limestone deposits, can be 

 explained by supposing it to have been formed as a sandbank at 

 the mouth of a river, while limestone (or, at a later date, fine silt) 

 was being deposited on every side. The deposition of the sand 

 ever farther eastwards as time elapsed indicates a slow rising* 

 of the land, and this might well happen towards the close of the 

 elevation of the Pennine axis ; an elevation generally regarded 

 as mainly post-Carboniferous and pre-Permian, 1 but probably not 

 completed until later. 2 



The diagram (fig. 4, p. 90) is intended to illustrate the hypothesis, 

 and also to explain the presence of an outlier of Marl in the western 

 part of Mansfield, opposite the gap in the main outcrop. It is 

 supposed that a river flowed south-eastwards from the Pennines and 

 built up a sand-bar in the shallow Permian sea some miles from 

 the shore. Limestone was deposited all round the bar on every 

 side. The slow rise of the sea-floor caused the sandbank to migrate 

 eastwards to a point where the necessary conditions of depth and 

 currents existed. As a result, it was deposited on the earlier- 

 formed limestone, and, at the same time, limestone forming behind 

 it spread over the earlier sand deposit. 



The presence of ripple-marks in the dolomitic sandstone at the 

 Chesterfield Road quarry, and the footprints of reptiles found in 

 the Rock Yalley quarry, support the view that the rock was 

 deposited as a sandbank near the land. Shipman (teste Mr. Hick- 

 Jing 3 ) was also of opinion that the sandstone was formed as a 

 sandbank. Further support is given to this theory of the re- 

 placement by sand of the Permian Marl, by the records of Crown 

 Farm Colliery, situated on Bunter Pebble Beds, a mile and a half 

 east of Rock Valley. Here, under 112 feet of Lower Mottled Sand- 

 stone, 16 feet of marly beds were met with resting upon limestone, 

 proving that the normal amount of Permian Marl is here present. 



The reptilian footprints, and the slight unconformity of the rocks 

 above, show that the sandbank reached the surface at the close of 

 the Magnesian Limestone period. After an interval, a slight 

 subsidence allowed the sandbank to be formed again ; but now marl, 

 instead of limestone, was deposited round it. 



To make this clearer, fig. 5 (p. 90) is appended. Here a, b, c 

 represent the successive positions of the sea-level, and a', b', c the 

 sand-bars formed at the corresponding times. The contemporary 

 deposits of limestone are represented by the letters a v b v c v The 

 convergence of the lines a, b, c represents the eastward tilting of the 

 sea-floor. 



The deposition of limestone on all sides of the sand-bar offers a 

 difficulty, but the difficulty is not due to the sand-bar theory, for 

 the fact remains that sandstone and limestone can be, and were, 

 deposited in contact and passing one into the other. The mode 

 of origin of the limestone is not understood, except that it is a 



1 J. J. H. Teall, 1880, p. 92; E. Wilson, 1880, o. 93. 



2 P. F. Kendall, 1902, pp. 510-13. 



3 G-. Hickling, 1906, p. 125. 



