420 Br. E. Greenly — 



the other hand, the same fauna suggests a limit ; for although it is 

 true that gulls fly many miles inland in winter, they would hardly 

 carry fish- vertebrae with them all that distance. The old level may, 

 accordingly, be estimated as exceeding the present level by a good 

 deal more than 20 feet, probably by nearly 50 feet ; but not probably 

 by much more than that. 



The Sources op the Alien Sands. 



This question is closely bound up with that of level. The Coal 

 Measures (Pennant) of Conygar have already (p. 363) been excluded. 

 Moreover, Mr. A. Heard has lately published (Geol. Mag., 

 February, 1922) a study of the petrology of the Pennant 

 Series. His list of its minerals, in approximate order of abun- 

 dance, is as follows : quartz,* muscovite,* coaly carbon, 

 chlorite, zircon,* pyrrhotite, siderite, sphserosiderite, pyrite, 

 rutile,* tourmaline,* cordierite, corundum, biotite,* felspar,* 

 glauconite, fluorite. Comparing this with the list given on 

 page 369, we find that, out of a total of some thirty minerals, only 

 seven (which are marked with an asterisk) are common to the two 

 formations. Moreover, muscovite, large and abundant in the 

 Pennant, is rare and small in these Pleistocene sands ; biotite, 

 rare in the Pennant, is not rare in the Pleistocene ; felspars, 

 indeterminable in the Pennant, are determinable in the Pleistocene ; 

 while certain cherts are of different characters in the two cases. 

 Zircon, rutile, and tourmaline are now known to occur in sand- 

 stones of many different ages. In fact, not a single mineral of 

 diagnostic significance is a common constituent of the two 

 formations, and it is evident that the Pennant sandstones cannot 

 have been the source of these Pleistocene sands. The narrow 

 outcrop of Old Red Sandstone is manifestly an inadequate source, 

 especially for the loams ; while, on the other hand, it would have 

 provided much more white mica ; besides which, at only a quarter 

 of a mile away, grains therefrom derived would hardly have lost 

 their characteristic haematite pellicle. 



But there is a far more probable source. While the Welsh ice- 

 sheet shrank back into the recesses of the mountains, we may be 

 sure that its melting waters distributed wide fluvio-glacial 

 accumulations. Most of these must have been levelled off and 

 redeposited by the sea since it was readmitted to the Channel by 

 the subsidence which let down the Forest-bed.^ During the retreat 

 of the ice, however, with the land standing at the level suggested, 

 the newly re-born Severn must have meandered along a 12 mile wide 

 plain of reassorted boulder-clay, fluvio-glacial gravel, sand, and 

 mud. From such a surface, under conditions of dry cold, storms 

 could easily sweep up vast quantities of dust and sand. 



^ Very likely the vast banks of mud and sand which embrown the waters 

 of the Channel for some 30 miles are part of the wreckage of them, incessantly 

 redistributed by the scour of the tides. For the Severn, Wye, and Avon are 

 not muddy rivers above tide-levels. 



