SKETCH OP THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 15 
is one by no means free from objection, as frequently the rock is 
not dolomitic, and it is more often a breccia than a conglomerate. 
Round the flanks of the Mendip hills the Dolomitic Conglomerate 
is often much in evidence extending, near Priddy, up to or above 
the 800 feet contour line, and the finer grained varieties which, in 
spite of their heterogeneous character, can be readily dressed, are 
quarried for building stone. One of the best localities for the study of 
this curious deposit is along the shore between Portishead and 
Clevedon, where it is seen resting with marked unconformity on the 
Old Red Sandstone. It may be noted that, as a rule, the blocks 
in the Dolomitic Conglomerate are in the main of purely local origin, 
consisting of Carboniferous Limestone when it rests on Carboniferous 
Limestone, Old Red Sandstone when it rests on Old Red Sandstone. 
Very few fossils have been found in the Dolomitic Conglomerate, 
but the bones of the reptiles Proterosaurus and Thecodontosaurus 
were many years ago obtained during quarrying operations at Clifton. 
A glance at a geological map of the district will show what a 
large part is covered by the Keuper marls. They are to be seen in 
various railway cuttings and quarries near Bristol, as at Pylle hill, 
and the Malago brick and tile works, but are best exposed at Aust, 
where the fine coast section shows the usual division into an upper 
thinner series of Grey or Tea-Green marls, and a lower thicker series 
of Red marls. Here, too, the gypsum occurring in irregular bands, 
veins, and lumps, is very well seen, though never occurring in masses 
approaching in size those exposed in the cutting at Hallen on the 
new Filton and Avonmouth railway. The Keuper beds of the 
Bristol district are further of interest from the fact that they con- 
tain two deposits of considerable economic importance, viz. : — celestine, 
and ochre or raddle, — red and yellow oxide of iron, in an earthy state. 
The former mineral, which is or has been, worked at Yate, Bitton, 
Winford, Abbot’s Leigh, and many other places in the Bristol area, 
is exported to Germany and used in sugar refining. The ochre or 
raddle is mined at Winford and at Wick rocks near Warmley. Red 
oxide of iron w^as also formerly extensively worked in the Dolomitic 
Conglomerate to the S.E. of Priddy in the Mendips. 
During the deposition of the Triassic beds in England, important 
physiographic changes were taking place in Europe, and these were 
soon to make themselves felt in the neighbourhood of Bristol. While 
the waters of the Keuper lake were creeping up the slopes of the 
Mendip Isle and Durdham Downs, an extensive mediterranean sea 
was giving rise to contemporaneous marine strata in the region which 
now forms the Eastern Alps. There, some 2,000 feet of Rhsetic 
strata 1 follow the marine equivalents of the Keuper of Northern 
^ Among earlier papers describing the Rhaetic strata of the district are 
those of T. Wright, ‘ Q. Journ. Geol. Soc.’, XVI. (1860), p. 374 ; C. Moore, 
ibid. XVII., p. 483, and H. W. Bristow, ‘Report Brit. Ass. (Bath) Trans. 
Sect.’, p. 50. A. R. Short’s paper, ‘ Q. Journ. Geol. Soc.’, LX. (1904), 
p. 170, contains a full bibliography with references to the numerous papers 
on the Rhaetic strata of the district that have appeared in recent years. 
