386 LAMPLUGH : EARTH-MOVEMENT IN NORTH-EAST YORKSHIRE. 
be deduced from the relations of the three planes to each other in passing 
across the district. 
At the top of the Lower Oolites, I think that we may take the base 
of the Cornbrash with fair confidence as having been originally hori- 
zontal. The position and persistency of the Cornbrash at the top of 
the Estuarine series leave no doubt that it represents a wide en- 
croachment of the sea over the estuarine or fluviatile flats ; and its 
relation to the overlying sandy Kellaways Rock points to the per- 
sistence of shallow and uniform conditions on the sea-floor of the period. 
In the Middle Oolites, the Oxford Clay affords little or no indi- 
cation of relative levels, but the Corallian series unmistakably denotes 
shallow-water conditions. Perhaps the best criterion in this series 
is the horizon of the coral-reefs — the Upper Corallian Umestone and 
Coral Rag — which must have been everywhere ciose to the sea-level 
of the period ; and although the reefs may not have been strictly 
contemporaneous throughout the district, the earth-movements 
within the narrow time-Hmits of the reef-formation were too small 
to be appreciable in the present discussion.* 
The remainder of the Jurassic sequence in Yorkshire is repre- 
sented by clays — the Kimmeridge Clay and the doubtful Portlandian 
of Speeton — which were deposited in seas of some depth and probably 
on a shelving floor, so that they contain no plane on which we can 
depend. They are, however, shaved off southward into a wedge, in 
common with all the Oolites, by the great unconformity of the Upper 
Cretaceous rocks. 
The conglomeratic Red Chalk under the Western Wolds, which 
passes unconformably across well-nigh the whole of the Jurassic 
sequence in approaching the crest of the pre-Upper Cretaceous anticline 
near Market Weighton, was probably at first nearly, if not quite, 
horizontal ; but although so thin, the accumulation of this band 
appears to have covered a long period of time, and may not have been 
exactly contemporaneous in all parts of the district. We cannot tell 
what thickness of the Kimmeridge Clay was deposited over the site 
of the anticline, where it is now absent ; but from the indications 
obtainable where the formation sets in again on the southern side of 
* For purposes of calculation of thickness it is convenient to include 
the Upper Calcareous Grit with the other Corallian rocks, as I have 
done in the summary, p. 393 ; but the plane shown in the diagram 
(Fig. 1) is drawn at the top of the lunestones. 
