216 
THE CRETACEOUS ROCKS OF BRITAIN 
CHAPTER XVIII. 
» 
THE LOWER CHALK IN LINCOLNSHIRE. 
General Description. 
The Lower Chalk of Lincolnshire presents a succession of beds 
which is similar to that of the northern part of Norfolk, hut as a 
whole it is thicker, being from 75 to 80 feet thick as compared 
with the 55 feet of North Norfolk, and this thickening is princi¬ 
pally in the lower part or zone of Am. varians. 
The Cretaceous escarpment is the dominant feature in East Lin¬ 
colnshire, and the Wolds form a tableland trenched by the valleys 
of numerous streams, which all drain eastward into the North 
Sea. No great thickness of Middle or Upper Chalk is present, so 
that most of these valleys cut into the Lower Chalk, and some of 
them expose the “ Red Chalk ” (Selbornian) and underlying strata. 
In consequence of this structure, there are numerous exposures of 
the Lower Chalk, not only along the main escarpment, but in 
the valleys which intersect the Wolds. 
The Chalk emerges from beneath the Drift-covered country 
between Gun by and Welton, not far from the Burgh Station on 
the East Lincolnshire Railway, and strikes thence to the north¬ 
west through Sheets 84, 83, and 86 of the Geological Survey Map. 
Fig. 51 is a tabular view of the vertical succession of the beds 
forming the Lower Chalk, with the underlying Red Chalk (Sel¬ 
bornian), as developed in the country round South Thoresby, Louth, 
and Withcall. It is only in this area that the portions indicated 
are coloured pink ; the succession is the same both to the south and 
to the north of this district, but the whole of the beds above the 
“ grey bed ” are greyish or yellowish-white in other parts of the 
county. 
The Zone of Ammonites varians is about 34 feet thick at the 
south end of the Lincolnshire Wolds, and rather more near 
Louth. It has two well-marked beds at the bottom which clearly 
correspond with those described under the names of “ Sponge Bed \\ 
and “ Inoceramus Bed ” at Hunstanton. Both, however, are 
thicker in Lincolnshire. 
The “ Sponge Bed ” is a hard compact limestone, either white 
or yellowish-white, and sometimes partly pink in colour. It is a 
very pure limestone, and is entirely devoid of quartz or glauconite 
grains. Its thickness varies from 18 inches to 3 or 4 feet, and 
sometimes it is difficult to separate it from the Red Rock below. 
