CRETACEOUS ROCKS OF BRITAIN. 
478 
CHAPTER XLI. 
THE MIDDLE CHALK IN LINCOLNSHIRE. 
General Description. 
The marly band which forms the top of the Lower Chalk through¬ 
out Lincolnshire is succeeded by beds of hard greyish or yellowish 
grey chalk in well-marked courses, which are separated by thin 
layers of grey slialy marl, some of which contain loose nodules 
of hard chalk. This greyish chalk is so full of fragments of Inocera - 
mus shell that it is often quite gritty to the touch, and more or less 
perfect shells of Inoceramus mytiloides Aire very common, so that 
in the Memoir on the Geology of East Lincolnshire (wherein it was 
first described, 1887) it was indicated as the zone of Inoc. mytiloides . 
Rhynckonella Cuvieri, however, is also common, so that there is no 
need to give the zone another name. Its thickness varies from 10 
to 13 feet. There is no bed which could be regarded as specially 
representing the Melbourn Rock, but the whole seems to be a con¬ 
densed equivalent of the zone of Rh. Cuvieri . 
The overlying Terebratulina zone is much more sharply defined 
from that of Rhynch. Cuvieri than it is in more southern 
counties; for its lowest bed is a thin course of white compact 
chalk, never more than 6 inches thick, which is traversed by a 
system of nearly straight vertical joints, so that the bed breaks up 
into a set of rectangular blocks; these joints are so numerous that, 
when a weathered exposure is seen, the bed seems broken into a 
number of small blocks like columns or bricks set on end, and this 
has suggested the name of “ the columnar bed.” As it generally 
rests on a layer of shale or shaly chalk, the horizontal lines of the 
one contrast strongly with the vertical lines of the other. 
The appearance of such a section is illustrated in Eig. &, which 
was sketched in a chalk pit south of Claxby. 
Section in a Chalk-pit south of Claxby. 
Fig. 81. — Junction of the Terehratulina Zone with that of Rnyncn. 
Cuvievi south of Claxby. 
feet. 
d. White chalk with three layers of flint nodules - - 12 
c. The columnar bed, jointed vertically 0| 
b. Greyish laminated chalk - - 0| 
a. Greyish chalk without flints - -I - 8 
