XXIII. 
NOTES ON THE CHALK HOCK. 
Ry John Morison, M.D., F.G.S. 
Read at Watford , 12 th April , 1889. 
The Chalk Rock is one of three hard beds which occur in the 
Chalk of this district. These beds are, from below upwards, the 
Totternhoe Stone, the Melbourne Rock, and the Chalk Rock. The 
Chalk Rock lies at the top of the Middle Chalk or Turonian Series, 
in the zone of ITo faster planus. It is a band of somewhat cream- 
coloured limestone rather rubbly in character, jointed at right 
angles to the plane of bedding, and breaking with an even or some¬ 
times slightly conchoidal fracture. It readily weathers and breaks 
down on exposure to the air. The upper surface is well defined, 
but the lower border shades off gradually into the soft chalk beneath. 
It contains numerous grains of glauconite and also a number of 
irregularly-shaped nodules slightly phosphatic in composition and 
coloured green on the outside by a coating of glauconite. Whether 
these nodules are of a coprolitic nature I cannot undertake to say. 
There are also found in the Chalk Rock many curious somewhat 
cylindrical branching cavities, which may be empty, or filled by a 
ferruginous sandy clay, or may contain nodules of iron-pyrites. 
These cavities Mr. Whitaker considers were originally occupied by 
tubular sponges. 
The Chalk Rock varies in thickness from a few inches to 10 or 
even 20 feet. In the section at Boxmoor* 1 it is 18 inches thick. In 
some places we can trace two rocky bands, one a few feet above 
the other, as in the section on the Midland Railway between Luton 
and Chiltern Green stations, while in the sections near Baldock 
three such beds may sometimes be seen, but the upper bands are 
always comparatively thin and much less defined. It is the lower 
or principal band which contains the green-coated nodules and the 
Gasteropoda and other characteristic fossils ; but it is the upper 
band which is considered to form the top of the Middle Chalk. 
Where there is more than one rocky band we find between the 
two layers from 7 to 15 feet of rubbly chalk consisting of hard 
yellowish lumps in a matrix of soft whitish marly chalk. Fossils 
are here abundant, especially Micraster cor-testudinarium , M. brevi- 
porus , Ananchytes ovatus , and Rhynchonella plieatilis. Mr. Whitaker 
calls this bed, from the abundance of Micrasters, “ the Micraster 
Red.” 
I have not seen any flints in the Chalk Rock. It is important 
to mention, however, that on one occasion I found a pebble of 
quartzite in it. 
It is the Chalk Rock which, resisting denudation more than the 
softer chalk, protects the slope of the Chalk escarpment to the 
north of our county, and is mainly instrumental in forming the 
outline of the Chiltern Hills. 
* It is here that the Chalk Rock was first observed, by Dr. John Evans, who 
pointed it out to Mr. Whitaker by whom the name was afterwards given.— Ed. 
