462 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XXXVII. 
THE MIDDLE CHALK IN BEDFORDSHIRE AND 
HERTFORDSHIRE. 
General Description. 
The tract described in this chapter extends from Tring and 
Berkhampstead on the west to the country south of Royston 
on the east, a distance of about 30 miles. It includes a small 
portion of Bucks, for between Tring and Dunstable the boundaries 
of the three counties are very irregular. 
The Melbourn Rock forms a conspicuous and continuous feature 
all along this tract, but the width of ground occupied by the over- 
lying beds varies very considerably. Between Tring and Hitchin 
the escarpment is deeply trenched by the heads of several valleys 
which drain to the south-east, so that the higher part of the 
escarpment is divided into a succession of bold hill ranges sepa¬ 
rated by broad and deep depressions. The upper beds of the 
Middle Chalk (those which underlie the Chalk Rock) pass into 
these valleys, and run down them for long distances, because 
the general inclination of the strata is very little greater than 
the slope of the valley floors. 
Thus in the valley of the Bulbourne the Middle Chalk runs 
down as far as Box Moor ; in the valley of the Gade it runs to 
Hemel Hempstead. 
From above Dunstable it passes into the valley of the Ver, and 
extends to near Redbourn. 
Through these counties the Middle Chalk occupies a consider¬ 
able width of ground, not because it is any thicker than in Buck¬ 
inghamshire, but because the escarpment of the Chalk divides 
itself into two portions, a lower slope crowned by the Melbourn 
Rock and an upper slope crowned by the Chalk Rock and a vari¬ 
able thickness of Upper Chalk. Between these slopes there is 
generally a plateau formed by the chalk of the Rhynchonella 
Cuvieri zone and the lower part of the Terebratulina zone. 
The estimated thicknesses of the zones are as follows 
feet. 
Zone of Terebratulina gracilis , var. lata - about 160 
Zone of Rhynchonella Cuvieri (including the Melbourn 
Rock).-60 
4 - 
Up to - 220 
