GEOLOGY 
sufficient depth to prove that it thus thinned out, his ‘ Geological Inquiry 
respecting the Water-bearing Strata or the Country around London’ is 
a classic of geological literature replete with valuable information. 
Near Leighton the Lower Greensand consists for the most part of 
white and light-coloured sand known as ‘silver sand,’ which has an 
industrial value.’ It is now chiefly obtained for filter-beds, but it is in 
places so free from iron and other colouring matter that it has been used 
for glass-making. The larger quartz grains show signs of attrition, being 
rounded and polished, like the sands of the sea-shore. 
At Leighton and near Silsoe a bed of dark-brown ferruginous sand- 
stone is exposed which is sufficiently indurated to be of service for 
building purposes, several churches in this part of the county being 
built of it, but it weathers rather rapidly and very unevenly. This 
‘ carstone,’ as it has been called, is a local feature which has been stated 
to be dependent on the presence beneath the sand of beds of clay which 
have arrested the percolation of ferruginous water.’ Opposite the Castle 
Hill near Clophill there is a very instructive section showing 10 feet of 
dark-coloured clay, called by the workmen ‘black clay’ to distinguish it 
from the ‘blue clay’ of the Gault, occurring in three distinct beds of 
about equal thickness with thin layers of sand between them, dark-red 
carstone being above the clay and light-coloured sand beneath it; but 
elsewhere the carstone is frequently seen resting on light-coloured sand 
with no trace of clay. The sand is, wherever exposed, seen to be 
partially or wholly false-bedded, showing that it was deposited in a 
shallow sea with shifting currents. The false-bedding is also evident 
in the carstone whether it occurs in a continuous layer or in isolated 
masses which are called ‘ doggers.’ Concretions of brown iron-oxide are 
of frequent occurrence. 
At the base of the series near Woburn, and at a rather higher 
horizon near Potton, there is a peculiar bed of variable thickness (6 inches 
to 2 feet) consisting for the most part of pebbles with water-worn fossils 
derived from a distant source, and containing also numerous (so-called) 
‘coprolites.’ The pebbles are of quartz, quartzite, limestone, ironstone, 
slate, etc., and they are sometimes cemented into a hard rock by car- 
bonate of lime. Sub-angular fragments of rock also occur. Most of 
the fossils are derived from older beds, the majority of Upper Jurassic 
and some of Lower Cretaceous age. With these may sometimes be 
found fossils proper to the formation, that is which lived in Vectian 
times. The indigenous fossils are chiefly brachiopods and lamellibranchs ; 
the derived fossils, teeth and bones of saurians and fishes. At Mill- 
brook have been found remains of the saurians Ichthyosaurus, Plestosaurus, 
and Dakosaurus, and of the fishes Spherodus, Pycnodus, and Acrodus. 
Most of these also occur in the neighbourhood of Potton ; and in 
addition a previously undescribed brachiopod, Terebratula dallasn, and 
1 Analyses of this sand from Heath near Leighton are given in E. W. Lewis’s Lectures on the Geology 
of Leighton Buzzard, p. 61 (1872). 
2 H. B. Woodward, Geohgy of England and Wales, ed. 2, p. 379 (1887). 
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