Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/301

 The north-east side of it is steep, and has at its base a large chalk pit, the top of which displays the Reading oyster bed one foot thick between the chalk and incumbent ash coloured Woolwich sand.

The plain at the summit of this bank is Boston Heath, where a well has recently been sunk about 200 feet; through gravel 65 feet, sandy beds 65, chalk 70. The water stands five feet deep in the chalk. I could get no accurate detail of the sinkings, but learnt that in descending they came to water far above the chalk though not in quantity sufficient to supply the well. The upper gravel in this well, and in the shafts at Plumsted, appears to be alluvial, though like that at Blackheath composed almost wholly of pebbles of rolled chalk flints, such as the neighbouring strata of the plastic clay formation contain abundantly, and from which they were probably derived.

The thickness of the alluvium in this district is exceedingly irregular, swelling suddenly, and as suddenly disappearing. It covers, however nearly the whole surface of the under table land extending from Blackheath to Plumsted Common and Boston Heath, and is found also on the upper table of the summit of Shooter's Hill, as well as on many parts of the slope of its sides. The slopes that fall from the under table to the valley of the Thames are so frequently and so completely covered by this alluvium that except in places where they are laid open by artificial sections, it is difficult to discover the existence of any strata of the plastic clay formation. A striking example of this fact may be seen in the Park at Greenwich, where, nearly all traces of the subjacent beds are concealed by a mass of alluvium along the steep slope where we might expect to see them exposed, and where there can be no doubt of their existence from the strength and regularity in which they appear at Charlton and Woolwich on the east, and near Deptford on the west of Greenwich Park along the continuation of the same escarpment.