publication of Hodge's book many more have been discovered such as the 60 million
year old 'Silverpit crater', a 3 kilometre diameter structure preserved 1,000 metres
beneath the floor of the North Sea. Although disputed by some geologists, this was
claimed to be the UK's first impact crater when the discovery was announced with much
media publicity in 2002. But what about impact structures on the British mainland?
Apparently, the statistics of cratering rates worldwide imply that an area the size of
Britain should have suffered numerous impacts. For example it is estimated that at least
two and possibly as many as seven impact structures with a diameter of more than 20
kilometres could have been formed in the British Isles over the last 600 million years.
Some structures will be deeply buried and many destroyed by erosion but not all. So
where are they?
The first book to attempt to comprehensively answer this question has recently been
published. 'Bombarded Britain: A Search for British Impact Structures' by Richard
Stratford (Imperial College Press 2004) describes the author's search for geological
evidence of these structures and claims to have found several, particularly in the Midlands,
where the geology is more favourable to their preservation. Among the possible impact
structures there is one in Essex, a nearly circular topographic basin about 6 kilometres
in diameter centred near Rochford and described as the 'Rochford Basin'.
The author describes not only the shape and drainage of the basin as evidence of an
impact but also the uplift of the Lower London Tertiaries and the Chalk beneath the area
as shown on the geological section on the 1:50,000 scale geological map. However, the
main evidence for the claim is the presence, near the centre of the basin, of a very small
'inlier of Chalk' on the north shore of the River Roach (TQ 893902), 600 metres east of
Stambridge Mills. This is inconsistent with the accepted geology of the area which
requires the Chalk at this point to be at a depth of at least 110 metres (clearly shown at
this depth in nearby well records). The author accepts the difficulty of explaining this
anomaly but says that the Chalk could represent part of the central uplift of the crater
which he suggests may have been formed in the Late Oligocene or Early Miocene (about
20 to 25 million years ago).
The Chalk 'outcrop' is certainly curious. It is a stratified mixture of chalk rubble, clay
and flint pebbles only about 15 metres by 20 metres in size and forming a low cliff, the
top of which is about 1 metre above high tide mark. It is exciting to think that this could
be evidence of the immense upheaval caused by a giant meteorite impact but close
inspection of the exposure reveals that there may be a more mundane explanation. Firstly,
the chalk appears to be contaminated by charcoal and pieces of recent animal bone and
secondly it is overlain by the remains an old concrete foundation. The 1:2,500 Ordnance
Survey Map of 1923 shows a former wharf ('Broomhills Wharf) at this exact point
which strongly indicates that the chalk simply originated as a consignment of agricultural
chalk brought up the river by barges and used to make the foundations of the wharf.
Essex Field Club Newsletter No. 47, May 2005
11