This is alongside the west wall of St Clement’s Church, next to the mass grave of sixteen cadets and an officer drowned on 30th August 1915 – see https://www.geograph.org.uk/photo/6570988 It is of the lozenge variety, more common in Kent, but it is huge and the largest I have ever seen.
TQ5977 : Very large grave in St Clement’s Church, West Thurrock
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