Essex erratic boulders: a gazetteer
it has been cut and polished for use in jewellery and other decorative items since at least the early 18th
century. In 1748, a great deal of puddingstone, including polished puddingstone snuffboxes, was
reported to be in the possession of Sir Hans Sloane, whose huge accumulation of natural and man-
made specimens formed the founding collection of the British Museum (MacGregor 1994).
Sarsens are effectively puddingstone without the pebbles but they are distinctly different in other
ways. The sarsens of Thurrock, for example, are undamaged by glacial or river erosion and display
remarkable mammilated growth surfaces formed as the sand grains were slowly cemented by silica.
An alternative name for sarsens is 'grey wethers', a name that originated in Wiltshire and is derived
from the resemblance of these stones to a flock of sheep, when they are numerous and seen from a
distance.
A rock that has been confused with puddingstone is ferricrete, an iron-cemented flint gravel, but
the two rocks are easy to tell apart. In ferricrete the flint pebbles are cemented together by iron-
stained sand whereas puddingstone is a much harder and more attractive rock whereby the sand
between the rounded pebbles has been completely silicified, i.e. turned to a solid mass of silica
(quartz) w ith no visible sand remaining (Curry 2000). These rocks are fonned from the evaporation
of groundwater that contains dissolved iron compounds or silica but if the groundwater is rich in
calcium carbonate a rock known as calcrete is formed. Ferricrete and calcrete boulders are true
Essex rocks as they have usually been quarried close to where they are found. They are therefore
not erratics but they are included in this gazetteer for completeness.
Other rock types represented are limestone and sandstone from the north of England, particularly
Carboniferous Limestone and Millstone Grit, but limestone boulders are difficult to examine as
they arc usually coated with moss and lichen. Septarian nodules from the Oxford Clay of the
Midlands are not uncommon but they contain cracks, or septa, which means that they break up
easily on exposure to frost. Igneous rocks such as fine-grained dolerite and basalt are present, the
dolerite probably originating from Northumberland. Very weathered dolerites and basalts can be
easily mistaken for sandstone. The rarest erratics of all are the coarse-grained plutonic igneous
rocks such as granite and syenite and the metamorphic rock gneiss, all of which probably originate
in Scotland.
This paper is concerned with the large erratic boulders but brief mention should be made of the
smaller erratics used in the construction of historic churches, walls and pavements. Apart from
flint, winch is present across the whole of Essex, the most common erratics used for this purpose
are the extremely hard, rounded cobbles of quartzite known as 'hunters'. Usually no more than
30 centimetres in diameter they are derived from the Triassic Kidderminster Formation of the
Midlands and are common in the Kesgrave gravels.
The Distribution of Essex Boulders
As expected, the majority of boulders are to be found in the north and west of Essex (the districts
of Braintree, Chelmsford, Epping Forest and Uttlesford) (Fig. 1) - the areas with a surface geology
consisting mostly of Kesgrave gravels and till. No attempt lias been made here to analyse further
the distribution of different rock types across Essex. This would no doubt be of limited validity
because the survival of a boulder depends on its composition. Basalt, for example, is under-
reprcsentcd in the list because it usually occurs in the till only in small boulders which arc easily
lost. This probably explains why a large number of previously recorded basalt boulders cannot
now be found.
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Essex Naturalist (New Series) 20 (2003)