June 20 2006. Sand sampled from around the 85 m above mean sea level in S.E. Mill Green Common, uppermost Claygate Member on the Geological Survey maps, did not look much like that Eocene marine sand, being angular with many sharp edges at 0.08 mm diameter and mixed with only few well rounded 0.14 mm grains familiar to me from the Claygates elsewhere. In addition there were varied pebbles and even an angular cobble, with the same orange sand inside their pitting and presumably moved down the hill from the mapped area of the Old Head Gravels (which should have a clay matrix). I was, however, mainly interested in what the local pebbles looked like and made the following observations:

a) Angular flint cobble. The cobble did not look like a flint to me until I broke it open to show a conchoidal fracture of dark grey (N3) to brownish grey (5YR 4/1) colour, inside a chalky cortex of typical 1.4 mm thickness. The unbroken dimensions were 97.3 by 66.0 by 48.1 mm, with a white hard vein of chert of around 9 mm width raised above the soft cortex containing a few similarly white and raised Cretaceous burrow fillings of 3 mm diameter. The exterior of the cortex did not look like chalk, or the normally much hard cortex of Chalk flints, since it was stained to a yellowish orange colour (10YR 7/6) like the sand matrix. However, in the freshly fracture the cortex was pure white chalk (N9) and not simply a white patina of the flint mineral, which according to textbooks is a microcrystalline silicon dioxide and water mineral (Chalcedony), rather than fragmented Cretaceous microplankton (coccoliths) composed of the softer mineral calcite (one form of calcium carbonate).

b) Flint pebbles. One showed the same dark grey (N4) freshly broken interior, like the cobble and flints inside the chalk mine at Grimes Graves. Others had moderate yellow brown (10YR 5/4) to orange (10YR 6/4) colour on similarly made fractures. This internal and relatively primary colouration is masked by the variable development of a white layer around the whole surface of the somewhat flattened ellipsoids, which is evidently produced by oxidation of organic inclusions in the flint, rather than by a primary cortex of chalk being preserved on this hard exterior during transportation by rivers. In a brown flint (10 YR 5/4) which had a bluish grey (5B 6/1) exterior of 44.2 x 86.6 x 24.1 mm orthogonal diameters, the white patina had a uniform thickness of 0.10 to 0.15 mm, with an underlying grey patina extending to 2 or 3 mm into the unaltered interior. Another flint found next to it had a more mottled exterior, with orange (10YR 7/6) and blue (5B 6/1) areas, and a generally thicker patina of 0.4 mm thickness extended to 0.8 mm in pipe-like structures. The interior was moderate yellow brown (10YR 5/4) to orange (10YR 6/4). A thinner patina can therefore show the interior paler colours, if present while the thicker patina can produce a dark mottling which is not indicative of how it varies in thickness over a similarly pale interior. On the surface of all these pebbles there were irregular rounded pits, which probably result from poor silicification of the parent chalk burrows and smaller triangular pits of 0.3 to 1.2 mm width produced by impact damage and or permafrost action. Between these pits there are a variety of surface cracks, generally slightly curved and with a width of 0.03 to 0.2 mm potentially made by the grinding action of the host sand. This quartz sand is slightly harder than the flint mineral and certainly less likely to shatter on impact.

c) Lithic quartzite pebbles. One sample collected as sand yielded seven of the usual flint patinated pebbles described above from various samples and three generally smaller and facetted, highly polished pebbles of quartzite. The largest of these was more irregular than triangular in shape, but with a highly polished exterior showing impact triangle of only 0.3 to 0.5 mm width, some 0.2 mm to 5 mm wide rounded pits and abrasion grooves of less than 0.02 mm width. The orthogonal dimensions were 44.8 x 23.6 x 18.0 mm and the external colour, which might be mistaken for flint, consisted of patches of darker grey (N5) in a paler grey (N7) matrix. When fractured the pebble split easily, without the flashes of flame and dangerous shards associated with splitting the flints, into a pair of parallel transverse and slightly rough cracks of similar colour (N7 and N4). There was a slight hint of greyish orange pink (5YR 7/1) on some parts of the paler fractures, Within the darker grey areas there were paler grey to white spots of 0.06 to 0.08 mm diameter, which were not sand grains, but sections through elongated objects associated with them in random orientations. They are something of a mystery without further investigation, but might be quartz pseudomorphs of the fridymite > 870° C phase of silicon dioxide, if that contact metamorphic texture was not so rare outside the North Ireland to Scotland Tertiary basaltic province. An alternative explanation is that the darker area are of trachytic of Palaeozoic lava containing elongated inclusion of glass and crystals before they cooled down. The host quartzite showed no obvious sand grains and other smaller inclusions except these 10 mm square grey clasts and a few dark 0.06 mm spots of iron oxide.

Quite apart from the different origin and provenance of these smaller pebbles, it is clear that they differ from the associated and generally more widespread flint pebbles (i.e. the latter were also seen along Mill Green Road opposite Hardings Lane and north of Mill House with a 55 mm maximum length) in being rounded and polished into facets by wind blown sand, and not by the rough sand not present around them. Ice is also required to bring them into Essex and it is not required to explain how rounded flint pebbles could be transported from say Reading, as roughly patinated ellipsoids, from the parent Chalk or Paleocene strata by a proto-Thames. However, I suspect that the softer chalk cobble and these polished quartzites actually came from the north by ice and were then polished in a dry but cold climate before being mixed with the rough sand and patinated flints.