Drifted L.E.C.A.

October 14 2006. The Light-weight Expanded Clay Aggregates (L.E.C.A.) found stranded on Chalkwell Beach on October 9 and sampled coming in with the tide at Westcliff the next day correspond to a sentence by G.C. Cadée in his article on Texel beaches in The Drifting Seed vol. 11 no. 2 p.5 (Sept. 2005) namely “building stones of this expanded clay are made and those much larger stones one can find also in drift on our beaches.” His report was mainly about 7 mm spheres found there, with yellowish surfaces around dark purple coloured shallow cracks. both were made by calcination of the middle London Clay Formation at 1200°C in the LECA (G.B.) pits and factory at Mill Lane, High Ongar, Essex c. 1968-1995. However they are potentially made from various clay sites around the world and I have assumed that the Ongar pits ran out of clay, at least from there. On taking both the drifted material and part of one currently being used on a building site to a builders merchant, I was informed that they are marketed as CELLON BLOCKS but not where they are currently made. They doubtless get put into the sea either by illegal flytippers, or as part of official dumping of hardcore to improve sea defenses, and then float away!! I had not noticed them before and initially assumed that Lower Greensand Cretaceous sandstone pebbles in the recently repaired seawall had already come loose. They might also be confused with concrete until picked-up and the following description is made here for beachcombers and geologists.

As seen on building sites the blocks are rectangular with a length around one foot or 30 cm (I am not sure exactly which; doubtless 30cm in the Netherlands) with easily broken edges showing a pitted, rather than actually porous, rough interior. Two colours were seen both on the beach and on house building sites at Southend-on-Sea this week. A dry inland fragment (110 by 85 by 60 mm) was light bluish grey (5B 6/1), changing on both the submerged and the emerged surfaces to dark bluish grey (5B 3/1) after about five hours in an oxygenated seawater tank. The other type is very light grey (N8) and represented by the sample taken from the sea (180 by 130 by 60 mm worn subtriangle) given a light greenish grey (5G 8/1) colour by a thin film of algae and having dark yellowish grey colour (5Y 7/1) on the less easily coated original surface of the block. The latter look very much like clay which has been cut and smeared by a spade or mechanical digger, while the rough surface of both colour varieties consists of rather square 0.5 to 0.8 mm wide pits, between thin walls of silty vitrified clay showing a few dark spots of what was once presumably pyrite. Perhaps the blue variety is a more originally pyritic clay, or more likely one processed in a more reducing environment. One of the yellowish blocks from the beach was placed in the deoxygenated seawater bucket for two days and seen to go uniformly dark bluish grey below the waterline. This stain differs from the hydrogen sulphide staining of the wood, bone and aragonite shells on Chalkwell foreshore in being entirely lost again when exposed to dry air overnight. Both varieties have 10 to 20% of the volume above the waterline and do not appear to be sinking like pumice initially does.