There was a modest loess fall over south-eastern England in around 20,000 BP. Thats a very rough date, and there were in fact two loess falls, with a short interval in-between, which had some effect on geomorphology and life in southern England. Loess provides agricultural enhancement so farming improved over a substantial area, an admixture is almost as good as a large loess-fall. This loess would provide material for the making of bricks and for most of the nineteenth century (particularly the early parts) it provided material for the bricks to build London. Many brickpits were opened, mostly small scale enterprises but some large and very productive such as Smeed-Deans at Murston near Faversham. One significant concentration of brick making material was at Crayford in north-west Kent; here the Crayford Brickearths were used, and for many years geologists sought Pleistocene fossils; the brickpits were great sources of material for geologists and collectors.
Map from Kennard (1944) pits labelled as W Whites, N Norris, F Furners, R Rutters, S Stoneham
The bricks to make Victorian London have been discussed by Peter Hounsell in his recent book (Bricks of Victorian London. University of Hertfordshire Press Hatfield 283). This is more of a social history than a text on brick manufacture, but Crayford does feature 13 times in the index (on pages 19 30 36 39 68 83 112 132 134 135 159 160 and 161). Most writing about Crayford is by the geologists and palaeontologists; the brick pits were very popular in the nineteenth century and much visited by Geologists Association excursions; the opening of Slades Green station on the South Eastern Railway in 1900 facilitated latter day visiting; the station opened as Slades Green but the name was changed to Slade Green in 1953.
The Benchmark paper on the Crayford Brickearths is Kennard (1944)- this discussion largely revolves around this paper. The title quotation comes from here (p.122).
"Thames reds" have given place to "Flettons" Kennard demarcates the ending of brick production in Crayford and the overwhelming arrival of the Fletton machine-made bricks. Kennard collected at Crayford between 1892 and 1900 and observed the decline of the industry.
Alfred Santer Kennard 1944. The Crayford Brickearths. Proceedings of the Geologists Association 55, 121-167. The definitive map is by Kennard- see above, related to relevant OS map.
That short quote is critical. Most of the paper is given up to fossil collecting and Pleistocene geology; there is a small part on actual bricks and brickpit geography. The quote suggests that Crayford was producing bricks that fired to red-the Thames Reds, which must mean that no chalk admixtures were used, and perhaps no spanish. There appears to be no records of spanish being used in the Crayford bricks. The great source of brickearth bricks was further down the river, at Smeed Dean at Murston. These bricks fired to yellow- the famous yellow of the London Stock brick. The yellow colour was obtained by adding chalk; the other key admixture was the so-called 'spanish' which was a mixture of cinders and ashes from London's rubbish. This made firing more economic by providing an internal fuel source. As Nature obligingly did with the Flettons, from the midland clays.
Whitaker, W. 1889. The Geology of London and of parts of the Thames Valley (Explanation of Sheets 1, 2 and 7). Memoirs of the Geological Survey of England and Wales. HMSO London 556p. POD book by Forgotten Books; copy from Harvard University
p.432. Prof. Morris seems.. to have been the first to describe the great sections of Erith and Crayford.. Unfortunately, of the four sections described in this paper, the precise locality of only one, near Crayford [Stonehams] is given; though presumably the others follow on in order, northward, to Erith.
p.436. At Northend, on the southern side of Colyers Lane, a new pit has been opened [Norris pit] since the six inch ordnance map was made, a little south of the Erith pit, and just NW of the Happy Home Inn. It was carried down to a depth of from 35 to 40 feet (in 1887), being deepest on the west.
p.439. When I last saw the Crayford section [Stonehams], in the summer of 1888, the northern part had been cut back much further, even to touching the road [Howbury Lane-near Slade Green], by Manor Farm.
