By the employment of suitable sampling and analytical methods a marked degree of vertical variability is shown to exist in two exposures of Oxford Clay. Very little of this variation was apparent on visual examination of the strata. Some lateral variation exists between the two exposures, which are thirty miles apart, but this is on a relatively minor scale.

Trends for the vertical variation have been determined and these are in the main common to both areas. The lowest beds are high in organic content and low in acid-soluble lime, whilst higher up in the succession the amount of organic material diminishes and the percentage of lime increases. When examined in greater detail these trends are found to be more complex: they are the resultant of innumerable small scale sedimentary cycles. Thermal methods of analysis have shown that the organic material present in the lowest sixty feet or thereabouts of the Buckinghamshire succession, differs considerably in its ignition range from that found in the next twenty feet. In some respects the characteristics of the higher-level combustible resemble those of lignite. D.t.a. examination suggests that the clay mineral illite is dominant in all layers of the succession in Buckinghamshire. Some montmorillonite may be present in certain of the higher zones either as a simple impurity or in the form of an interlayering with the illite.

The effects of some aspects of these variations on the processes of Fletton brickmaking are discussed.