In studies of particle-size distribution of the less than 20-mm fraction of glacial clays in eastern Durham it is noted that one particular characteristic seems common to them all. When the particle-size data obtained by sieving and hydrometer analysis are plotted on semi-logarithmic graph paper (BRITISH STANDARDS, 1961) it is seen that almost all the curves are made up of two shallow concave or straight sections separated by a small section of varying convexity. What is perhaps most surprising is that this convexity or marked break of slope is nearly always found in the range 0.2–0.8 mm. The break of slope shows that the material in the finer grain sizes is more abundant than the coarser material.
The question arises as to what is the cause of this break of slope and whether it is a sorting characteristic of sub-glacial processes or is explicable in terms of parent-material lithology.