4 R. M. Deeley—Traivl and Underplight. 
rests. The contorted nature of the Underplight is not of a character 
to suggest that the troughs are eroded hollows as Fisher supposed. 
Fig. 1 is a copy of Spurrel’s section xii, plate i. Here the surface 
flow is as shown by the arrow. There are two well-marked layers 
of Trail (T’), the upper layer of Trail resting upon an Underplight of 
rainwash (UP), whilst the lower Trail rests upon an Underplight 
of brickearth (UP). ‘he Trail has the appearance of being due to 
the sinking of beds of heavy clayey gravel and sand into lighter beds 
of sandy clay. Whilst this sinking of the upper bed was taking 
place the whole surface was flowing very slowly in the direction of 
the arrow, the upper surface moving farthest down the slope. Indeed, 
the phenomenon is one which we should expect to find in a fluid of 
high viscosity rather than in a plastic one such as clay. 
It is very generally agreed that Trail and Underplight are not 
now forming in this country, and that although beds of heavy sand 
or gravel may be now at times spread over beds of lighter clay by 
surface floods, the plastic nature of the clay prevents the heavier 
gravel from sinking into it. However, if the clay were wet and 
were alternately frozen and thawed, then for a short time after each 
thaw the clay, brickearth, silt, loam, or soft chalk would be in 
a slightly fluid condition, and any beds of superior gravity which 
rested upon them would sink into the deposit below. , 
Although some regard a plastic substance as being a highly viscous 
liquid, technically it is not. A plastic substance is a solid which, 
if not overloaded, will carry a load put upon it for an indefinite period 
without yielding. A highly viscous liquid, however, yields slowly. 
under any load, however small, and yields the more rapidly the 
greater the load (stress). Thus, if one stone be placed upon a piece 
of clay or soft plasticene, and another stone be placed upon a piece of 
hard brittle pitch, after an interval of a few months the one stone 
will be found resting as it was placed upon the clay or plasticene, 
whilst the other stone will have sunk more or less deeply into the 
brittle pitch. The pitch is a very stiff liquid and the clay is a soft 
solid. Under the conditions of climate now existing in this country 
the ground is seldom very deeply frozen, and consequently the soft 
beds of clay or brickearth are almost continuously in the plastic 
condition, and heavy material in or upon them cannot sink. But 
when the ground is annually deeply frozen and contains much 
moisture, for a short time after each thaw the thawed clay or brick- 
earth virtually becomes a liquid for a time and allows heavier 
materials in or resting upon it to sink some distance. 
It is clear that if the process of the sinking of a layer of gravel 
and sand into a lighter layer of soft clay went on long enough, the 
gravel and sand would eventually all collect beneath a layer of clay 
at the level to which the thaw penetrated each summer; indeed, 
some sections show beds of gravelly clay resting upon and covered by 
loam, clay, or brickearth, oblique streaks of small stones remaining 
in the upper deposit. 
No doubt there is generally a slow bodily movement of hills 
composed of plastic clays, ete. Such movements, however, are not 
of the nature of fluid movement (viscous flow). They. probably 
