EARTH, SAND-CLAY, AND GRAVEL ROADS. 37 
a natural mixed sand-clay. The other is where the roadbed consists 
of a good quality of plastic clay and sand is available for the surface. 
In either case the appearance of the roadbed material may some- 
times be very misleading under certain weather conditions, and be- 
fore deciding whether it is suitable for use in the surface its behavior 
should be observed for all weather conditions to which the surface 
will be subjected. 
For example, an earth road which appears very sandy in dry 
weather in reality may be composed of soil that will be converted 
into mud by even a very moderate rain. l This is particularly true 
of soils that contain a high percentage of. either very fine sand or 
silt. Where the roadbed is composed of such soil, no part of it should 
be incorporated in a sand-clay surface. The sand contained in a 
sand-clay surface is supposed to supply stability to the surface in wet 
seasons, and the roadbed material, to be adapted for this purpose, 
must be composed of sandy soil that will increase in stability with an 
increase in moisture content, and will not become muddy under any 
circumstances. 
On the other hand, a roadbed that in wet seasons appears to be 
composed of clay suitable for use in a sand-clay surface may crumble 
into fine dust on drying out under even very light traffic. While a 
road surface constructed by mixing sand with soil of this kind might 
be a considerable improvement over the old earth road in both wet 
and dry weather, it would be very much inferior to a surface con- 
structed by mixing together a good quality of nonslaking clay and 
sand. 
TOPSOIL. 
In the Piedmont and upper Coastal Plain regions of the South 
Atlantic States it happens often that the soil over limited areas of 
land is of such quality that a road surface made of it will possess all 
the desirable characteristics of a well-constructed sand-clay surface. 
A rather large mileage of roads has already been surfaced with soil 
of this kind and much of it is giving excellent satisfaction. Such 
roads are known locally as " topsoil roads." 
The best topsoil for road surfacing usually is found in fields which 
have been under cultivation for a number of years. The probable 
reason for this is not only that cultivation produces a more intimate 
mixture of the soil constituents, but that the repeated aeration tends 
to improve the stability of the soil by causing the oxidation of certain 
component minerals. Also, where the original soil contains a too 
high percentage of clay, cultivation may have improved its quality by 
increasing the rate at which clay is leached out. Since cultivation 
ordinarily extends to a depth of only a few inches below the ground 
surface, the layer of soil suited for use in road surfacing is, in most 
