re referred to as clay, gumbo,
sandy or caliche roads as local custom may elect. In each case,
however, the wearing surface consists of the natural soil, which may
have been shaped and smoothed for traffic or may be in its natural
state except for a trackway formed by the vehicles that have used it.

=Variations in Soils.=--The nature of the existing soil will obviously
determine the serviceability and physical characteristics of the road
surface it affords. That is to say that even under the most favorable
conditions some earth roads will be much more serviceable than others,
due to the better stability of the natural soil. Some soils are dense
and somewhat tough when dry and therefore resist to a degree the
tendency of vehicles to grind away the particles and dissipate them in
the form of dust. Such soils retain a reasonably smooth trackway in
dry weather even when subjected to considerable traffic. Other soils
do not possess the inherent tenacity and stability to enable them to
resist the action of wheels and consequently grind away rapidly. Roads
on such soils become very dusty. These are the extremes and between
them are many types of soils or mixtures of soils possessing varying
degrees of stability, and, in consequence, differing rates of wear.
Similarly the various soils exhibit different degrees of stability
when wet.

It is to be expected that soils will differ with the geographical
location, for it is well known that there is a great variation in
soils in the various parts of the world. But wide differences are also
encountered in the soil on roads very near each other and even on
successive stretches of the same road. It is for this reason that
earth roads often exhibit great differences in serviceability even in
a restricted area.

=Variation in Rainfall.=--The stability of a soil and its ability to
support the weight of vehicles varies greatly with the amount of water
in the soil. A certain small amount of moisture in the soil is
beneficial in that practically every soil compacts more readily when
moist than when dry because the moisture aids in binding together the
particles. But most soils also become unstable when the amount of
water present is in excess of that small amount referred to above and
the stability decreases very rapidly as the amount of water in the
soil increases.

The serviceability of an earth road will change continually as the
moisture content of the soil changes and consequently