18 BULLETIN 463, U. S. DEPARTMENT OF AGRICULTURE. 
roads. Such roads sometimes are improved temporarily by mixing 
sawdust, straw, pine leaves, or other similar material into the sur- 
face, but, in general, the sand-clay crust is the most satisfactory im- 
provement for roads of this character. This is true especially where 
clay may be obtained readily and only limited funds are available 
for the improvement. 
LOAM. 
Loam is a soil composed of clay and sand, mixed with a consider- 
able percentage of finely divided vegetable matter or humus. The 
quality of loam from the standpoint of road building depends very 
largely upon the proportions in which sand and clay are present and 
on the character of these materials. Loam that contains about 60 per 
cent of moderately coarse sand and from 20 to 30 per cent of good 
cementing clay usually will make an excellent road surface for light 
traffic. Material of this kind is drained easily and is fairly stable, 
even when wet. Another advantage is that it will not become very 
dusty under traffic in dry weather and frequently will cement together 
into a very hard, compact surface. Roads surfaced with such ma- 
terial are commonly called " topsoil " roads, because the topsoil of 
cultivated fields possesses the desired characteristics more frequently 
than that to be obtained from any other source. Topsoil is considered 
further in this bulletin in connection with the discussion of sand-clay 
roads. 
GRAVEL. 
Gravel is made up of small rounded particles of stone which occur 
in nature and are sufficiently large to be retained on a ^-inch mesh 
screen. Gravelly soils are distributed widely in the United States, 
and vary widely in quality. In general, when a soil contains as much 
as 40 or 50 per cent of gravel and sufficient clay or other cementing 
material to bond the gravel particles together, it proves a very satis- 
factory material for construction of roads, because it is drained easily 
and is very stable when compacted. The various characteristics of 
gravel are considered at much greater length in the discussion of 
gravel roads. 
Marl is clay containing a relatively large percentage of carbonate 
of lime. It grades into calcareous clay and argillaceous limestone, 
and its value for road building is variable. In general, this material 
has few advantages over the best varieties of ordinary clay for use 
in constructing a road, unless the percentage of lime carbonate is 
sufficiently high to give it, when compacted, approximately the char- 
acter of limestone. The best varieties of marl become very hard 
