THE FOREST AS A SOIL-FORMER 47 
reaches sand containing shells. This indicates that 
the depression in which the swamp was formed was 
once an arm of the sea. It began, no doubt, as a 
salt-marsh; later a few trees, which could endure 
such conditions, found a foothold, and now it is coy- 
ered by a dense forest-growth of many species. Little 
by little the forest detritus has increased in thick- 
ness, until the swamp has reached an elevation higher 
than the surrounding land. In fact, elevated swamps 
are common in many parts of this country. A swamp 
is usually defined as a tract of land, with or without 
trees, lower than the surrounding country, and so 
saturated with water as to be unfit for cultivation. 
The term “swamp ” in the South usually refers to a 
wooded region which is often higher than the sur- 
rounding land, and, when cleared and drained, forms 
excellent farm-land. 
The bald-cypress (T’axodium distichum) is a won- 
derful tree in its ability to grow in water and mud. 
It is not preference on the part of the tree, however; 
it is endurance. It grows just as well, if not better, 
on the upland, but is able to endure these conditions 
better than other trees. Al] roots must have air, and 
this is provided in the case of the cypress by knees 
which extend upward above the surface of the water. 
In addition to forming soil by the fall of leaves and 
twigs, the roots collect sediment and flotage, and 
