I am inclined to believe that large areas of our wet woods and the open border-lands along our streams are completely overturned to the depth of two feet or more in the course of half a century by the actions of these animals. It is not impossible, indeed, that the very fine division of the soil which-characterizes the regions inhabited by these creatures may be in good part due to their action. In this manner the creatures may have in part worked to bring about the very conditions which best serve their needs.
In open grounds, in natural prairies or grass-plains, the smaller species of ants are extremely effective agents in overturning the soils. Wherever the ground remains for some time unplowed it becomes occupied by these creatures. In the sandy soils of Eastern Massachusetts, the overturning accomplished by these creatures assumes a geological importance. For many years I have been puzzled by the fact that the glacial terraces and plains of this region were extensively covered to the depth of a foot or more by a coating of fine sand and very small pebbles, while below the depth of a foot pebbles of larger size are very numerous, and the spaces between them but imperfectly occupied with any material. It is obviously impossible to explain these conditions through the action of earth-worms, for the reason that these creatures are rarely found in soils of this description. From much observation I have become convinced that this coating of sandy material is, to a great extent, to be explained by the action of various species of ants in the forest condition by the work of the larger black ants, and in the condition of open plains by that of the smaller species.
The amount of material which these creatures bring to the surface in a single season is surprising. At several points in Eastern Massachusetts I have found the surface to contain at least one ant-hill to each square foot of area, or about forty thousand hills to the acre. This is, probably, an exceptionally great number; it will, perhaps, be safer to estimate the number at twenty thousand to the acre. The incoherent heaps of excavated matter which these creatures form are quickly washed away by the rain, or in many cases, are blown away by strong winds, and so scattered over the surface. As soon as destroyed they are, in most cases, rebuilded, the result being that a single hill is reconstructed at least half a dozen times during a season. I have estimated that the amount of material brought to the surface often exceeds three cubic inches to each square foot of surface in a single year, or about a fiftieth of an inch of the whole area each year. Thus, in the term of fifty years, the accumulation of material on the surface would amount to as much as an inch, and reckoning the soil as having an average depth of one foot, a total overturning would be accomplished in less than a thousand years. It is likely that in some cases, over considerable areas, a tolerably complete overturning is brought about in less than a quarter of this time.