44 BULLETIN 141, U. S. DEPARTMENT OF AGRICULTURE. 
a high percentage of carbonate of lime. Analyses of numerous sam- 
ples have shown the lime carbonate content to range from 1 or 2 
per cent to as high as 20 or 25 per cent. This arises from the close 
association of the largest areas of the type with areas where the 
local limestone was first reworked into the glacial till and later rede- 
posited as a part of the glacial lake sediments which constitute such 
a large proportion of the total area of the type. 
In all areas where it occurs the Clyde clay occupies level or slightly 
saucer-shaped depressions which are usually below the level not only 
of other upland soils but even below that of other members of the 
Clyde series occurring in the same area. In fact, the Clyde clay 
represents the quiet-water deposition of the ancient glacial lakes, and 
it was formed in the central portions of the basins of the larger lakes 
and in depressions in other glacial lake sediments. In such loca- 
tions the deposition of mineral matter was not usually as great as 
nearer shore lines or stream deltas, and the deposits were finer 
grained than in the case of the materials giving the other members 
of the series. 
The surface of the Clyde clay is almost universally flat or but 
slightly inclined, and there is abundant evidence that the areas of this 
type constituted shallow lakes or at most swamps until about the 
time of the pioneer occupation of the general region. The presence 
of numerous fresh-water shells, the high organic matter content of 
the surface soil, and many historical accounts of the original aspect 
of the country all bear out this conception of the immediate origin 
of the Clyde clay. 
Not until artificial drainage was undertaken either by individuals 
or by county or State authorities was the greater part of the total 
area of the Clyde clay susceptible of agricultural occupation. It 
has only been after the opening of large main ditches, along the 
boundaries of land sections or along natural drainage ways through 
which farm drainage might find an outlet, that the land has been 
brought under even the more extensive forms of cultivation, and the 
production of intertilled crops has frequently become profitable only 
after the installation of tile drainage. It is certain that many 
thousands of acres of this soil type would be very greatly benefited 
by the extension of tile drainage. The value of this form of im- 
provement has been abundantly demonstrated by numerous cases 
where the value of the land has been quadrupled through the laying 
of tile. Usually the increased value of the land has more than paid 
for the expenditure within 5 to 10 years from the installation of 
the drains, while it has even been the case that the increased crop 
production for the same period of time has more than paid the total 
cost of drainage. It is certain that the crop adaptations of the type 
