The tufa (fig. 23) and travertine occurring as surficial deposits in
Illinois were formed by springs. The deposits usually occur at or near
the outcrop of a layer of porous water-bearing earth material, such as
gravel, sandstone, or fissured limestone, that is underlain by a
nonporous clay or shale formation. Water moving down through the porous
layer cannot sink through the clay or shale and so is forced to move
laterally. Where valleys have cut into the layers of gravel or rock, the
water emerges as springs.
If the material through which the water has passed is limestone, or
gravel containing limestone as most Illinois gravels do, some of the
limestone is likely to have dissolved in the water. When the water
issues as a spring, conditions may be such as to cause precipitation of
the dissolved limestone as tufa or, more rarely, travertine.
Tufa is generally highly porous and more or less impure, whereas
travertine is harder and less porous.
The middle or lower slopes of bluffs are common sites for
spring-deposited tufa or travertine. Deposits have been seen in various
parts of Illinois, but are not known to have been worked, except in
Calhoun County where small quantities of tufa were produced for use as
agricultural limestone. It is thought that the tufa and travertine
deposits of Illinois are relatively small, but they may be of local
importance.
[Illustration: Figure 23—Calcareous tufa from Pike County. In
addition to the large visible pores there are numerous tiny ones.]