Environmental Engineering Reference
In-Depth Information
FIGURE 7.79
Driven tube sample in clayey till from a
depth of 2.1 m (Linden, New Jersey).
Tills of North America
Four stages of glaciation and their associated till deposits have been identified in North
America. The exposed limits of the various stages are shown approximately in
Figure 7.76.
A block diagram of the stratigraphy of the last three stages representative of locations in
the midwest United States is given in
Figure 7.80.
Nebraskan
, the oldest till, has not been found clearly exposed, but underlies most of the
area mapped as Kansan. It is a thick sheet of drift spread over an irregularly eroded rock
surface and averages more than 100 ft in thickness. Gumbotil, a dark, sticky, clayey soil,
averaging about 8 ft in thickness, has resulted from the weathering of the till during the
interglacial age. Scattered deposits of peat have been found sandwiched between the
Nebraskan and Kansan tills.
Kansan
till is exposed over a large area of northern Missouri, northeastern Kansas,
Nebraska, and Iowa, averaging about 50 ft in thickness. It has been encountered east of the
Mississippi beneath the younger drift sheets. Its surface also weathered to form gumbotil
averaging about 12 ft in thickness, and peat deposits were formed on the surface during
the following interglacial age. The peat and gumbotil were subsequently covered with a
moderately thick deposit of loess over much of the area.
Illinoian till
, exposed mainly in Illinois, southern Indiana, and central Ohio, and in small
areas of Wisconsin, Pennsylvania, and New Jersey, is composed chiefly of silts and clays.
Its weathered zone of gumbotil averages 3 ft in thickness, and its surface contains numer-
ous deposits of peat and areas of stratified sand and gravel. In the Midwest it is covered
by a loess sheet.
