Environmental Engineering Reference
In-Depth Information
FIGURE 7.78
Exposure of bouldery till: a matrix of sand, silt, and clay (Staten Island, New York).
is typically gravelly or bouldery; in areas of sandstones, sandy; and, in areas of gneiss,
shales, or limestones, clayey. A clayey till derived from shale is shown in
Figure 7.79.
In
the till formations of the northeast United States, lenses of sand indicating occasional flu-
vial activity within the glacier are found.
Boulders
Throughout the northeast United States, boulders are found strewn over the surface as
well as distributed throughout the till, or as concentrations or “nests” at the bottom of the
deposit at the bedrock contact. Boulders ranging up to 20 ft across, or larger, are common,
either on the surface or buried.
Classes by Mode of Deposition
Lodgement or basal tills
have been plastered down beneath the actively moving glacier to
form an extremely hard and compact mass. Seismic velocities from geophysical surveys
can be
as high as
for some rock types, making identification difficult. Basal till is at times
referred to as tillite, but the term is more correctly applied to a lithified till.
Ablation or superglacial till
has been dropped in place by stagnant, melting ice, and is a
relatively loose deposit. Where flowing water has removed finer particles the material is
coarse-granular.
