Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 88

FIGURE 3.26
Example of a poorly sorted sedimentary material.

Scale
1 meter

Lacustrine silty clay deposit

Groundwater

Ground moraine/
lodgment till deposit

Unconformity

FIGURE 3.27
Presence of groundwater due to presence of an unconformity. (Photo by Daniel T. Rogers.)

An example of this type of porosity is presented in Figure 3.27. This figure is a photo-
graph of an excavation through a fine-grained lacustrine deposit composed of silty clay
overlying a ground moraine or lodgment till deposit of glacial origin. The unconformable
contact between the two different and distinct geologic units is represented at the floor of
the excavation. Groundwater is not encountered in the upper lacustrine deposit nor is it
encountered within the ground moraine deposit. Instead, groundwater is encountered at
the unconformable contact between the two geologic units.
This type of porosity is significant because it represents an apparent anomaly when
evaluating the porosity and permeability of the two geologic units individually. By them-
selves, the geologic units may not exhibit significant porosity, but the contact between the
two units creates enough porosity for groundwater to be present and for flow to occur.

3.5.6 Secondary Porosity

Secondary porosity refers to the porosities formed within a geological unit after the mate-
rial has been deposited (Freeze and Cherry 1979). Types of secondary porosity include