Geology Reference
In-Depth Information
Exercise 1 • Earth Materials, Geologic Time, and Geologic Processes
13
The properties of surficial materials are deter-
mined largely by grain size, sorting, and moisture
content. Generally fine-grained sediments (silts and
clays) are less suitable for construction substrates
because of loss of strength and resulting deformation
that occurs, particularly with high moisture content.
But such fine-grained materials also provide benefits
as they are more suitable for trapping subsurface pol-
lutants and restricting groundwater flow. Coarse-
grained sediment such as gravel is generally a good
substrate for construction because of its strength and
well-drained character. Gravel units often are good
groundwater reservoirs. The strength of till (an
unsorted component of the regolith) depends on the
range of grain sizes and the dominant grain size. In
glaciated regions of North America, much of the infra-
structure is built on glacial till or gravel, so under-
standing their properties is important. If clayey and
unfractured, till serves as a barrier to movement of
toxic materials and polluted groundwater. Clay min-
eral type is also important in understanding the
behavior of regolith (e.g., some clays expand more
than 50 percent when wet). More information on the
engineering aspects of the materials on bedrock is
available in engineering geology topics (e.g., Rahn,
1996; West, 1995).
the samples, their possible origin, and their geologic sig-
nificance. For example, a dry, compact mass of well-
sorted, very fine-grained sediment that can be abraded
with a knife might be a clay, formed in a lake bed, and
have low strength, slow movement of water through it,
and thus not a good aquifer (but useful for containment of
wastes, e.g., a potential landfill site). Coarser-grained
sorted sediments reflect higher energy environments. See
Table 1.5 for grain-size information.
2.
Define each of the following terms and any others
assigned by the instructor.
Regolith
Soil (engineering)
Soil (biological or soil science)
Till
3.
List four possible environments of formation or origins of
regolith. (Hint: See the introduction to the section "Regolith.")
4. Think of the types of materials that make up the regolith
and determine which material or materials would be most
useful for:
a.
a groundwater reservoir (aquifer)
To minimize the structural losses from hazards
and maximize availability of the resources in the
regolith, land-use planners use information from
two basic sources: surficial geology maps (often
known as glacial geology maps in the northern
states and Canada) and soil surveys. The surficial
maps provide information on geologic materials at
the surface, stratigraphy of the regolith, depth to
bedrock (which might range from less than lm to
more than 200m), and landforms. The soil surveys
typically focus on the upper meter of the regolith,
but also give us a clue to the substrate beneath. Soil
surveys also contain site-specific aerial photographs
marked with soil types and important tables
that contain resource and engineering limitations.
Groundwater resource maps provide additional
information on the nature of the regolith and
groundwater. By combining these information
sources in what are known as stack maps, it is possi-
ble to obtain a 3-D picture of the regolith that aids us
in planning land use. For more information on surfi-
cial materials consult Graf (1987), Easterbrook
(2003), Walker and Cohen (2006), and state soil sur-
veys (many on the Web).
b. protecting an aquifer from contamination from a gaso-
line spill on the land surface
5.
If the local surficial geology map and county soil survey
reports are available in lab or online, determine:
a.
the material and origin of the regolith beneath your
building or campus
b. the soil type beneath your building or campus. (Soil
maps are available for most counties; they contain a wealth of
data on a site, often including aerial photos.)
PART C. GEOLOGIC TIME
One of the basic concepts underlying geologic stud-
ies is that of geologic time, a term which implies an
exceedingly long duration of Earth history. Geologic
time encompasses the total age of the Earth, about
4.6 billion years, from the time of formation of the
primitive Earth to the present. The immensity of geo-
logic time is difficult to comprehend because we
must consider time spans that far exceed our com-
mon experiences.
Geologic processes acting on the Earth since its
beginning have produced tremendous changes that
are possible only because of almost unlimited time.
QUESTIONS 1, PART B4
1.
Your instructor will provide samples of sedimentary
materials for you to identify. Use information in the
description of regolith types above, and descriptions in the
sedimentary rocks section of the manual, to help identify
