What are geological maps?
A geological map shows the
distribution of different types of rock in an area. It usually consists of a topographic
map (a map that gives information about the shape of the earth's surface) that
is shaded or colored to indicate where various rocks are located. Units occur
on or just below the surface of the earth. Figure 1.1 shows a geological map of
an area in the Cotswolds. It tells us, for example, that clays form the bedrock
at Childswickham and Broadway, but if we move east up the Cotswold Escarpment
up to Broadway Hill we can find oolithic limestones. Lines are drawn on the map
to show the boundaries between each rock unit.
How do you create such a
geological map?
The geologist on site first
records the nature of the rock where it is visible on the surface. Rock
outcrops are examined and features such as rock composition, internal structure
and fossil content are recorded. They can be distinguished and shown separately
on the basemap. The rocks are of course notexposed everywhere on the surface.
In fact, much of the area in Fig. 1 is covered by soil and alluvial debris
deposited by younger rivers. Soil type, land surface shapes (geomorphology) and
borehole information. Enable geophysical methods the properties of rocks (such
as their magnetism and density) need to be measured remotely and are therefore
useful for mapping rocks in poorly exposed regions. This additional information
is taken into account when the geologist determines the position of the
boundaries of the rock units to be drawn on the map. However, there are always
parts of the map where there is more uncertainty about the nature of the
bedrock, and it is important that the card reader realize that there is a lot
of interpretation used in the mapping process.
What
is a geological map used for?
The
maximum apparent use of a geological map is to signify the nature of the
near-floor bedrock. This is simply of great significance to civil engineers
who, for example, have to suggest at the excavation of avenue cuttings or at
the siting of bridges; to geographers reading using land and to agencies
exploiting minerals. The skilled geologist can, however, extract extra from the
geological map. To the skilled observer the capabilities on a geological map
reveal crucial clues approximately the geological records of an area.
Furthermore, the bands of coloration on a geological map are the expression at
the floor of layers or sheets of rock which expand and slant downwards into the
crust of the earth. The regularly complex sample on a map, like the graininess
of a refined timber desk top, affords tell-tale proof of the shape of the
layers below the floor. To make those deductions first calls for know-how of
the feature shape of not unusual place geological systems such as faults and
folds.