Page 298 - Laboratory Manual in Physical Geology

HOW TO CONSTRUCT A GEOLOGIC CROSS SECTION

Geologic
map 45 45 D Sh Devonian shale
S Ss Silurian sandstone
A
O Ordovician limestone
N Ls
O
Ss Ordovician sandstone

Step 1 (red): Project contact lines onto the
topographic profile (red dashes).
Topographic profile

Step 2 (green): The sandstone dips 45°E in the
eastern half of the map and 45°W on the
west side of the map. So from the point
where its contacts occur on the topographic
45° 45° profile, use a protractor and ruler to project
the contacts into the subsurface.

Step 3 (black): Project the other contacts into the
subsurface parallel to the contact lines
drawn in step 2, and color the formations if
desired.

Step 4 (blue): Use dashed lines to project the
geologic contacts above the land to visualize
the part of the structure that has been
eroded.

FIGURE 10.6 Geologic cross section construction. Follow the four steps in the illustration to construct a cross section.

The fold axis may not be horizontal, but rather If a fold is tilted so that one limb is upside down, then
it may plunge into the ground. This is called a the entire fold is called an overturned fold ( FIGURE 10.8H ).

plunging fold ( FIGURE 10.8C , D ). Plunge is the angle Monoclines have two axial planes that separate two nearly

between the fold axis and horizontal. The trend of the horizontal limbs from a single, more steeply inclined limb
plunge is the bearing (compass direction), measured in ( FIGURE 10.8G ).

the direction that the axis is inclined downward. You can Domes and basins ( FIGURE 10.8E , F ) are large,

also think of the trend of a plunging fold as the direction somewhat circular structures formed when strata are
a marble would roll if it were rolled down the plunging warped upward, like an upside-down bowl (dome) or
axis of the fold. downward, like a bowl (basin). Strata are oldest at the
center of a dome, and youngest at the center of a basin.

268 ■ L ABOR ATORY 10