Geology Reference
In-Depth Information
Principle of original horizontality:
States that sediments forming sedimentary
rocks are usually laid down in a horizontal position (due to gravity). Therefore,
rock layers that appear vertical have been moved from their original, horizontal
position by some natural force (such as an earthquake).
Principle of lateral continuity:
States that when sediments are laid down, creating
sedimentary rocks, they spread out until they reach some other object that con-
fines them. This principle is illustrated when you fill your bathtub with water. The
water spreads to fill all the space, confined only by the edges of the tub. Pour that
same amount of water on the bathroom floor, and it spreads out until it hits the
bathroom walls. Sedimentary rocks, like water, continue laterally until they are
stopped by some other object.
Principle of cross-cutting relationships:
States that where one type of rock cuts
across or through another type of rock, the rock being cut is older and the rock
cutting through is younger. After all, a rock must already exist in order to be cut
through by another rock. This principle is illustrated in Figure 3-1 where rock unit
D is younger than the rocks A, B, and C that it cuts through.
Figure 3-1:
In this
sketch of rock lay-
ers, the oldest is A
and the youngest
is E.
These things take time! Hutton's hypothesis
Scottish physician James Hutton is considered the father of modern geology. While
Plutonists and Neptunists were arguing about the origin of earth's rocks, Hutton was ob-
serving the rocks in his native Scotland and thinking about the different rock types and
rock layers. He proposed theories about their relationship to one another and how cur-
rent rock formations came to be.
