Figure 7.6: Snow accentuates the fold exposed in these rocks in Provo Canyon, Utah. ( 34 )Once rocks are folded, they do not return to their original shape. If the rocks experience
more stress, they may undergo more folding, or even fracture. Folds often occur in groups.
There are three types of folds: monoclines, anticlines, and synclines. A monoclineis a
simple bend in the rock layers so that they are no longer horizontal but are inclined (Figure
7.7). In a monocline, the oldest rocks are at the bottom and the youngest are at the top. In
the Grand Canyon geologic column, the rocks in group 2 have been folded into a monocline.
Ananticlineis a fold that arches upward. The rocks dip away from the center of the fold
(Figure7.8).
The oldest rocks are found at the center of an anticline and the youngest ones are draped
over them at the top of the structure (Figure7.9).
When rocks arch upward to form a circular structure, that structure is called adome. If
the top of the dome is eroded off, the oldest rocks will be exposed at the center.
Asynclineis a fold that bends downward. The rocks curve down to a center (Figure7.10).
In a syncline, the youngest rocks are at the center and the oldest at the outsides (Figure
7.11).
When rocks bend downward in a circular structure, that structure is called abasin. If the
rocks are exposed, the youngest rocks will be at the center. Basins can be enormous. For
example, the Michigan Basin is centered on the state of Michigan, but extends into four
other states and a Canadian province (Figure7.12).
Folds are sometimes, but not always the cause of geographic features such as hills or valleys.