Notes on Folding and Geologic History
Rock deformation is closely tied to the Theory of Plate
Tectonics:
 Converging Plate Boundaries—compressional
forces
 Diverging Plate Boundaries—tensional forces
 Transform Plate Boundaries—shearing forces
 Deformation manifests itself in the form of different
folds and faults
STRUCTURAL FEATURES IN ROCK
All rock may undergo both folding and faulting
Folding=plastic deformation (Bending of Rock)
Faulting=brittle deformation (Breaking of Rock)
This week, we will examine folds:
FOLDING
Anticline—arch-like structure; older rocks near center
flanked by younger rocks
Syncline—basin-like structure; younder rocks near
center flanked by older rocks
Axial Surface—divides a fold into equal parts to form
hinge line
Refer to figure 12-2 in Lab Manual
 Upper portion of folds have been removed by
erosion at earth’s surface
see figure 12-3 in lab manual
 Use strike/dip to describe orientation of folds and
of beds
Strike=trend of rock formation with respect to
North
Dip=inclination of rock unit from the horizontal
in a direction perpendicular to strike
Plunge=folds inclined away from the horizontal;
produces U-shaped pattern at earth’s surface
See figures 12-4 and 12-5 in lab manual
RULES FOR INTERPRETING FOLDS
1) Principle of superposition: In undeformed
sedimentary rock, youngest formation will overlie
older formations
2) In deformed sedimentary rock, the older rock
formations will dip in the direction of younger rocks
3) Older rocks are exposed in the center of anticlines,
while younger rocks are exposed in the center of
synclines
4) Non-plunging folds have straight contact lines that
are paralled to each other, while plunging folds
have contact lines that curve around its axial trace.
GEOLOGIC HISTORY
All geologic time has been organized into Eras,
Periods, and Epochs in a geologic time scale: refer to the
condensed version, Table 12.2, p. 192 in lab manual.