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Structural Geology Lab 6: Analysis of Folds in Outcrop
I. Lab Exercise on Fold Description and Geometry.
A. Review of Fold Terms
Hinge/ hinge line inflection point line of inflection median surface
crest/trought crestal /troughal trace cylindrical noncylindrical
fold axis symmetric/ asymmetric axial surface axial plane
axial trace synform/antiform anticline/syncline overturned
vertical fold reclined fold recumbent fold interlimb angle
parallel fold similar folds dip isogons Ramsay's Classif.
B. Drawing dip isogons
1. dip isogon: lines connecting points of equal dip between bedding planes in
a fold
2. Method 1 using protractor
3. Method 2 using two see-through drawing triangles
a. pick point on bedding plane
b. use triangle to define line tangent to point
c. slide triangle paralle itself til it touches tangent to next bedding
plane
d. make point on bedding plane 2, connect two points to form dip
isogon
e. repeat numerous times limb to hinge to limb of fold to derive set of
dip isogons
f. compare to Ramsay's visual classification system and identify
C. Looking at Folds in Cross-section
1. profile plane: best drawn perpendicular to fold axis, perpendicular to
strike of beds... i.e. looking parallel to fold axis
2. Using map patterns to see fold shape in x-sectional view
a. using strike and dip symbols, determine the type of fold being
examined in map view
b. determine if fold is plunging or nonplunging, use law of v's for
outcrop pattern
(1) determine plunge direction
c. Examine fold map pattern with an acute, oblique line of sight,
looking in a down plunge direction along the hinge trace of the
fold
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