Teaching Earth Sciences - Earth Science Teachers' Association

The foregoing is a summary of terms that can be used to
describe and record folds. Once students are familiar with
the ways in which folds can vary, it is possible to consider
their interpretation in terms of mechanisms of formation.
In this article, I would just like to deal with the topical
problem of explaining the development of symmetric and
asymmetric folds, which the OCR AS/A2 Geology Author
Team seem to suggest is too complicated to be explained to
A level students, without employing a fallacy. The following
is not a full explanation, and will leave the able student
asking more questions, but it takes a step in the direction
of the truth and will not have to be unlearnt if the student
progresses to a higher level of study.
In each of the following examples, only the maximum
principal stress axis (σ max
) is shown. In the first scenario,
the three layers have the same competence (ductility) and
lie at right angles to the axis of compression. The layers all
get thinner, and extend in a direction at right angles to this
axis, however, no folds develop (Figure 13a).
In the second case, the axis of compression lies parallel to
the layers, but once again, all three layers are ductile, and
have identical physical properties. Consequently, they all
behave in exactly the same way: thickening and shortening
without folds developing (Figure 13b).
If a layer cannot accommodate the shortening by a
consequent thickening, it will buckle. In the third situation,
the axis of compression is parallel to the layers, and leads to
the development of symmetric folds Figure 13c).
However, if the axis of compression is oblique to the
layering, as in the fourth case, the resulting folds will be
asymmetric (13d).
Figure 12 The locations of crests, troughs and hinges in an overturned antiform and
corresponding synform.
In summary: if the axis of compression is more or less
perpendicular to the layers, shortening will not produce
folds. If it is parallel to the layers, shortening generates
symmetric folds. If it is oblique to the layers, the result will
be asymmetric folds. At A level, I do not think it needs to
be any more difficult than that. Nevertheless, this limited
explanation can be used to explain why the two limbs of a
developing fold may develop parasitic folds with opposite
senses of asymmetry: the erroneous description of the
development of asymmetry offered by the OCR team offers
no help in explaining the phenomenon.
If an individual teacher conveys a misconception to a class,
it disadvantages a few individuals. When an author does
the same, more students are exposed to the error, but the
teacher can overcome it through lectures or handouts.
However, when examiners publish misconceptions in
textbooks, specifications and mark schemes, teachers
are faced with a dilemma: do they teach the untruths,
knowing that they will be rewarded in the exam, or do they
teach the truth and run the risk of their students being
penalised when the examiners’ mark scheme offers no
credit for the correct responses? Ultimately it is up to all
teachers of Geology to feedback to the examiners through
formal channels whenever errors are identified. Equally it
is the professional responsibility of examination boards to
respond formally in such a way that all teachers are aware
of the arguments and resolutions, so that no candidates are
disadvantaged.
Figure 13 The ways in which layered sequences respond to compression are
controlled by many variables. The development of asymmetric folds can be
explained very simply in terms of the axis of compression being oblique to the
deforming layers.
Alan Richardson
as.richardson@virgin.net
www.esta-uk.net Vol 35 No 1 2010 Teaching Earth Sciences 55