Video Transcript
Which of the following is a secondary geological structure? (A) Cross-bedding, (B) ripple marks, (C) mud cracks, (D) a fold, (E) graded
bedding.
Geological structures can be broadly classified as primary and secondary. Primary geological structures are nontectonic structures that form prior to or during
the deposition of a rock. Secondary geological structures are structures that are formed after the deposition
of a rock in response to applied tectonic forces. Let’s examine each of the answer choices to identify which is a secondary geological
structure.
Cross-bedding forms when sand grains are transported by wind, rivers, or marine
currents. The sand grains pile up and they avalanche down the side of the pile. Repeating avalanches form thin cross beds that are inclined to the bedding plane. As cross-bedding is a nontectonic structure, it is an example of a primary geological
structure and is not the answer to the question.
Ripple marks are formed when sediment grains are transported by water currents or
wind in environments such as coastal shorelines and shallow seas. The sediment grains form structures with peaks and troughs, which can become
lithified. Like cross-bedding, ripple marks are nontectonic structures and are therefore not
secondary geological structures.
Mud cracks form in hot, clay-rich, depositional environments. When water evaporates from these environments, it causes the sediment to contract and
crack. The tops of the cracks are wider because evaporation is greatest near the
surface. Further evaporation can widen the crack, which can then be filled with other sediment
that lithifies, preserving the crack. Mud cracks are yet another example of primary geological structures.
Folds are geological structures that arise due to the curving or bending of Earth’s
crust in response to compressional stress. The compressional stress creates a wavelike structure. As folds are structures that have formed after the deposition of a rock in response
to applied tectonic forces, they are secondary geological structures.
We appear to have found the answer, but for completeness let’s take a look at graded
bedding. Graded bedding can form when a current begins to slow. The heaviest grains can no longer be carried by the current and are deposited
first. As the current loses its ability to support its load, progressively finer grains are
deposited, forming graded bedding. So graded bedding is another primary geological structure.
In conclusion, the answer choice that is a secondary geological structure is a
fold.
