The construction of a geological cross-section from a map, the projection of dipping beds, folds, faults and unconformities into the section, and the reconstruction of the full sequence of geological events of an area from the map and section.

Set out the procedure in order, because the marks track the construction steps.

First draw a topographic profile along the section line: mark where each contour crosses the line and plot its height to build the ground surface, using a vertical scale (ideally the same as the horizontal to avoid exaggeration).

Next mark on the profile where each geological boundary and fault crosses the section line, transferring them from the map.

Then project the beds down into the section at the correct dip: use the dip direction and angle read from the map (from dip symbols or the rule of Vs) to draw each boundary continuing below the surface, keeping the beds parallel and of constant thickness.

Draw in the faults at their dip, displacing the beds across them, and show any folds by curving the beds according to the outcrop pattern, and any unconformity as a surface truncating the older beds beneath it.

Finally label the beds in order and add the key, so the section shows the subsurface structure.

Markers reward drawing the topographic profile, transferring boundaries and faults, projecting beds at the correct dip, and representing folds, faults and unconformities correctly.

Explain the principles and how they order the events, because the question asks for the method of reconstruction.

Superposition orders the deposition of the sedimentary beds: in an undisturbed sequence the lowest bed is the oldest and each bed above is younger, so the order of deposition is read from the bottom up.

Cross-cutting relationships order the later events: any fault, dyke or intrusion that cuts the beds is younger than the beds it cuts, so faulting and intrusion are placed after the deposition and any folding they disturb.

Unconformities mark episodes of uplift and erosion: everything below the unconformity is older than the beds above, and the structures truncated by it (folds, faults) happened before it.

Putting these together gives the full history: deposit the lower beds in order, then fold them, then intrude or fault them (each dated by what it cuts), then uplift and erode to form the unconformity, then deposit the younger beds, and so on. Folding is dated by the beds it affects and the unconformity that truncates it.

Markers reward superposition giving the order of deposition, cross-cutting placing faults and intrusions after what they cut, unconformities recording uplift and erosion, and the assembly of these into a full ordered sequence.