Why Do the Rock Layers Sag in the Middle? Something Squeezed Them

Sedimentary rock forms in flat horizontal layers, so any layer found tilted or bent has been deformed after it formed. Compression of the crust folds those layers, and the folds come in two complementary forms. Where the layers arch upward the structure is an anticline, and the oldest rock sits at the centre of the arch. Where they sag downward into a trough the structure is the one described here, and the youngest rock sits at the centre. Identifying which is which in the field therefore depends on working out the age order of the layers rather than on which way the ground slopes.

Several features identify the structure where the fold itself is not visible:

• •Layers on either side dipping towards a common centre line
• •The youngest rock exposed along that centre
• •Progressively older rock outcropping away from it in both directions
• •A symmetrical banded pattern on a geological map
• •That pattern closing at each end where the fold dies out
• •Fractures concentrated along the hinge, where bending is greatest

The intuitive expectation is that a downward fold makes a valley and an upward one makes a hill, and across long periods the opposite is frequently true. Rock in an upward arch is stretched at the crest, which opens fractures and lets water and weather in, so the arch erodes faster than its surroundings and can become a valley. Rock in a downward trough is compressed and tightly closed, resists erosion and can end up standing as a ridge above the eroded arches on either side. That reversal is common enough to have a name and is one of the first things a geology student is taught not to assume.

Describing a fold precisely requires a small vocabulary that geologists use constantly in the field. The hinge is the line of maximum curvature running along the fold. The limbs are the two sides sloping away from it. The axial surface is the plane containing the hinge lines of successive layers, and whether it is vertical or tilted distinguishes a symmetrical fold from an inclined or an overturned one. Dip is the angle at which a layer slopes and strike is the compass direction of a horizontal line on that layer, and a geologist records both at many points with a compass and clinometer, from which the shape of the whole structure is reconstructed.

The structures are central to finding both water and hydrocarbons, and the reasoning differs between them. Water collects in the trough, since a permeable layer folded downward between impermeable ones forms a basin that fills, and the pressure in such a basin can be sufficient to drive water to the surface without pumping where a well penetrates it. Oil and gas do the reverse, being lighter than water and migrating upward through permeable rock until something stops them, which is why they accumulate under the crests of upward folds rather than in troughs. Coal is frequently preserved in troughs, where later burial protected it from being eroded away.

Compression folds flat-lying layers into arches and troughs, and a trough is identified by layers dipping towards a common centre with the youngest rock along it. A stretched arch fractures and erodes faster, so troughs frequently end up as ridges and arches as valleys, which reverses the obvious expectation. Water collects in troughs while oil and gas migrate upward and accumulate under arches.