Effects of Horizontal Compression on Rocks: When rocks are subjected to deforming forces, what happens to them depends upon the circumstances. Let's look at some flat, layered rocks that are being squeezed. Assume the rocks are at a high temperature, but are not hot enough to melt.

As the force is applied the rocks, made 'soft' by the high temperature, slowly begin to bend. The original flat layers are 'folded' into a series of up-arches and down-arches.

With continued slow squeezing, the folds become tighter and tighter, like an accordion.

Landforms That Result From Horizontal Compression: The results of squeezing rocks may be seen in many places. Rocks that are bent (folded) are very common. What effect do you think such deformation might have upon the landscape?

What landform occurs at 1?

What landform occurs at 2?

Note that the layers are parallel to the surface of the land.

Landforms and Differential Erosion of Folded Rocks: It is important to distinguish between landforms created by folding and those formed by differential erosion of already folded layers. In this picture are hills and valleys formed when an area underlain by flat layers was folded. Note that the land surface everywhere is parallel to the layers. Hills are located over up-arched layers, valleys over down-arched layers.

The brown layers are more resistant to erosion than the green layers. As yet, however, only the brown layers are exposed to erosion. As the land surface is eroded, the high places are worn down faster than the low places.

Now erosion has removed the brown layer completely from the hilltops. In those places, the weak green layer is exposed and is subject to erosion.

The green layer, which is less resistant to erosion, is much more quickly eroded than the more resistant brown layer. As a result valleys (V) are formed where the green layer is exposed. The immediately adjacent areas that are underlain by the brown layer are left as hills (H). Some valleys (R) are remnants of valleys originally formed by folding.

As this process continues, with the land surface wearing down faster where the green layer is exposed and more slowly where the brown layer is exposed, the locations and numbers of hills and valleys keep varying. The process is known as differential erosion.

Hills formed by differential erosion are always located over resistant (brown) layers that are exposed at the surface. Some of the hills are located over up-arched rocks (U); others over down-arched rocks (D). Some of the valleys may be located over up-arched rocks, others over down-arched rocks.

Note that in this case, in which the hills and valleys formed by differential erosion, the land surface in general is not parallel to the layers. Where hills and valleys are formed by folding of originally flat layers, the land surface everywhere is parallel to the deformed layers. Parallelism of the land surface and deformed layers is an important criterion by which hills and valleys formed directly by deformation may be distinguished from those formed later by differential erosion.

Landforms That Result From Deposition Onto Folded Rocks: Consider an area where the rocks have been folded and then differentially eroded. Look at what happens when this area is deeply but unevenly buried by unlayered sediments. The hills and valleys on the surface of the land are not parallel to the up-arches and down-arches of the buried rocks. Nor are they parallel to former high and low areas that were the result of differential erosion. The shape of the land surface is independent of the character of the buried rocks.

Look at what happens when this area is deeply and evenly buried by layered sediments. The surface of the land is essentially horizontal; it is not parallel to the up-arches and down-arches of the buried rocks. Nor is it parallel to the former high and low areas that were the result of differential erosion. The shape of the land surface is independent of the character of the buried rocks.