For arches to form, there needs to be three main criteria present.
1. first a thin wall of sandstone (as seen in picture) need
to be present. These thin "walls" of sandstone are called fins (see fin formation on blog). These fins allow for the arch to open up all the way through the wall.
2. The next criteria that must be present is that the overlying layer of sandstone must have an underlying layer of shale. Having the less resistant rock underlying the more resistant layer causes the underlying shale to be eroded beneath the sandstone. When the underlying shale is eroded away, there is no support under the sandstone. This causes the sandstone to slowly collapse.
3. The contact between the sandstone and the shale is the third criteria necessary for arch formation (arches form at a contact between overlying sandstone and underlying shale). The second picture shows the contact between the two layers very well along with the early stages of arch formation.
This picture shows the contact between the sandstone
and the shale very well. You can also see, in the
center of the photo the beginning formation of an arch.