What do you mean by the term Structural Plain?



If there is a nearly horizontal resistant bed of rock, it is relatively protected from down wearing and may form a higher erosion surface with reference to the level adjoining level or grand base level. It is called structural plain. If there were weaker, beds above the hard stratum the weaker material will be worn down and washed away. As an example we may consider an extensive masonry platform surrounded by earth surface filled to the level of the platform. In course of time, the surrounding softer material will be worn down to lower levels whereas the masonry platform will maintain a higher surface.

Probably the Vindhyan tableland south of the Ganga plain, such as Kaimur upland, is a structural plain. This 300-meter surface standing abruptly above the Ganga plain is composed mostly of horizontally bedded resistant sandstones partly silicified into quartzites. This structural plain, however, is not so distinctly elevated on the south because of quite resistant rocks like Dharwars, Gondwana sandstones and basalt.

Generally, structural plains are of limited local extent. They are differentiated from peneplains and other erosion surfaces in the sense that the latter truncate varying structures while the structural or stripped plains occur over a single horizontally disposed stratum.

In structural plains, it is the upper surface of the hard bed, which serves as a (temporary) base level because of its resistance. The adjoining lower ground or local base levels may help in the maintenance of the structural plain by the greater amount of degradation of the surrounding softer materials. Fenneman, the noted American physiographer, however, thought that no hard stratum can maintain itself as a local base-level particularly at a high level. It will soon be demolished by erosion from sides. Moreover, the formation of a structural plain is subject to the stripping or removal of the debris overlying the hard structure. He doubts clean stripping.