An inselberg is defined as an isolated hill that protrudes from a flat extensive land called a pediplain/ peneplain. Inselbergs can be in the form of isolated hills called monadnocks or small to medium features such as bornhadts, tors, whale-backs (dwalas), or even mesas.

NB: Don’t confuse inselbergs with mountains, they are formed completely different, although some mountains are isolated.

Several theories on inselberg formation has been envisaged by several geomorphologists such as Charles King, Twidale, Linton and many others. Here we’re going in detail with King’s theory of Scarp retreat and the Etchplanation theory

Pediment: A gently sloping extension of a hill formed by retreating scarps (other explanations have been put in place for pediment formation).

Pediplain: According to King, a vast and flat land formed from several merging pediments which are left when scarps retreat. This term can be loosely used to explain any flat and vast land. According to W.M Davis, this is called a peneplain

Interfluve: An upland area between two scarps or valleys of adjacent watercourses

Scarp: A hard steep face of a hill, plateau or valley uncovered by debris.

Pediplanation Theory/ Scarp Retreat (Charles King)

King states that inselbergs are created when scarps retreat backwards leaving a vast pediplain in the wake. Initially for scarps to back wear or retreat, interfluves (see diagram) or coastal cliffs must be present. Lateral erosion by rivers, sea waves, wind, or weathering acts along valley sides or coastal scarps wearing them backwards leaving a pediment behind which then coalesce with other pediments to form a pediplain.

As time goes on the interfluve is reduced to an inselberg called a monadnock (from Mt Monadnock in New Hampshire, USA, although Mt Monadnock is not formed from scarp retreat, but denudation of softer rocks leaving hard stands of rock).

Inselberg formation from scarp retreat diagrams
Inselberg formation from scarp retreat

Several monadnocks can be seen protruding in the plains of Africa including Tanzania, Zimbabwe, South Africa, New Hampshire, USA and many others. In Zimbabwe isolated hills can be seen in areas such as Banket.

Houtkop An insleberg in the Free State South Africa

King’s theory practically doesn’t apply to inselbergs such as bornhadts or tors rather, it applies to medium and large scale hills. Other researches suggests that bornhardts are formed as scarps retreat which is mostly argued with.

However, King’s theory has some criticism.

  • The assumption that scarps always retreat is not true. Lateral erosion by rivers on valley sides or erosion by sea along cliff foot causes slopes to retreat, however, not all valleys or regions with rivers eroding laterally or sea waves eroding scarps backwards.
  • In addition, hard resistant rocks may restrict back-wearing than soft rocks. He assumed a homogeneous landscape composed of the same rock type which is not always the case.
  • Less emphasis or ignored tectonic processes

Etchplanation / Exhumation Theory

Several geomorphologists envisaged the etchplanation theory including Twindale, Linton and Ollier. This theory takes into account deep weathering and mostly occurs in sub-tropical climates where successive periods of wetness and dryness exists.

During the rainy season deep weathering is more pronounced which produces deep regolith. During dry periods erosion and denudation then strips away the regolith revealing seated basal surfaces or weathered boulders as tors. This theory implies that the rate of denudation and stripping must overtake that of deep weathering. If deep weathering is faster then the basal surface will not be be exposed to the surface, but rather keeps on lowering

Etchplanation makes more sense in explaining inselberg protrusions such as bornhardts, tors or dwalas but not hilly inselbergs.
However, some argue that deep weathering and stripping occur at the same rate a process called double planation.

Etchplanation process diagram

These and other theories have been put forth to try to explain the formation of inselbergs. Another theory state that inselbergs are formed during the old stages of landscape evolution (Davison theory of slope decline).

When surrounding softer rocks are eroded and denuded, protrusions of hard rock can be left standing as inselberg. No theory is widely accepted as each has its own criticism, thus it still remains a topic of much debate among gemorphologists.

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