Dear Ijaz,

In addition to what Richard and Wolfgang explained, the actual landform of the slope is (1) very much material dependent and (2) also depending on the actual erosional processes and partly the history of the denudation.

Concerning Point (1): calcareous material will behave differently as the precipitation partly solves the material. Well-cemented, thick bedded, layered material (can be also siliciclastic) or an existing resistant layer inbetween the pile tends to form little "bumps" in the landscape, breaking this convex/concave pattern. The layers can be also tilted, complicating the pattern.

Concerning Point (2): if the area has been glaciated for a long time (inherited oversteepening), it will last for long to reach the "equilibrium" state you are asking about. In other cases, e.g., if soil creep dominates (this is also depending on the material), the surface will be smoothed and therefore the inflexion point (actually a curvilinear boundary) will be moved off.

Vegetation, land use/land cover can also play a role, but maybe this would be an off-topic here.

Kind regards, Balázs

I think the underline structures could have effect. For example embedded normal faults or anticline - syncline systems ... check the geological map of the study area...

I agree with most propositions. Convex/concave hill slopes also (generally) typify soil-mantled forested tropical landscapes and/or multi-convexo-concave rolling pediplains upon vast granitoid or tabular sandstone terrains (e.g., in West Africa...). Kind Regards, Anicet

The feature may be a domal structure. Please give me the location. Adhir Kumar Basu 6.2.2019

Dear Ijaz,

May I refer you to a book on slopes - Slopes, by Andrew Young, 1972, publ. by Longman - pages 91 - 102. It deals very succinctly with the theory of slope convexity/concavity. The second book worth visiting is The Report on the Geology of the Henry Mountains by G.K.Gilbert, 1877, also G.K.Gilbert, 1909, The Convexity of Hilltops, Journal of Geology, 17, p.344-350. I will return to the subject shortly.

George Strachan

In A Report on the Geology of the Henry Mountains (1877), p.118 Gilbert writes ‘Every slope is a member of a series, receiving the water and waste of the slope above and discharging its own water and waste on the slope below . . . And as any member of the system may influence all the others, so each member is influenced by every other’ (Young 1972).

Let us now follow Gilbert’s observations to the top of the hill where the divide or watershed is. Here, water plays a minor role except in weathering processes. If, for example, frost shattering breaks a rock fragment off, the fragment can only fall sideways or down and only then if there is no rock debris to stop it. This underlies the convexity found at divides. Creep of soil and rock debris downslope then clears the summit ridge allowing further weathering to continue. Further downslope water takes on an extended role as sheet wash begins to combine with soil creep to increase the convexity of the slope.

There comes a point when sheet wash and mass wastage such as debris and mud flows, slides etc. gradually comes into play. The change is transition, as any sudden change would result in a break of slope. With increasing water and waste movement the slope begins to develop a concave form, unless a river is eroding into the base of the slope.

Think carefully on what G. K. Gilbert said all these years ago. They laid the basis for Process/response geomorphology and also introduced the idea of dynamic equilibrium. I hope these thoughts help you Ijaz

I agree with George Strachan - have a look at the older literature. Another example is Savigear, R.A.G. 1952. Some observations on slope development in South Wales. Transactions, Institute of British Geographers 18, 31-51. Slopes have fallen out of fashion in the last few decades but there is a vast literature of older papers and books.

The evolution of a subaerial slope bounding fluvial drainage systems of whatever size and pattern is first and foremost a climatical-morphological process and only second in order controlled by the lithology and structural evolution as far as “true sculptural landforms” are concerned. If you deal with “true structural landforms” (such as volcanic edifices) the shape of the slope and its stability are under the control of the structure-building processes. In the course of waning endogenous activity or when getting dormant or totally extinct, the processes mentioned at the beginning more and more prevail. In conclusion, it is the interplay of endogenous and exogenous forces that dictates the shape of slope and shape of a landform at the very end.

H.G.Dill

Totally agree with Colin Pain. Slope geomorphology was big until WW2, then gradually there was a shift to process/response geomorph. Strange to say it was a re-discovery of work by folk like G.K.Gilbert. Maybe the time has come when some reconsideration of slope theory is needed in the light of the great advances made in understanding slope processes.

In the passing Ijaz, the oldest literature is often a good starting point. Researchers such as Gilbert, Powell, Holmes and Rapp wrote lucidly and to the point, an excellent model to follow for one’s own writing style.

George Strachan

Dear George J. Strachan ,

You wrote: "Maybe the time has come when some reconsideration of slope theory is needed in the light of the great advances made in understanding slope processes. "

Yes, indeed. Mostly because now you can have dm- or cm-resolution DTM of the slopes, if you want (and can afford) daily (using e.g., UAV-based laser scanning). Even the effect of the vegetation can be analysed better than ever as individual plants can be identified and monitored now (position, growth, growing attitude), e.g., in alpine slopes.

Gentlemen, I am delighted and feel honoured to be in your virtual company here.

Kind regards, Balázs

Dear, Ijaz.

It could be a classic upstream migrating knickpoint in a steepened reach zone, where the limit between the two topographic profiles could be a response to differential uplift rates for two tectonic blocks.

To corroborate this hypothesis you must look at map of the study area, checking the presence of both lithologic changes and fragile or ductile structures near to the topographic break. Morover, you have to calculate the Ksn index along the stream profile for successive segments.

Regards.

Dear José González-Alfaro ,

Don't you think that stream channels (mostly linear erosion) and (valley) slopes (many other shaping processes) should be discriminated in this case?

The original question tackles the slopes. I would not exclude a lithological contrast as a possibility, but a quite normal, homogeneous slope also shows the described pattern. Applying Occam's razor principle, I would not assume any tectonic or lithological reason unless there is some evidence for that.

Thank you and kind regards, Balázs

Dear Balázs Székely

I think that now I am little confused. I don't know if Ijaz Ahmad is referring to a particular slope of a mountain hillslope or to a regional slope of a topographic profile? If this is the first problem, you are right. Factors involved in hillslope evolution will depend on the particular conditions of the study case (geological, climatic, biological, gravity stability, geotechnicals, structural, colluvial mass transport, etc.), according to the features and characteristics observed in the field.

Kind regards.

Fully agree with Balazs. Of course there is an interface between slopes and stream/river processes and forms. But the question posed by Ijaz deals specifically with slope process and form in which the role of water flow is of much less importance than in river flow; though water is crucial in weathering processes, in soil creep, in landslides, slumping and in debris flows, indeed even in rockfalls in most cases. Yes, lithology and geological structure play a role in determining slope form including convexity and concavity, but the starting point here is why slopes develop convexity and concavity in the first place.

George Strachan

Very interesting thing is that the question asked have reverted to a fruitful discussion. I posed this question after reading a paper/article about landslides from Sichuan province china.

Dear Mr. Ahmad,

One should also pay attention to the geological composition of the underlying rocks. Their properties may produce different natural angle of repose, hence slopes with complicated morphology being formed.

Regards, Sergei

Dear Ahmed,

Sometimes the stress different in same area for example from extension to compression give us as a results different geometries of structures for example parasitic fold .

Regards