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Tuesday, 1 December 2015

Cliff faces and talus cones

Iceland -- a cliff face partly buried beneath talus cones.  These cones are nowadays not very active -- and so vegetation has been able to establish itself.

How do cliff faces in high latitudes and high mountain environments get destroyed and incorporated into a new landscape of rolling hills and sediment-filled valleys?  Well, there are very many processes that come into play, but here is a model of what should happen in an "ideal" world in which we have a coherent and geologically uniform cliff, an environment where frost-shattering is able to operate, and a foreground in which scree or talus accumulation is able to operate unhindered.  And, of course, we need a stable environment for many thousands of years, since the average rate of cliff retreat is unlikely to be more than a few centimetres per year.

The diagram shows how cliff retreat is accompanied by talus buildup and the gradual evolution of a convex rock slope.  This convexity develops because that part of the rock face buried beneath talus is effectively protected from further frost shattering, while erosional processes continue to operate on the rock face still exposed to the atmosphere.  So the highest part of the cliff retreats over a far greater distance than the base of the cliff.  Until this process is complete, the overall profile of the surface will be concave, with a steep cliff above grading to a shallower gradient on the talus slope below.

But things are never this simple.  Take a look at some of these photos:

Victor Point, Baffin Island.

Lake Louise, Canada.

 Lechtaler Alps.

In the real world rock faces tend to break up into buttresses and gullies, so that instead of a real apron of talus developing along a rock face we see the development of cones or fans, which coalesce either high up or lower down.  Often they overlap in a very complex way, with the deeper gullies feeding the biggest fans, which overwhelm or incorporate those which develop more slowly.  Some fans may be made largely of large blocks and boulders from rockfalls, while others may be made of smaller fragments or clasts which may have shallower "resting angles" -- so quite violent rearrangements might occur across a series of linked fans.  If you have ever walked across a talus slope and set it in motion, you will know what I mean...........

In some cases, as we see in the photos above, talus surface angles may remain more or less constant from tip to toe, but in others (as in the Iceland example above) there is a clear concave profile.  That is caused by the lodgement of the largest blocks at the top of the cone at a relatively high angle, grading down to the settlement of the finest fragments on the outermost part of the cone.  This implies that there is a certain amount of lubrication and water transport involved within the cone -- and this is what happens, for example, in the UK, in Norway and in Iceland.  In the arid high arctic, scree slopes may be essentially dry internally, or else stabilised by permafrost.

And in the real world it would be very rare indeed for an environment -- even in the high arctic -- to remain stable for 10,000 years or more.  A warming or a cooling, or an increase or decrease in precipitation,  will set off a string of changes which will affect sediment supply, the talus angle of rest, and the operation of biological processes.  

And what happens at the foot of the talus cone is also of crucial importance.  If an adjacent valley floor is effectively filled with talus, with virtually nothing being removed, the rate of talus accumulation and cliff face burial will be enhanced.  If there is a river or coastline which effectively saps away at the base of the come or talus apron, then the top of the talus slope may remain remarkably static for many thousands of years, with the rate is sediment supply matched by the rate of sediment removal by rivers or waves.

Then it gets even more complicated........ but you get the general idea.

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