Jump to content

Geology of the Gers

Recommended Posts

The Gers has a group of rivers descending from the Pyrenees, and ridges of wooded hills between them, like the fingers of a hand. And in other parts steep rolling hills like the waves of the sea.

We live in one of the valleys, and see another feature which is large smooth rounded stones, very heavy, which are embedded in the clay soil.

I've often wondered about how the hills and valleys were formed, but have been unable to find any information locally.

I expect a geologist would know where to search.

Or a theologist - could be the result of Noah's flood?[geek]

Can anyone find a link to explain it?

Link to comment
Share on other sites

The Pyrenees, like the Alps and Apennines, are formed as a result of tectonic plate movement.  Basically Africa is moving Northwards relative to Europe and the crumple zone in the collision are the mountain ranges.


On top of that we then have glaciation.  During the ice ages, precipitation falls as snow, and summer temperatures (especially at height) lead to a gradual increase in the snow and ice layers over years and then glaciers form.  These gradually flow down the hills scouring out the rock into boulders, rocks, stone and sand.


Then the ice age ends.  The ice begins to melt and carries with it the debris scoured out by the glacier - like a liquid sand paper the valleys are cut doen by water and the flowing debris.  When the water reaches flatter land te heavier rocks settle out - first the boulders (now rounded) then the stones, sand and fine ground down powder rock - which then sets as clay.


A rather simplified overview but essentially the processes that have formed what you find around you today.


Incidentally the Pyrenees are still at some risk from Earthquake as a result of this tectonic collision - which is still going on as evidenced by the large Italian earthquake last year.

Link to comment
Share on other sites

That's fascinating, Andy, thanks for the explanation.

So it's the ice age and its consequences which created the landscape. And as the Gers is below the centre and highest point of the Pyrenees, we get the extremes of these effects.

I haven't travelled all over France, but have never seen this type of landscape in any of the areas that I have been to. It seems very unusual.

Link to comment
Share on other sites

That is a very interesting question.

I live on one of the ridges radiating from the central area somewhere South of Lanemazan.

I did a bit of hunting and came up with this ....

The syn-tectonic deep-marine Ainsa fans (Eocene of the Ainsa basin, Spanish Pyrenees) were confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. Mass transport complexes (MTCs) constitute a major component of the stratigraphy and represent an integral part of the evolution and depositional style of the deep-marine clastics. Using an integrated study of outcrop data from sedimentary logs and mapping, with core data from eight wells and micropalaeontological and palynomorph analyses, we demonstrate the lateral step-wise migration of sandy channelized submarine fans, as a foreland-propagating clastic wedge on a time-scale of hundreds of thousands of years. The deep-marine expression of the inferred tectonic pulses began with the large-scale basin-slope collapse as sediment slides and debris flows that formed much of the seafloor topography for each fan and contributed to their lateral confinement. The uppermost slope and any shelf-edge, including the narrow shelf, then collapsed, redepositing unconsolidated sands and gravels into deep water. This is overlain by an interval of mainly channelized and amalgamated sandy deposits, passing up into several tens of metres of less confined, non-amalgamated, medium- and thin-bedded, finer grained sands and marls. These deposits represent the phase of most active fan growth, initially by erosional channel development, sediment bypass and backfill (in several cycles), giving way to non-channelized, finer grained sandy deposition, interpreted as a response to the flushing out of the coarser clastics from the coastal and near coastal fluvial systems. During this latter stage in active fan growth and when sediment accumulation rates probably remained high, the degraded submarine slope was regraded and healed by finer grained depositional events. The high amount of woody material and the high non-marine palynomorph signal in these sandy deposits suggest direct river input as both turbidity currents and hyperpycnal flows for the silty marls. In the upper few metres, a thinning-and-fining-upward sequence shows a return to background marl deposition, representing fan abandonment. Many sequences are overlain by intraformational sediment slides that attest to the increasing seafloor gradients associated with the regrading and healing stage in slope development. These organized, predictable vertical sedimentary sequences provide a testable generic model for submarine fan development where fan growth is strongly influenced by tectonic processes.

.......... and then I lost the will to live!

Link to comment
Share on other sites

As you say, Mik, too much technical jargon, but thanks for the reference.

 I found Ainsa on the map and it's just behind the Pic du Midi.

Seems that the whole area was a sea at one time?

Radiating fans is a good way of describing the landscape.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...