Re: Where do gorges come from?


Jo Schaper wrote:
Sorry for the mis-send above.

Near as I can tell, the flat part at Lake Tana flows across Tertiary
basalts (a fairly hard rock) which caps the softer sandstones and
mudstones of the gorge itself. hence, the river, flowing along its merry
way, at some point encountered a fault or a joint or perhaps the edge of
the basalt, got beneath the hard rock and began chewing through the
softer rock below. The gradient increased, as did the rate of erosion.

The concept of water flowing across a tough layer of cap rock and then
encountering softer rock below and forming a gorge is pretty common.

This is a best guess-- I know very little about East African geology.
Maybe someone else more familiar has another explanation.

Thanks. I didn't think of the "hole" in the ground being worn out by
the river as the aerial shot made the "hole in the ground" look much
much larger than the river. Also, the edge is rather sharp. So I didn't
think of the large gorge having been made by the river, as I would have
done for a narrow gorge. But your tough layer of cap rock explanation
explains everything. Particularly with the floods making the river much
wider. As well as cracks, can there be big holes in the cap rock, or
can it suddenly "run out"?

I've tried to identify the falls. I think they are Tis Isat Falls. The
following picture makes the river look larger and the gorge smaller
than the aerial shot did.

http://www.adum.com/photos/index.html?RollID=ethiopia&FrameID=113-1318_img

This one shows a much wider waterfall. Perhaps during the flood.

http://www.amhara-development.org/photos/amhara01.JPG

I can't find an aerial view that looks like what I saw on the
television program.

.

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