Simien Mountains Geology

Wading through the tangled hyperlink skein today, I came across a very fine article by Simon Winchester about "one of the most remarkable geomorphological spectacles existing on our planet." It concerned Wulingyuan National Park in the Hunan province of China. Mr. Winchester deploys his considerable geological background to explain the weird, otherwordly formations that give the park its pedigree:


"Sixty million years ago there were tropical seas there; sometimes they were deep, leaving soft and fossil-rich limestones, sometimes shallow, leaving hard beach-sandstone. Then the land rose under tectonic pressure...[the] limestones dissolved over millions of years into fissures and immense caves, the sandstones cracked into knife-edged pillars, some them needle-shaped mesas, gully 1,000 feet high."

With Ethiopia perpetually on my mind, it won't come as a surprise that my thoughts turned to the similarly singular pillars of rock in the Simien Mountains. I became curious as to whether the Simiens--whose dramatic escarpment serrations are, at first perception, nearly rejected by the mind--had anything in common with the needle-shaped mesas of "China's Ancient Skyline."

First things first: much of the Simien Mountains' most dramatic scenery centers around an escarpment many kilometers long where a blade of lofty peaks falls 3,000 feet. You could move from the frigid Afromontane belt to arid lowlands in a matter of steps, should self-preservation not be on your shortlist of favored pastimes. The escarpment, like much of the massif, is carved by deep river-bearing gorges. The end result is spectacular and something like this:






What exactly would produce such a precipitous drop over so short a distance? From my limited understanding of geology, I inferred that there must be some kind of erosion at work...

I raced around the internet in search of geological references to the Simiens, keen to find out if sandstone and limestone were at work as in Wulingyuan. As with almost everything I find interesting about Ethiopia, little had work had been done on the matter. After a couple hours of dredging, the only pertinent information I could come up with was from a tour company called World Expeditions:

"The Simiens dramatic topography is a result of the erosion of basalt lavas which have been calculated to be nearly 3,ooo meters thick. The rocks beneath the lava spread were horizontal layers of sandstone and limestone. Here and there weaknesses and cracks developed, opening the way for points of erosion. The cracks in the hard, resistant basalt once begun were widened and deepened by floods that poured into them, creating deep trenches and leaving hard cores of volcanic outlets from which the surrounding material has eroded away. Thus leaving an incredible array of jagged carvings, reminiscent of America's Grand Canyon." [sic]

I remembered previously reading that the Simiens were technically a massif that predated the creation of the Rift Valley, the Abyssinian arm of which cut through the landscape a couple hundred miles to the east. Some 40 million years ago the area was host to violent primeval volcanic activity that left behind a 9,800 foot layer of lava.

But I still didn't get to the bottom of what precise processes were responsible for such awe-inspiring spires of rock, such as these below: