Quoted from the Afarensis blog:
I'm a geologist so I'll give my qualified opinion. From the photos (I wish I had a hand sample), it looks like basalt with some foliations. Simply put, it's basalt that's undergone some metamorphosis and is part way to turning into slate. Foliations often have a "woody" texture because elongate mineral grains will align their long axes when stresses are applied. Given that this rock formation is exposed in a mountain chain, it's likely that it has undergone some heat and pressure in its history, explaining this woody texture. If it's basalt, there is no possibility that this ever was wood. Building an ark out of basalt would be incredibly stupid.
http://scienceblogs.com/afarensis/2006/06/29/more_crappy_archaeology_report/