That's pretty much it. Sandstone? Granite? I'm guessing nothing igneous. I've run this through Google and Wikipedia. Can any geologists answer this one for me? Please say how you know.
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There is a great deal of sandstone in Mongolia, from the Alagtegian, djadokhtan, barun goyot, nemegt--the deposits are continental so no limestone I'm aware of. Long ago, Gradzinski mentioned a pediment consisting of ancient paleozoic(?) rock, possibly igneous--granite(?) I have part of the ref but can't seem to find the description...