Limestone (he says, having just Googled). Sadly, from a photo or a distance one big grey lump of rock often looks like another big grey lump of rock. Geologists can be very cagey about this thing, as many things look alike from a distance, and they have a fragile ego. They'll have their and lenses up close to a sample for ten minutes and still not give you an answer.
If you're ever on an excursion, and a geologist says from a great distance, hey, look at that XXXXX, he's either been there previously or read the guidebook the night before. Most of us spend great time practising disguising our WTF is this stance...
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