The one in Russia might be high enough in altitude to be pika habitat, and if that is the case it explains the lack of overgrowth--those little critters carefully and thoroughly harvest vegetation to tide them over the winter. They even eat toxic plants, because it turns out most of the toxins help preserve the dried plant material and its nutritional value.
The one in Bulgaria might be a totally different stone, or it might just be very, very old. If it's a softer stone and older than the Russian one, that would explain the rounder stones. And the Bulgarian rock river might very well be pika habitat as well--they're found in pretty much every tall mountain range in the world from the North American Rockies to the Himalayas and the Andes (European and South American). Pikas need high altitude, cool summers, access to minimal drinking water, as much piled-up stone for warrens as they can find, and lots of low-growing grass and herbs to harvest. They go into a semi-hibernation state for winter and need little to eat or drink, but they don't go fully under like bears do because they're too small to pack on sufficient fat reserves.
And I agree with lumberjim--if these places were more accessible, I think they'd be all over album covers! I can imagine a black metal band showing up with a photographer and a bucket of blood to be thrown over them as they strike mighty Norwegian poses...
I didn't get enough sleep last night...