I had a look at the maps, but couldn't find the place you're talking about. You described the geology of the location, but not the ecology. Often this can have an effect on description. My ideas (and others already mentioned):
(if mostly treeless) meadowlands, grasslands, savanahh, plains, arable plains, black soil country, red soil country, moor, heathlands, tundra
(if undulating landscape with overall declination, with streams and copses) steppes, dales, glens, fells, dells, slopes
basin has a watery feel about it to me, you could try bowl? Valley would also work.