Those aren't sand dunes, they're just rounded mountains. Their soil might be a little sandy but it's not all loose like a sand dune. And, yes, they're big, probably a few hundred feet tall from their bases at least. This is the desert... click here for more info on where I took the pic from... http://www.nps.gov/jotr
"I am in love with this world. I have nestled lovingly in it. I have climbed its mountains, roamed its forests, sailed its waters, crossed its deserts, felt the sting of its frosts, the oppression of its heats, the drench of its rains, the fury of its winds, and always have beauty and joy waited upon my goings and comings."