Spongelike layers of sandstone, ranging from 300 feet [100 m] to almost two miles [three km] in thickness, are sandwiched between layers of water - resistant rock.
Porainie smilšakmens slāņi, kuru biezums vietām ir tikai 100 metri, bet vietām trīs kilometri, ir ieslēgti starp ūdensnecaurlaidīgiem iežiem.