Where the rock projects it more usually appears in low crags and knolls, from which long trails of grey or purple debris descend till they are lost among the grass.
I wandered around the ponds, up hills and down grassy knolls.
The direct passes across it from Herat (the Baba and the Ardewan) wind amongst masses of disintegrating sandstone for some miles on each side of the dividing watershed, but farther west the rounded knolls of the rain-washed downs may be crossed almost at any point without difficulty.
Some outcrops of limestone in the Clitheroe area, for example, have been derived from reef knolls formed in a prehistoric sea.
The fairies (see Fairy), located in the fairy knolls by the inhabitants of the Shetlands, may also be put under this head.