There is a great variation in the hardness of the petrified wood, from soft lignitic coal to consistently hard pieces replaced with silica.   The grain is well preserved in most pieces, while others appear to be casts of wood with clear agate chalcedony and opaline portions.  The wood is found from almost completely black, through tan, brown, yellow, to almost pure white, mottled specimens being more common.  This diversity in color and material undoubtedly reflects its multiple original sources.  Some of the wood looks quite similar to Oligocene wood from the Cypress Hills, Saskatchewan, as well as many other Tertiary and Cretaceous sources to the west and south.  Samples of some of the wood and associated chalcedony is indistinguishable from Knife River Flint from North Dakota.  Some from this latter location seems to be silicified lignite from the Eocene, Golden Valley Formation and shows widespread utilization by Indians of the past for flint tools.  Frequently, cavities either in the original wood, or vugs formed as the quartz solution was replacing the wood, are filled with quartz crystals.