I was recently perusing through Lewis and Clark in North Dakota by Russell Reid (1948) which contains the journals of Lewis and Clark as well as the Biddle texts and annotations by the author.
I found Clark’s journal entry from March 21, 1805 interesting. The expedition had spent the winter at Fort Mandan and would soon continue their travels upstream on the Missouri. Here is an excerpt from the Biddle texts for Clark’s journal that day. It is easier to follow than Clark’s original entry.
“…passed along the points of the high hills, where he saw large quantities of pumicestone on the foot, sides and tops of the hills, which had every appearance of having been at some period on fire. He collected specimens of the stone itself, the pumicestone, and the hard earth; and on being put into the furnace the hard earth melted and glazed, the pumicestone melted, and the hardstone became a pumicestone glazed.”
That “pumicestone glazed” was apparently Clark’s introduction to scoria. Scoria, or more accurately clinker, is formed when a seam of lignite catches fire and burns hot enough to bake an overlying layer of clay, forming a red brick-like material.
Western North Dakota has an abundance of lignite deposits that are exposed in the buttes and badlands. Those lignite beds are known to occasionally spontaneously combust. Lightning strikes can also serve to ignite the lignite. And historically there were frequent prairie fires, some of which were deliberately set by Native Americans to attract the bison or as a backfire to protect villages. If the fire burned hot enough, an overlying layer of clay can be baked, forming scoria. And we still occasionally hear of a lignite vein burning in the western part of the state.
Most North Dakotan’s are familiar with this scoria as the conspicuous red colored sediment layers visible in the buttes and badlands in the western part of the state. However, the color can vary due to differences in chemical composition (for example iron content). And because it is often more resistant than other sediments, scoria often forms the caprock on the rougher landscapes. And of course scoria is also widely used in the western part of the state as surface material for roads and also landscaping.
Chuck Lura
Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Dakota College at Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.
