Named stone
Rainy Butte chert
Also called Rainy Butte silicified wood, Rainy Buttes silicified wood
A kind of silicified wood and other silicified sediments
Rainy Butte chertSilicified wood and other silicified sedimentsPlains3 photos
Description
Rainy Butte chert is a chocolate-brown, iron-rich rock from the badlands of southwestern North Dakota. It is mostly fossil wood in which silica has replaced the cells, so pieces can show growth rings, and even some stones that look plain turn out to be wood under a microscope. It outcrops in lenses that break into scattered fragments and sometimes large branches or trunks. The type area is West Rainy Butte in eastern Slope County, and the state geologist found more of it in southeastern Billings County. The Survey's photo plate shows a Clovis biface about 8 inches long made from it. Knife River flint can also be chocolate brown, but wood structure separates the two.
How to identify it
- Colour
- Chocolate brown, iron-oxide rich (North Dakota Geological Survey).
- Texture
- Earthy to waxy in appearance.
- Inclusions
- Fossil wood: growth rings in hand specimens, and cell structure replaced by reddish-brown chert. Pieces that do not look like wood may still be fossil wood in thin section (Loendorf and others 1984, as summarized by the Survey).
Geology
Found in the upper Sentinel Butte Formation. Silica and iron oxide appear to have precipitated from groundwater along an organic-rich layer. The rock occurs in several lenses that scatter pieces over the outcrop face as they weather.
Where it comes from
Plains
States
Quarries and outcrops
- West Rainy Butte area, eastern Slope County
- Southeastern Billings County (deposits found by the Survey's geologist, about 22 miles northwest of the type area)
Used by
- Clovis: a biface of Rainy Butte chert is shown in the Survey's plate of western North Dakota artifacts (the plate does not give the site)
Look-alikes
Stones that are easy to confuse with Rainy Butte chert.
- Knife River flint (similar chocolate-brown colour, but no wood structure)
- Taylor Bed and Rhame Bed silcretes (gray to tan)
Sources
- Murphy, Ed. 2014. Knife River Flint and Other Siliceous Rocks in Western North Dakota. Geo News (January 2014), North Dakota Geological Survey, pp. 1-7.dmr.nd.gov (opens in a new tab)