Named stone
Tongue River silicified sediment
Also called TRSS, Rhame Bed silcrete, Tongue River silicified peat (older name)
A kind of silicified wood and other silicified sediments
Tongue River silicified sedimentSilicified wood and other silicified sedimentsPlainsDescription
Tongue River silicified sediment, usually shortened to TRSS, is a gray to light tan silica-cemented rock from southwestern North Dakota. Geologists now read it as a silcrete, a silica crust that grew in an old soil, not a true flint. It caps the Rhame Bed and is often only 1 to 2 feet thick. The surface is earthy to waxy, and wind-blown sand can polish exposed faces until they shine. Plant stems, some of them horsetail, are common inside it. Quality swings widely: fine-grained pieces flake with good control, while coarse pieces barely flake at all. A warning from the state geologist matters here: the nearby Taylor Bed silcrete looks alike and is far more common, so many artifacts labelled TRSS may really be Taylor Bed.
How to identify it
- Colour
- Gray to light tan (North Dakota Geological Survey).
- Texture
- Flat to sugary on fresh surfaces, earthy to waxy on exposed surfaces. Fine-grained pieces knap with a controlled conchoidal fracture; coarse-grained pieces are barely knappable (Gregg and Bleier, as summarized by the Survey).
- Luster
- Waxy where wind-blown sand has polished the exposed surface.
- Inclusions
- Stems, some identifiable as horsetail (Equisetum), are common.
Geology
A silcrete forming the top 1 to 2 feet of the Rhame Bed, at the contact between the Slope Formation and the overlying Bullion Creek Formation (Paleocene). The Rhame Bed is read as an ancient soil (paleosol); the silica cap probably formed in a hot, dry climate where silica dissolved and was redeposited as a layer. Older papers called it quartzite or pseudo-quartzite. It is extremely hard to break and survives as float or lag after softer rock erodes away.
Where it comes from
Plains
States
Quarries and outcrops
- Rhame Bed outcrops and float, southwestern North Dakota (Little Missouri River country)
Used by
- Paleo-Indian and later: the Survey says people used the siliceous beds of western North Dakota for at least 13,000 years (a statement about all the beds together, not TRSS alone)
Look-alikes
Stones that are easy to confuse with Tongue River silicified sediment.
- Taylor Bed silcrete (gray to cream, often iron-stained, with wood and horsetail molds; the Survey says it is much more common and that most tools called TRSS were probably made from it)
- Knife River flint (glassy, darker, with a cream rind)
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)
- Gregg, Michael L., and Allen Bleier. 2021. The Little Missouri River Study Unit. In Historic Preservation in North Dakota (statewide plan). State Historical Society of North Dakota.history.nd.gov (opens in a new tab)