Striking red rock caps many of the hills of eastern Montana. Some of the rock looks volcanic, so it has incorrectly been called scoria. Lewis and Clark attributed the red rock to burning coal beds and called the red areas "burnt hills." This rock develops when coal burns from the surface into a hill, where it cooks, fuses, and melts the adjacent rock, forming new, completely different types of rocks. Sandstone is baked to a brick-like rock. Shale may be fused like a ceramic in a kiln. Other rock may melt to look like hardened lava. The heat from burning coal rises, so most of the clinker develops above the burning coal bed.
Members of the Corps of Discovery included a number of geological observations in their journal entries during their journey through the area that is now North Dakota in 1804-1806. Several of these entries related to observations of burning coal or the presence of clinker, rock that is baked or fused by heat generated from burning of coal. Meriwether Lewis and William Clark used volcanic terminology to describe these rocks, referring to the vesicular type of clinker as pumice stone and the molten clinker as lava. Using this terminology reflects their preconceived notions that these rocks were formed by volcanic processes. They sent a sample of each of these clinker varieties back to President Thomas Jefferson in a shipment from Fort Mandan. These clinker samples are housed at the Academy of Natural Sciences in Philadelphia and are among only a handful of the original 68 rock, mineral, and fossil specimens collected during the Lewis and Clark Expedition that are extant.
The Expedition first observed burning coal on April 8, 1805 in exposures of the Paleocene Sentinel Butte Formation along the Missouri River just north of Fort Mandan in what is now central North Dakota. Patrick Gass noted in his journal that the area had recently been a burning volcano, again indicating preconceived thoughts that volcanism had occurred in this area. The Corps also encountered burning coal on April 10, 1805 near present day Beulah Bay on Lake Sakakawea. Although Gass’s entry and the use of volcanic rock terminology indicates the explorers initially thought clinker was formed by volcanic processes, Lewis (April 16, 1805) correctly attributed the creation of clinker to burning coal. Clark accurately and succinctly described the process in an undated journal entry likely written the following winter at Fort Clatsop. As such, Lewis and Clark were the first to record the phenomenon of burning coal on the Northern Great Plains, the first to collect clinker specimens for scientific study, and the first to make scientific observations regarding how clinker is formed.
Coal-bed fires have baked and fused overlying sediments to form clinker, a hard red or varicolored rock, through much of the northern Great Plains of the United States (USA). The gently dipping coal beds in the region burn when regional downwasting brings them above the local water table. The resulting clinker forms a rim along the exposed edge of the coal bed in an ongoing process through geologic time. The resistant clinker is left capping buttes and ridges after the softer unbaked strata erode away. Clinker outcrops cover more than 4100 km2 in the Powder River basin (PRB), which lies in Wyoming (WY) and Montana (MT). The clinker in place records tens of billions of tons of coal that have burned, releasing gases into the atmosphere. The amount of clinker that has eroded away was at least an order of magnitude greater than the clinker that remains in place. Fission-track and uranium–thorium/helium ages of detrital zircon crystals in clinker, and paleomagnetic ages of clinker, show that coal beds have burned naturally during at least the past 4 million years (Ma). The oldest in-place clinker that has been dated, collected from a high, isolated, clinker-capped ridge, has a fission track age of 2.8±0.6 Ma. Evidence of erosion and downcutting is also preserved by clinker clasts in gravel terraces. One clinker boulder in a terrace 360 m above the Yellowstone River has a fission track age of 4.0±0.7 Ma.
To see a good example of this rock formation walk to the other side of the wayside and look over the interstate. You will see this towards the NW.
In order to claim this find, please email me the answers to the following questions:
The Red-Capped Hills of Eastern Montana
- A specific type of clinker rock is formed from welded shale producing a ceramic-like rock; what is it called by geologists?
- What is the name given to this formation of "burnt hills" rock?
- Clinker is formed from veins of burning coal near the surface of the earth. What are a couple different ways that could start the coal burning?
- Bonus question: On the plaque on the Yellowstone River what is the error that is written there?
