Photo by the author
The Calumet Bluffs, comprised of yellowish orange exposures of the Niobrara Formation upriver of the Gavins Point Dam, were the concluding geological highlight along an extraordinary stretch of terrain that the expedition traversed between August 22 and September J, 1804, where nearly half of all the date-documented mineral specimens from the 1804 leg of the expedition were collected.
One of the primary geological highlights in this region is Calumet Bluffs, the site of an important council with the Yankton Sioux on August 30-31, 1804. Upon embarking on the following morning, Clark noted:
[Sept. 1, 1804:] pass Calumet Bluff of a yellowish read [red] & brownish white Hard clay, this Bluff is about 170 or 180 foot high here.
proceeded on pass the Bluffs Compsd,of a yellowish red, & brownish White Clay which is a hard as Chalk this Bluff is 170 or 180 feet high.
Finding the Bluffs Today
Construction of the Gavins Point Dam in 1952-1957 significantly disturbed the chalk cliffs located immediately downstream. A few exposures remain on the Nebraska side, including a narrow bluff immediately east of the spillway and a low-lying cliff hidden in a birch tree woodland between the boat ramp parking area and Route 12. At this latter outcrop, the Niobrara Formation is comprised of medium gray to white chalk that was fractured extensively into tabular sheets and blocks. It has the appearance of a well-worn white masonry wall.
The Niobrara Formation is spectacularly exposed above the waterline on the upstream side of the dam all the way past the Bon Homme-Yankton County line. These upriver exposures are, as Clark noted, not pure white but yellowish orange, suggesting either depositional impurities or post-depositional staining.
'A Hard as Chalk'
The Niobrara Formation is one of the most extensive rock units in the Great Plains, and was deposited between 87 and 82 million years ago during the Late Cretaceous Period.2 Most chalk deposits worldwide were formed during the Cretaceous Period, the most famous being the White Cliffs of Dover. They record a time when global sea levels were at one of their greatest heights in the last 570 million years, due to several factors including the break-up of the supercontinent of Pangaea. Chalks are so representative of this time period that the word Cretaceous was derived from the Latin word for chalk—creta.
Lewis and Clark knew how to recognize chalk, even though they opted to initially identify these chalk exposures cautiously as "Clay which is a hard as Chalk."3 When the captains had the opportunity to closely examine the formation, it's easy to imagine them chipping off a piece and perhaps running it across one of their writing slates to find it produces a distinctive streak as strong and dense as schoolhouse chalk. Whether or not they did so, they subsequently labeled a specimen as Fort Mandan mineralogical specimen No. 52. It became one of the key mineralogical samples sent back East.
John W. Jengo
John Jengo is a professional geologist and licensed Site Remediation Professional who works for an environmental consulting firm in Pennsylvania, specializing in hydrocarbon remediation and dam removals to restore migratory fish passage. He has published numerous articles in We Proceed On since 2002 on the subject of Lewis and Clark's mineral collection and the significance of scientific influence of their geological discoveries.
Articles on this site by John W. Jengo:
- 1. John W. Jengo, "'Blue Earth,' 'Clift of White' and 'Burning Bluffs'," We Proceeded On, Volume 37, No. 1 (February 2011), the quarterly journal of the Lewis and Clark Trail Heritage Foundation. Editorial additions include page titles, side headings, and graphics to assist the web-based reader. The original printed format is provided at http://lewisandclark.org/wpo/pdf/vol37no1.pdf#page=8.
- 2. Michael J. Everhart, "Revisions to the Biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas," Transactions of the Kansas Academy of Sciences, 2001, Vol. 104, 56-75.
- 3. Moulton, Journals, 3:38. I wonder if the absence of the technical rock description of chalk in Clark's Field Notes and Notebook Journal on August 26th (and Ordway's and Whitehouse's journals also) indicates that they observed a clay facies of the Niobrara Formation on August 26th or simply observed a chalk bluff from afar but did not get close enough to ascertain its true lithology until the next day.