Where did it get that name?

Every now and then (especially at work) I am asked:

"Why is a formation named that?"
"How did that formation get its name?"
"Is that [town/landmark/place] named for the formation?"
"Where is the original [or type] location for that formation?"

All interesting questions!

The US Geological Survey maintains a great resource online - the GEOLEX, or "Geologic Names Lexicon." You can use this site to search for the answers to questions like the ones above, and many other things! It is a great resource I utilized when working on paleontological resource reports for the National Park Service over the past four years.

There have been two of these questions recently brought up on the Dinosaur Mailing List (or DML). The first asked about the origins of the Late Jurassic Morrison Formation:

"When was the Morrison Formation named? It was named after the town and not vice versa, right?" [link]

Great question!

Dr. Tom Holtz gives a great explanation:

"Yes, the Formation is named after the town. Standard stratigraphic practice is to name a formation or other lithostratigraphic unit after a geographic locality (typically a town) near the stratotype section (the lithostratigraphic equivalent to a holotype specimen)." [DML link][USGS link]

Another great question was asked the next day:

Does anyone know how the Dewey Bridge Formation got its name (was there an actual bridge? where? when was it named?)? [link]

This one was easy for me, as Dewey Bridge is less than an hour from my house and I drive by it often when I go to Moab, Utah. It is off Highway 128 and crosses the Colorado River.


View Larger Map

So I went to the GEOLEX website to see when it was named and to see if it was indeed named for this particular bridge. The entry reads:

"Usage: Dewey Bridge Member of Entrada Sandstone (CO*,UT*)...Type section: measured near Dewey Bridge, near center of sec. 8, T. 23 S., R. 24 E., Grand Co., CO (Wright and others, 1962)." [link]

There is a Grand County, Colorado, but I am not familiar with a Dewey Bridge there. Oddly enough, the Dewey Bridge in Utah is also in Grand County, which got me to thinking there might be a mistake. So I went back to the USGS, this time looking at their Map Locator tool in the store area. I typed "Dewey Bridge" into the search box provided and *bam!* it takes me to Grand County, Utah. Not yet fully convinced, I click on the Dewey Quadrangle the locator found and download the quad. When I zoom into the Dewey Bridge area (see picture below), sure enough, the locality information given by the USGS matches. So I think, in this one small case, there is a tiny, but easily fixed and figured out, mistake on the USGS page.

Another way I could have done it was just to look for the Entrada Formation on the geologic map of Colorado in Grand County, but the only Jurassic I saw there upon quick look is Morrison Formation. I know from personal observation that the Entrada Formation is present in the Dewey Bridge area of Grand County, Utah. While the USGS recognizes the Dewey Bridge as the oldest member of the Entrada Formation in this area, the Utah Geological Survey includes it in the underlying Carmel Formation (it seems to vary between the two depending on you you talk to).

Picture from the Grand County, Utah webpage, photo Courtesy of Rim Rock Road Runners

Some fun info about Dewey Bridge (pictured above) - it was built in 1916 and at that time connected this then remote portion of Utah with Grand Junction, Colorado, the areas largest city. It was listed on the National Register of Historic Places and was also the longest suspension bridge in Utah until its destruction. On April 6, 2008, a six year old boy who was playing with matches at a nearby campground accidentally set the bridge, and surrounding area, on fire (read the full account here). The county is now in the process of raising funds to restore the bridge [link to more information].

Picture from the Grand County, Utah webpage, of the April 6, 2008, fire at Dewey Bridge.

A picture I took this summer (August 25, 2009) of what is left of Dewey Bridge with the Entrada Sandstone in the background.

I hope these two tools, the USGS map locator and GEOLEX, are either something you already knew about and get use from or will help you to answer your own geologic questions someday!

Post and images © ReBecca K. Hunt-Foster, unless otherwise stated. Thanks to Tom Holtz and Donna for permission to use their quotes.