Tuesday, March 13, 2012

Not So Seriously Mysterious Rock

Mystery question of the day:
What is this crumbly white rock scattered all over the red sand, and what do you think it’s doing along the Gila Trail? 

IMG_1109ScatteredMysteriousWhite Rock
What is all this white rock doing in this red sand?

The history of the mystery:
Three cheery friends get together to take a lovely hike one recent sunny winter day… 

Three cheery friends – COC, JC, NF

Our trail meanders for seven miles through boulder–strewn hillsides and rolling sandy flats of the Red Cliffs Desert Reserve in (where else?) southwestern Utah. 

IMG_1100GilaTrail RedClifffsDesert Reserve
Gila Trail
We are surrounded by ancient red sandstone of fossilized dunes. No surprises there! 

IMG_1107Cross BeddingIn Sandstone
Fossil dunes with toupee of black basalt

Along a portion of the trail, we notice the previously mentioned crumbly white rock in all this red sand.
We all take a closer look. 

IMG_1108 MysteriousWhite Rock
Mystery white rock

JC picks up a sandwich of the mysterious white rock. 

IMG_1119Mystery Sample
Inquiring minds want to know!

What’s this? He inquires quizzically. 

It looks like limestone to me, I say authoritatively. 

But secretly I wonder – What is limestone doing in all this sandstone?
Now it’s time for fun with chemistry! 

I subject this mystifying sample to careful chemical analysis in order to determine its likely composition. 

When in doubt, bring out the dilute hydrochloric acid

A mere drop makes the rock fizz and bubble like surf breaking on a shoreline.

Limestone reacts readily to hydrochloric acid

It certainly is limestone. 

What is limestone doing here in all this red rock sandstone? 

All is revealed (well, maybe):
Horizontally bedded red siltstones and sandstones of the Early Jurassic Kayenta Formation are seen in many places in southern Utah. They outcrop as crumbling ledges and slopes just beneath the massive cliffs of the world famous Navajo Sandstone. These ledges and slopes tell a story of an environment that existed around 190 million years ago. In a Utah that existed during a different geologic time than our own, slow–moving rivers and streams coursed across broad floodplains and undulating lowlands far from their headwaters.  

In these ancient mudstones and siltstones the presence of small freshwater lakes and ponds is indicated by thin lenses of limestone found scattered between the layers of rock.

So could our limestone be an eroded remnant from one of these ancient ponds or lakes? 

That definitely seems like a reasonable explanation to me. 
If someone has any other bright ideas I would love to know about them!

Is there anything you have unexpectedly discovered during your adventures both near and far, geologic or otherwise?  Leave a comment!

IMG_9562 ChuckwallaTrail RedCliffsDesert Reserve
What new discoveries lurk along the trails?


Once again…

My Excellent References:
Biek, R.F. et al, 2007, Interim Geologic Map of the St. George 30’x60’ Quadrangle and the East part of the Clover Mountains 30’x60’ Quadrangle, Washington and Iron Counties, Utah, Utah Geological Survey Open–File Report 478
Hamilton, W.L., 1984, The Sculpturing of Zion with Road Guide to the Geology of Zion National Park, Zion Natural History Association
Higgins, J., 2000, Geology of Snow Canyon State Park, Southwestern Utah in Sprinkel, D., Chidsey, T., and Anderson, P., editors, Geology of Utah’s Parks and Monuments, Utah Geological Association Publication 28, p. 478-494


  1. Sometime you'll have to come critique my Yellowstone Rocks Jr. Ranger program for me. It's getting better, but I'm not quite 'there' yet.

    1. Marianne - I would love to! Just let me know when. This gives me a good excuse to play at Madison.

  2. I thought limestone right off as it's so prevalent in the SW.

    I found great zeolites of blue agate in basalt while hiking the Drakensburg in South Africa. My guide new nothing. Fun to figure this stuff out.

    1. People look but they don't see what is there, or even wonder why. I was intrigued about "why" the limestone is there, in all that sand and sandstone.

      There are no other outcrops anywhere nearby, that I know of.

      Cool about your figuring out the zeolites!

  3. It's caliche! Maybe. As groundwater evaporates, calcite is precipitated in the sands.

    1. Sam - Yes it seems like it might be caliche, although more broken up and weathered than is often seen in the desert. Thanks for your comment!