Grand Canyon’s Blacktail Canyon…….bottoms up!

Grand Canyon ExpeditionsGCEX Main Leave a Comment

Photo Tom Blagden (tblgden@bellsouth.net) Above photo, Tapeats Sandstone, this layer averages about 545 million years old.

Lying on your back,  looking up at Blacktail Canyon (RM 120) on the Colorado River  in the Grand Canyon is totally surreal.  Not only a grand concert hall for acoustics, it also displays a truly grand view of “The Great Unconformity”.  In  addition to actually touching this geological phenomenon,  I have seen many a geologist literally kiss this 1.2 billion years of missing time!

Inside Blacktail Canyon,  the Great Unconformity is at head height, 1.2 billion missing years right at eye level.  This is a change between rock types so unmistakable and spectacular, so conceptually overwhelming, that once it engages your attention, you watch it weave throughout the Canyon, a reminder of the human ability to conjecture about what has gone before, the briefness of life on Earth compared to eons recorded in this rock, and the need to stretch the gray cells into new dimensions. The Great Unconformity first appears high on the walls of the Upper Granite Gorge at Mile 77.5, marked by what Stanton saw as a “black beaded fringe” of horizontal Tapeats Sandstone  dramatically truncating the vertical and diagonal fins of schist beneath it.  At Blacktail Canyon, the ending of the Upper Granite Gorge drops the Great Unconformity to river level.

This geologic feature is a dramatic break in the rock record.  Tapeats Sandstone forms the layer just above the Unconformity. One such place is Blacktail Canyon, where the Great Unconformity represents about 1.2 billion years of missing time.  This amount of time adds up to 25% of Earth’s history! What went on in the Southwest during this vast time gap? Luckily, the Grand Canyon Supergroup records parts of this history and offers us a look into a time when very significant changes were happening on our planet.

References Cited: Ann Haymond Zwinger, “Downcanyon”: a Naturalist Explores the Colorado River through the Grand Canyon. www.gcrg.org/bqr/12-1/supergroup.html

Leave a Reply

Your email address will not be published. Required fields are marked *

For security, use of Google's reCAPTCHA service is required which is subject to the Google Privacy Policy and Terms of Use.

This site uses Akismet to reduce spam. Learn how your comment data is processed.