The Bass Formation is the lowest formation of the Grand Canyon Supergroup, resting on top of the Vishnu Complex and below the Hakatai Shale. It forms layered, stair-stepped cliffs and slopes. The easiest way to identify the Bass Formation, at least for me, is in relationship to the Vishnu and Hakatai. Wherever you see layers beginning to appear on top of the Vishnu, which is unlayered, you are looking at the Bass Formation. And when you see stair-stepped layers beginning to appear beneath the bright red-orange, gentler slopes of the Hakatai, you are entering the Bass.
Few stretches of trail within the Grand Canyon spend much time in the Bass, usually just in quick transit between the Vishnu and Hakatai. The Tonto Trail, near its eastern end, does dip into it for about a mile beginning here, in Mineral Canyon. The Tonto Trail begins at river level at Hance Rapids, and then follows a platform of Hakatai Shale gradually upward for 4 miles to reach its usual place on the Tonto Platform. In Mineral Canyon, though, the trail drops down to cross the wash at the bottom of the canyon, and that drop brings you down into the Bass Formation for a time.
Some years ago the Bass Formation had its name changed from its original name of Bass Limestone. In Mineral Canyon you can clearly see why. There are lots of different kinds of rocks in the layers that make up the Bass, not just limestone. As you descend down to the wash on the eastern side of Mineral Canyon you pass through a thick layer of loose rocks and cobblestones, called the Hotauta Comglomerate, that could not look any less like limestone. Some ancient river system ground down, polished, and deposited those cobblestones as just one of the many layers in the Bass Formation some 1.2 billion years ago.
Here is an example, on the North Kaibab Trail, of the Bass Formation dramatically changing the lay of the land. When you head up the North Kaibab Trail from Phantom Ranch, you are in what is called the Box for four miles. The Box is a narrow canyon cut by Bright Angel Creek through the hard, resistant Vishnu basement rocks. Near the end of the Box, you see some layered rocks beginning to appear high up in the walls of the inner canyon, which are in the Bass Formation. As you climb up, at this spot, through the Bass Formation, the inner canyon immediately begins to spread out from the narrow confines of the Box, and soon spreads out even further in the Hakatai.
The lower portion of the Thunder River Trail, as it follows along Tapeats Creek, does spend a fair amount of time in the Bass Formation. Starting at Upper Tapeats Campground, the trail passes through the Shimuno Quartzite, Hakatai Shale, and Bass formation on its way down to the Colorado River. As you head downstream, the formations are tilted upward, and present you with a series of shelfs and cliff faces that you have to pass through. Here we are looking at a cliff face band within the Bass Formation that has emerged from the creek about a half mile upstream. As with each of these bands, the trail, upon reaching the band, follows the platform above the cliff for some time before finding a break, then drops down quickly to reach this spot, back down near the creek.
Here we are further down Thunder River Trail, just above the mouth of Tapeats Creek at the Colorado River. Between the last spot and this, the trail has lurched up and down several more times as it has passed through bands in the Bass Formation, which can be seen clearly from here on the far side of the canyon across the creek. The contact line between the Bass Formation and the Vishnu Complex is quite dramatic here, with the light, layered Bass above and the dark, amorphous Vishnu rocks below.
In most places the layers of the Bass Formation, though tilted, still appear to be straight. But not always. The layers of the Grand Canyon Supergroup, including the Bass, have been twisted and turned by faults for over a billion years. Here, from a spot on the Tonto Trail just east of the South Bass Trail, we are looking down at Wheeler Fold, a stretch of Bass Formation that has been bent nearly 90 degrees in the space of a few hundred yards. Look around at the walls of the inner canyon and you can spot lots of folds like this, though usually not quite this dramatic. But look around in the panorama at the Devil’s Corkscrew on the South Kaibab Trail, and you can see another quick 90 degree fold in the Bass Formation like this, but tilted up instead of down.
To read more detailed geological information about the Bass Formation, visit the excellent Wikipedia page for the Bass Formation.