When Charles W. Cook first viewed the canyon after traveling west from the Lamar Valley on September 20, 1869, he subsequently wrote these words in his journal: I was riding ahead, the two pack animals following, and then Mr. Folsom and Mr. Peterson on their saddle horses.

[2]A year later during the Washburn expedition, on August 30–31, 1870, Lt. Gustavus C. Doane described the canyon with a bit more scientific detail: As we approached the Grand Cañon a dull roaring sound warned us that the falls were near at hand.

I had descended the cañon at a point where the creek joined the river, precipitated into a gorge just above its juncture in a lovely cascade of three falls, in the aggregate 100 feet in height.

At the foot of the gorge and on the margin of the Yellowstone stood a high promontory of concretionary lava, literally filled with volcanic butternuts.

The walls of the cañon are of gypsum, in some places having an incrustation of lime white as snow, from which the reflected rays of the sun produce a dazzling effect, rendering it painful to look into the gulf.

In others the rock is crystalline and almost wholly sulphur, of a dark yellow color, with streaks of red, green and black, caused by the percolations of hot mineral waters, of which thousands of springs are seen, in many instances, flowing from spouts high up on the walls on either side.

In some places masses of the rock have crumbled and slid down in a talus of loose material at the foot; in others, promontories stand out in all manner of fantastic forms, affording vistas of wonder utterly beyond the power of description.

There are perhaps other canons longer and deeper than this one, but surely none combining grandeur and immensity with peculiarity of formation and profusion of volcanic or chemical phenomena.

The canyon below the Lower Yellowstone Falls was at one time the site of a geyser basin that was the result of rhyolite lava flows, extensive faulting, and heat beneath the surface (related to the hot spot).

The chemical and heat action of the geyser basin caused the rhyolite rock to become hydrothermally altered, making it very soft and brittle and more easily erodible (sometimes likened to baking a potato).

According to Ken Pierce, U.S. Geological Survey geologist, at the end of the last glacial period, about 14,000 to 18,000 years ago, ice dams formed at the mouth of Yellowstone Lake.

When the ice dams melted, a great volume of water was released downstream causing massive flash floods and immediate and catastrophic erosion of the present-day canyon.