The structure at right leads to Clark's name.
It is part of the wooden stairway and walkway to the top.
At four in the afternoon of July 25, 1806, Clark and his contingent of nine men, plus York, Toussaint Charbonneau, Sacagawea, and little Jean Baptiste, arrived at "a remarkable rock Situated in an extensive bottom, on the Star[boar]d. [south] Side of the river & 250 paces from it." That landmark can hardly be seen from the river today, for cottonwood trees, their seedlings no longer fodder for Indians' horses, have filled in the broad riverbank between.
Clark measured the rock's circumference at 400 paces, or about 1,200 feet, and estimated its height at 200 feet. The first dimension was fairly accurate, but the height was off by about 40 percent. Even today, having apparently suffered but little erosion, or even perhaps grown a few inches with humus from dead grass and shrubs, it reaches 121.88 feet or 127.42 feet, depending on the side from which one measures it.1 Climbing to the summit of this "lightish Coloured gritty rock"—actually sandstone of the Cretaceous Hell Creek Formation—Clark found "a tolerable Soil of about 5 or 6 feet thick Covered with Short grass," and took notes on the "most extensive view in every direction."
"Pompy's Tower" from the air, with "romantic Clifts" upstream.
Clark named it "Pompy's Tower," in honor of the Charbonneaus' young son, who sometime that summer had acquired the nickname Pomp. On his map, he named that large brook "River Baptieste" after Sacagawea's 17-month-old son, adding parenthetically, "almost dry."
1. Sixty years later another river traveler, who carried a copy of the 1814 edition journals, and watched for landmarks Clark had mentioned, estimated it to be more than 300 feet high! He didn't stop to climb it. See J. Allen Hosmer, A Trip to the States by the Way of the Yellowstone and Missouri, with a Table of Distances (Virginia City, Montana Territory: Beaver Head News Print, 1867), 19.