“Massive in scale, each monument towers hundreds of feet above the floor of the North Platte Valley. Each is made of stacked layers of sandstone, siltstone, mudstone, and volcanic ash. Differential erosion of these layers-some soft, some hard-gives each monolith a distinct shape. Courthouse Rock and Castle Rock are blocky and rectangular, reminiscent of colossal buildings. Chimney Rock looks like an upside-down funnel. Its lower section consists of soft strata that erode into slopes, whicle the upper chimney section is composed of sandstone tough enough to stand as a vertical column.
The local Sioux knew what they saw in the towering phallus of Chimney Rock. They called it Elk Penis. This was too graphic for most white sensibilities, even those of the rough fur traders who, in the 1830s, were among the earliest whites to report on the rock.”