South Boulder Creek is the most prominent hydrologic feature and carved the sheer cliffs of Eldorado Canyon during the uplift of the Front Range. Other tributary drainages include Rattlesnake Gulch, South Draw, Johnson Gulch and an unnamed drainage north of Johnson Gulch.
The cliffs and slopes of Eldorado Canyon provide excellent exposures of rocks that enable us to unravel the complicated geology of the area. The sheer walls of the canyon provide world-class rock climbing routes due to the significant relief and the presence of natural fractures that expose near-vertical rock faces.
Ridges of erosion-resistant sedimentary rock form hogback; thus, the park is characterized by a series of folded and faulted linear geologic features.
Precambrian Boulder Creek Granite occurs as outcrops in the Crescent Meadow area of the park and also underlies formations of the Inner Canyon. A metamorphic quartzite dike is exposed near the Rattlesnake Gulch trailhead, forming the Quartzite Ridge and Supremacy Rock features.
Paleozoic Lyons and Fountain Sandstones, tilted into "flatirons" by uplifted Boulder Creek Granite, are present in the Inner Canyon. Fountain Sandstone is exposed in formations known to climbers as Bastille, Wind Ridge, Redgarden, West Ridge, Peanuts and Rincon. Lyons Sandstone is exposed in the Inner Canyon forming the feature known as Rotwand.