The Rampart Fault divides the Garden of the Gods Park. Rocks on the west side of the park are at an angle of 45 degrees or less. It is here that the rocks of the Fountain Formation, such as Balanced Rock, are on display. To the west were the Ancestral Rocky Mountains, formed 300 million years ago. Erosion washed down unsorted sand and pebbles of many sizes from the nearby Ancestral Rocky Mountains. By 250 million years ago these mountains were eroded away, leaving behind sediments piled up as gravels in layers that formed the Fountain Formation. This rock unit, up to 4,500 feet thick, has a dark red color from the chemical alteration of iron minerals.
Rocks east of the Rampart fault have been tilted more than 90 degrees from their original, horizontal position, such as the North Gateway Rock, which is formed from ancient sand dunes when the area was much drier and windier 280 million years ago when all the continents were joined into one giant landmass known as Pangaea. Today, geologists call this rock formation the Lyons Sandstone which is composed of uniform sized grains of sand. The Lyons Sandstone was deposited largely in a desert environment, and oxidation of iron to hematite caused the red color.