Located about 10 miles southwest of Denver, Colorado, Red Rocks Amphitheater is one of the most beloved concert halls in the world. From the classic symphonies and John Denver’s Beatles and U2the list of artists who have played at Red Rocks since it began hosting concerts in 1908 is almost as long as the amphitheater’s 2.5 miles of seats.
The reason for his high regard? A combination of natural geology, man-made architecture and near-perfect acoustics. But let’s start with the rocks themselves, which take hundreds of millions of years to form.
Benjamin Burke, an associate professor at the Colorado School of Mines, explains that the Red Rocks are essentially an ancient delta—a 300-million-year-old accumulation of sedimentary rock that once belonged to what is known as the ancestral Rockies. Like today’s Rocky Mountains, these mountains were very high. But the forces of erosion (wind, rainfall, and temperature) worked to bring silty, sandy, muddy waters downstream along a series of large rivers.
These materials created numerous deltas in the area, including the Flatirons region west of Boulder, Roxborough State Park near Littleton, and the Garden of the Gods west of Colorado Springs. Together, the Red Cliffs and other areas form the Fountain Formation. Although the composition of the region varies, the material that forms the Red Cliffs is mainly sandstone of the poorly sorted variety; it ranges from sand grains to larger cobbles, Burke says.
After ancient storms sent mud and sand flowing down the mountains, they were slowly compressed into rock through the geological process of lithification. Subsequent erosion of this rock has carved out the area where the amphitheater stands today. “The two rocks on either side of the amphitheater are a bit more stable [to erosion], for whatever reason,” says Burke, referring to Ship Rock — the iconic redstone face to the audience’s left — and Creation Rock to the right. Both stunning, sloping vertical rock formations are taller than Niagara Falls.
(Credit: Megan Mahoney Photography/Shutterstock)
The upper portion of the Red Rock seating area, which provides the best view from the amphitheater of Denver and the surrounding landscape, includes a bronze plaque marking where Fountain Formation rock meets the 1.7 billion-year-old Precambrian rock that formed bottom of the Denver Basin, Burke says. Older rocks in the area were mined for silver and gold during various bullion rushes between about 1858 and 1893, he added.
In 1911, three decades before it opened to the public in its current architectural form, the world-renowned Scottish opera singer Mary Garden performed the first solo concert in the Red Rocks. She remarked that she had never heard more perfect acoustic properties in any opera in the world.
Burke, who attends at least one show a year at the venue, admits he’s not an audio engineer. But he offers a basic explanation: Both Creation Rock and Ship Rock, combined with the stage infrastructure built directly in front of the disc-shaped rock known as Stage Rock, create an ideal transfer of sound.
“The building and the rocks in front of it really send the sound out to the audience,” he says. “The [rock] the walls essentially support that sound without having a roof where there should be a silencer. So you get a lot of projection out without the reverb, if you will, of the enclosed space for [sound] to jump around.’
The role of rocks in the acoustic profile is probably due more to size and angle than composition, he continues. While sandstone generally lacks any special acoustic properties, it may have some minor sound-absorbing properties compared to harder rock such as granite. “In general, rock is rock,” says Burke. “The fact that those walls are there allows the sound to stay in the amphitheater, but not get trapped and bounce back and forth.”
The Beach Boys perform in concert on July 9, 2012 (Credit: TDC Photography/Shutterstock)
There is also the aspect of height. At 6,450 feet, sound actually travels a little slower than at sea level, thanks to both air pressure and temperature. Although this in itself is not noticeable to the human ear or brain, a singer’s performance in thinner air can be.
“It’s always fun, as a Denver native, to see artists rise from sea level,” says Burke. “If the performer doesn’t know what they’re getting into, it kind of changes the show a little bit. They end up a little out of breath.”