Glacier National Park Colored Rocks: Where to See Them and Why They Glow

Glacier National Park Colored Rocks: Where to See Them and Why They Glow

Why Glacier's Rocks Turn Red and Green

All of it comes down to iron. Roughly 1.5 billion years ago, this part of Montana was the floor of an inland body of water called the Belt Sea, and layer after layer of silt and mud settled into it. Over about 100 million years those layers compressed into rock: quartzite, mudstone, limestone and argillite, together known as the Belt Supergroup. The argillite carries a lot of iron, and iron is what changes color depending on its surroundings.

Where the sediment sat in shallow water and was later exposed to air as the Belt Sea pulled back, the iron oxidized, the same chemistry that rusts an old car bumper, and the rock turned red and purple. Where the sediment stayed underwater on the sea floor, starved of oxygen, the iron bonded with silica instead and formed a mineral called chlorite, which reads as green and blue. Geologists name the two main layers after features in the park: the red layer is the Grinnell Formation, the green layer is the Appekunny Formation. Both are argillite. The only difference is what the iron in each one was doing 1.5 billion years ago.

Where to See the Colored Rocks

• Lake McDonald shoreline, near Apgar Village. The most reliable spot. Walk out onto the beach at the foot of the lake and the pebbles underfoot are a mix of red, green, gray and purple argillite, worn smooth by the lake. This is the location the park's own geology page points to first.
• McDonald Creek. The creek that feeds Lake McDonald carries the same rock. Where it runs shallow and clear along the road between Apgar and Lake McDonald Lodge, you can see the colored stones on the creek bed without leaving the car pullouts.
• Going-to-the-Sun Road roadcuts. The road climbs straight through the Lewis Range, where both the Grinnell and Appekunny formations are exposed in the cliff faces above the pavement. You will not stop and pick through gravel here, but you can see the red and green bands in the rock itself as you drive.

Get the Colors to Show

Dry argillite looks like ordinary gray rock. Wet argillite is where the color comes out, the same reason a wet stone always looks more saturated than a dry one. At Lake McDonald, that means either working right at the waterline where wave splash keeps the pebbles wet, or carrying a water bottle to splash a patch of shore before you shoot it. Overcast light or the hour after sunrise works better than full midday sun, which washes out the color in a photo the way it does on any wet rock.

Leave Them Where They Are

Taking rocks out of a national park is against federal regulations, and Lake McDonald's beach has thinned in places from decades of visitors doing exactly that. Photograph them, and put back anything you pick up to look at closer.

Plan the Rest of the Trip

Lake McDonald works as a stop on its own or as the first afternoon of a longer trip. Our 4 day Glacier National Park itinerary starts on the west side at Lake McDonald before crossing Going-to-the-Sun Road, so the colored rocks and the drive through the rock that made them happen back to back.