Lake McDonald: Montana’s Colorful Rocks

Lake McDonald looks like an ordinary mountain lake from the road. Step to the shore, though, and the shallows become a display case: pebbles in red, green, purple, and rust, each one sharp and clear under the water. They aren’t painted or brought in. Their colours come from iron, from an inland sea that disappeared over a billion years ago, and from the ice that later carved the valley. Here is how it all fits together.

Multicoloured stones line the clear shoreline of Lake McDonald in Glacier National Park.

Where Is Lake McDonald?

Lake McDonald lies on the west side of Glacier National Park in northwestern Montana. According to Wikipedia, which cites park service figures, it is the park’s largest lake, about 10 miles (16 km) long and more than a mile wide, in a valley formed by erosion and glacial activity. Depth figures vary slightly between sources, but it is close to 470 feet (about 140 metres) deep.

Why Are Lake McDonald’s Rocks Colourful?

The answer begins with the Belt Supergroup, a thick stack of Precambrian sedimentary rock. The National Park Service says you can see these layers above the treeline across about a third of the park. Estimates put their deposition roughly 1.4 to 1.6 billion years ago in a shallow inland sea known as the Belt Sea. Much of the rock is argillite, a fine-grained rock that held a lot of iron. Iron is reactive, and what it did next decided each pebble’s colour.

How Iron Makes Red Rocks

As the Belt Sea retreated, the argillite was exposed to air, and the iron in it oxidised, according to the park service. It is the same basic chemistry as rust, producing iron oxide minerals such as haematite. That is where the reds and purples come from. A guidebook quoted by Amusing Planet notes that red rocks often keep ancient ripple marks or mud cracks from the shallow water where they formed.

Why Some Rocks Are Green

Argillite that formed underwater without oxygen went through the opposite process. The NPS geologic formations page explains that iron bonded with silica compounds instead, and under heat and pressure those minerals became chlorite, which gives the green and blue-green shades.

What About the Darker Pebbles?

The same guidebook says the dark rocks near the upper end of the lake and along McDonald Creek are red and green rocks that were subjected to even more heat and pressure.

First Image source / Flickr

Red, green, and earthy-coloured pebbles line the shore of Lake McDonald in Montana.

Source/flickr

How Glaciers Shaped the Lake and Its Stones

The colours came first, and the ice came much later. During the Ice Age, glaciers ground through these mountains and carved wide U-shaped valleys. The park service says the ice retreated from the area about 10,000 years ago. Lake McDonald fills one of those valleys, and glacial action broke and moved the rock that water later wore into smooth pebbles. Two separate processes, chemistry and ice, explain what you see.

Why Are the Stones So Easy to See?

A Scripps News report citing the park service says the lake’s surface rarely rises above 50°F. Those cold temperatures limit plankton growth, which keeps the water clear enough to see details on the bottom as deep as about 30 feet. Wet stones also tend to look richer than dry ones, which is why the colours seem to jump at the waterline.

[Image/flickr]

Can You Take the Rocks Home?

No. Removing rocks and other natural objects from national parks is not allowed, and the stones are part of the lake’s natural shoreline. Refer to the park’s Lake McDonald Valley page for current rules.

Lake McDonald Quick Facts

• Location: Glacier National Park, Montana, USA
• Size: about 10 miles long, more than a mile wide
• Depth: roughly 470 feet
• Rock: Belt Supergroup argillite
• Red and purple: oxidised iron
• Green and blue-green: chlorite
• Valley: shaped by glaciers

More Natural Wonders on Ritebook

Another lake known for its colour is Hanging Lake in Colorado, while Al Naslaa Rock in Saudi Arabia is a geological puzzle of a different kind. You can find more lakes and geological wonders like these under the Geology and Lakes labels.

[Image/flickr]

Best Time to Photograph the Colourful Rocks

Lake McDonald looks different throughout the day and across the seasons. The colours also appear differently in photographs, depending on the light, cloud cover, water conditions, and the wetness of the pebbles. Soft daylight can help reveal differences between red, green, and darker stones without creating harsh highlights. Cloudy conditions can also help even out colour capture.

If you want to photograph individual pebbles, try a close-up composition with the shoreline in the background. For a broader photograph, include the clear water and mountain scenery to show where you can find the stones. Wet pebbles can look more vivid, but you do not need to move them or disturb the shoreline to get a striking photograph. Take care near the water, especially when the stones are slippery. There is no single guaranteed time when the colours are at their best. A little patience and attention to the light can have a bigger impact than the season alone.

Lake McDonald Facts

• Location: Glacier National Park, Montana, United States.
• Setting: The Rocky Mountains of northwestern Montana.
• Length: Approximately 10 miles (16 kilometres).
• Depth: About 464 feet (141 metres), according to the National Park Service.
• Geological history: Many of the park’s rocks originated from sediments deposited in the ancient Belt Sea.
• Cause of the colours: The colours are caused by iron-bearing minerals and the chemical conditions in which the rocks formed.
• Landscape formation: Glaciers carved the valley that holds the lake today.

For additional information about the park’s geology, the National Park Service provides a useful overview of Glacier National Park’s ancient rocks and geological history.

[Image/flickr]

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The pebbles’ varied colours reflect the mineral composition and the ancient geological history of the region.

[Image/flickr]

Conclusion

Lake McDonald’s pebbles are a record of ancient seas, iron chemistry and ice, all visible in a handful of stones. The colours are natural, billions of grains old in their story, and best enjoyed in photographs and left where they lie.

Sources: NPS: Geology of Glacier National Park · NPS: Geologic Formations · NPS: Lake McDonald Valley · Wikipedia: Lake McDonald · Amusing Planet · Scripps News via WMAR