Lake McDonald in Glacier National Park features brightly colored rocks scattered across the lakebed, visible from the surface. The reds, greens, and occasional purples create a striking mosaic under the clear water. This unusual palette results from the region’s ancient geological and chemical history, not pollution or algae. The visibility of these vibrant hues is amplified by the exceptional clarity of the cold, clean water filling the glacially carved basin.
The Geological Foundation of Lake McDonald
The foundation for Lake McDonald’s colorful stones is the Belt Supergroup, an immense sequence of ancient sedimentary layers. These rocks were deposited between 1.6 and 1.4 billion years ago during the Mesoproterozoic Eon, making them some of the oldest sedimentary rocks in North America. The original material consisted of fine mud, silt, and sand that accumulated in a vast, shallow inland sea covering much of what is now western Montana.
Over millions of years, the weight of the overlying sediment compressed these fine-grained deposits into rock types like argillite, mudstone, and quartzite. Argillite is a slightly metamorphosed mudrock. The colors seen today are locked within the mineral structure of these formations, which were later exposed and tumbled into the lakebed by glacial action.
How Iron and Oxidation Create the Colors
The specific colors embedded in the argillite are determined by the chemical state of iron compounds present in the original sediment. Iron is a common element, and its reaction with oxygen dictates the resulting rock color. This process, known as oxidation, is a chemical reaction where iron loses electrons, changing its form.
Red and reddish-brown rocks contain ferric iron (Fe³⁺), which is essentially rust. This coloration developed when sediment layers were deposited in shallow water environments, such as tidal flats, where they were repeatedly exposed to atmospheric oxygen. The oxidation of iron created the stable mineral hematite, which imparts the vibrant red hue to the rock.
The green and gray rocks formed in deeper water where oxygen levels were extremely low, or the environment was anoxic. In these conditions, the iron remained in a reduced state known as ferrous iron (Fe²⁺). This ferrous iron bonded with silica compounds, forming greenish minerals like chlorite under heat and pressure.
The presence of purple rocks often indicates a transitional zone where the oxidation of iron was incomplete, or a mix of both ferrous and ferric iron states exists.
The Environmental Factors That Enhance Visibility
Lake McDonald’s unique environmental factors ensure the visibility of the colorful rocks. The lake is oligotrophic, meaning it has very low levels of nutrients like phosphorus and nitrogen. This low nutrient concentration limits the growth of algae and other aquatic organisms that would otherwise cloud the water or coat the rocks.
The primary source of the water is glacial melt and snowmelt, which is naturally low in suspended sediment and organic matter. This low sediment load and minimal biological activity result in a remarkably high degree of water clarity. Deep light penetration allows sunlight to reach the lakebed and illuminate the brightly colored stones, making the geological tapestry visible from the surface.