Thursday, 13 October 2016

The Colored Pebbles of Lake McDonald

In The Glacier National Park of U.S. state of Montana, close to the border with Canada, is home of over 700 lakes. However, out of them only 131 of these lakes have names and about 200 lakes are over five acres in size, and a dozen of them surpasses thousands of acres, which in rare for mountain lakes. Moreover, the lakes waters are extraordinarily clear, owning to the yearlong low temperatures that forbid the growth of planktons. It is not rare to see details on the bottom of the lakes at depths of 30 feet or more. 

Furthermore, the one of most prominent feature of some lakes is the existence of a variety of colored rocks and pebbles just beneath the water surface and on the shores. Thus the rocks series color varies from maroon to dark red, and from blue to green. However, colored pebbles are seen in plenty on the shores of Lake McDonald on the western side of the park. The stunning lake McDonald is the largest of the lakes of Glacier National Park with a surface area of 6,823 acres, also the longest, at over 15 kilometers, as well as the deepest lake at 141 meters. The Lake McDonald is home to various native species of trout, and other game fish. However, catchable species include, but are not limited to west-slope cutthroat trout, rainbow trout, bull trout, lake trout, Lake Superior whitefish, mountain whitefish, kokanee salmon, and suckers.

The beautiful green rocks can be seen at “Otokomi Lake”, whereas the dark-colored rocks found at the upper end of Lake McDonald, along McDonald Creek and around Trout Lake are the result of subjecting the red and green iron-rich rocks to heat and pressure. These natural rocks are actually all around Glacier National Park, and were created at different eras. Because, when the glaciers came, it broke down the rocks into miniature fragments and the rivers washed them away. Several of these got deposited onto the lakes and "tarns" lakes formed by filling the bottoms of ice-scoured amphitheaters. Although the water erosion then rendered them into smooth pebbles!

Quoting from the book “Glacier-Waterton International Peace Park” by Vicky Spring and Tom Kirkendall:

The rock color is determined by the presence or absence of iron and the bright red rocks found along the Grinnel Glacier trail were deposited in a shallow ocean environment where the iron was oxidized by the tidal exposure to the air. Further, rocks with this coloration frequently have old ripple marks or ancient mud crack lines. The rich green-colored rocks were shaped in deeper water than the red rocks. Though these rocks comprise the same quantities of iron-bearing minerals, they did not have the same revelation to oxygen and the amount of oxidization was limited.

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