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The Geology of Mount Rainier National Park



Within the Park the type of rocks exposed are almost endless in variety. Although volcanic and plutonic rocks predominate, the lesser types include sediments and metamorphics. For the purpose of mapping, all these types have been placed into four, more or less distinct, formations. For two of these formations, the writer has followed the groupings suggested by Smith and Calkins. (34) The other two are somewhat individual and they will be defined later.

The oldest and smallest formation in the Park is an extremely small outcrop of carbonaceous sediments located near the snout of the Carbon Glacier. The base of these sediments has been replaced by the invasion of a granodioritic batholith and the top is covered by a concordant series of siliceous rocks resembling felsitic tuffs. Tentatively the carbonaceous sediments have been referred to the Puget Series.

The second group, known as the Keechelus andesitic series, occurs above the first formation and, in places, overlies it with an angular unconformity. The rock types consist of altered and indurated massive tuffs and breccias with subordinate amounts of flows, porphyries, hornfels, sediments, and many other heterogeneous varieties. This group extends in an almost complete circle around Mount Rainier and forms the most important areal unit inside the Park boundaries.

The third formation is granodioritic in character and has been correlated with the Snoqualmie granodiorite. Because of its invasion into both of the above mentioned groups it may be regarded as the basement rock in this vicinity. The outcrops are limited to regions where glacial action has been effective in removing the older rocks above. Exposures are found near the termini of the larger glaciers, such as the Emmons, Winthrop, and Nisqually, as well as at the base of the Tatoosh Range.

A considerable erosion interval followed the invasion of the granodiorite and, as a result, a distinct unconformity separates these three older groups from the youngest, and fourth group, the lavas and pyroclastics of Mount Rainier.

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Last Updated: 28-Mar-2006