Some Geological Features at Whitecap Mountain, Rumford, Maine

The relatively easy and short hike up Whitecap Mountain in Rumford affords the traveler spectacular views of the Ellis River valley and surrounding mountains. Maintained by the Mahoosuc Land Trust, this 751 acre preserve includes the summit (2,214 feet) and south slope of the mountain. The nearly bald summit extends for almost a mile from southwest to northeast and provides an excellent opportunity for examination of granite, pegmatite, and glacial features.

Pegmatite

Pegmatite is a very coarse-grained igneous rock, like granite in composition, but with much larger crystals, usually measured in inches across and sometimes in feet! On Whitecap, the pegmatite consists of mostly large crystals of clear quartz, white feldspar, and silver muscovite. On a sunny day, this rock brilliantly reflects sunlight, justifying the mountain's name and necessitating sunglasses!

Some examples of pegmatites:

Figure 1

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The Summit

Most of the summit of Whitecap is pegmatite along with some medium-grained granite. There are some interesting textures in the pegmatite that bear on the conditions of its origin.

Some examples of the granite and pegmatite from the summit area:

Figure 7

Figure 8

Figure 9

Glacial Features

All of Maine experienced several glacial episodes, with ice many thousands of feet thick covering all of the state periodically during the past 2 million years. This area most recently became ice-free about 12,000 years ago, but the glacial ice left a profound impact on the landscape: scraping and rounding the mountain tops, deepening and broadening the valleys, and choking the valleys with enormous quantities of eroded materials.

Here are few examples of glacial features that can be seen on Whitecap:

Figure 10

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Figure 13

Don't pass up the blueberries in the late summer and fall!

Additional Information

Moench, R.H., and Hildreth, C.T., 1976, Geologic map of the Rumford quadrangle, Oxford and Franklin Counties, Maine: U.S. Geological Survey Map GQ-1272, 1:62,500 scale.

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Text and images by Robert Marvinney

Originally published on the web as the January 2012 Site of the Month.

Last updated on January 9, 2012

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