Stebbins Gulch Glacial Erratic

Glacial erratic boulder half buried in the streambed

Close examination of this large boulder shows a texture of interlocking light and dark-colored crystals typical of granitic rocks. The enormous boulder was transported to the Gulch by Ice Age glaciers sometime during the last 1.8 million years, perhaps as recently as 15,000 years ago. The rounded shape is typical of granitic rocks, even when they haven’t undergone stream transport. The less stable minerals in the rock are the first to decompose under chemical weathering, especially in exposed areas like corners and edges of the rock. Over time, the rock takes on a rounded appearance. Another interesting question is, how much does this boulder weigh? Granite has an average density of 2.7 grams/cubic centimeter. The rock is shaped roughly like an oblate ellipsoid with radii of 3 feet x 2 feet x 1 ½ feet. Doing the math and conversions yields a weight of over 6,300 pounds.

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