Broken Terrace Wall
November 20, 2003
The photos above were taken last month in Joshua Tree National Park, California (west of Jumbo Rocks) and show two views of a dike formed by molten rock pushing into fractures in granite. This dike feature is also known as a broken terrace wall. Light-colored aplite, pegmatite, and andesite dikes formed as a mixture of quartz and potassium minerals cooled in cracks and later broke into blocks after continuous exposure to the extreme desert weather. The dike pictured above is about a foot thick.