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Our transcription: The mountain building process causes continents to increase in size over time. We know from radiometric dating of rocks that the central portion of the North American Continent is composed of very old rocks, all of which formed over a billion years ago. The Appalachian Mountains were then built on to the eastern margin of the craton in a series of collisions ending about 250 million years ago. The Sierra Nevada Mountains were then added to the western margin of the continent in a process ending about 80 million years ago. Followed by formation of the Cascade Range, which continues even today. In this way, the North American Continent has incrementally grown by accretion in a concentric pattern with the oldest rocks in the center surrounded by younger and younger mountain belts.
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