Easternmost is the Foreland Belt, which at the latitudes covered by the guidebook is represented by the Rocky Mountains. It forms the eastern wall of the Cordillera, extends for 50 to 100 km into Alberta, and towers over the Alberta plains. The belt contains a minimum thickness of about 15 km of Precambrian, Paleozoic, Mesozoic and early Tertiary sedimentary rocks. These rocks mostly were laid-down between 700 and 50 million years ago, although in the Purcell Mountains there are strata up to 1.5 billion years old). Igneous rocks are rare in this belt. The rocks were deposited just west of the old continent, and during mountain building between 100 and 55 million years ago were thrust eastwards for at least 150 km, onto the edge of the old continent.