Horizontal layers if sedimentary rocks are found primarily in the Interior Plains, the Great Lakes, St. Lawrence Lowlands, the Hudson Bay Arctic Lowlands and the Rocky Mountains. Portions of the Appalachians and the Innuition Mountains are composed of sedimentary rock. This rock was once at the bottom of some prehistoric sea. Metamorphic Rock Geologists know that igneous and sedimentary rock are often changed into metamorphic rock. Metamorphic rocks are formed when igneous or sedimentary rocks are subjected to heat, pressure and certain fluids.
Heat allows the chemical components of the rock to recombine more easily.
Pressure does the same, but only at great depths, such as 10 km. Fluids that are often mixtures of hot gases and chemical solutions allow rocks to recrystalize. Metamorphism often occurs when magma or fluids are forced into cracks in existing igneous or sedimentary rocks. The surrounding rocks are changed into metamorphic rocks. The Canadian Shield is composed of large numbers of metamorphic rocks.