The Rocky Mountains lie to the west of the Great Plains. If there were no Rocky Mountains, would the Great Plains exist? Yes, the Great Plains were formed from the sediments from the Mississippi River. Yes, the Rocky Mountains and the Great Plains are located on the different tectonic plates and were formed independently. No, the Rocky Mountains are former volcanoes that filled the area around them with magma to form the Great Plains. No, the Rocky Mountains are the source of the sediment that built the Great Plains.