7. Between the Rocky Mountains and the Appalachian Mountains is the central lowland, which is called the prairie. 8. Missouri is one of the many tributaries of the Mississippi. 9. In the Pacific Northwest, dense coniferous forests of spruce, tsugi and pine grow. 10. The rocky mountains are considered young, tall, hard to reach and uneven in form. 11. The United States is located in the central part of the North American continent. 12. Mixed forests of birch, beech, maple and pine predominate in the northern part of the Appalachian Mountains and in the highlands of the Great Lakes.