major physical barrier to crossing. Political borders are often classified by whether or not they follow conspicuous physical features on the earth. Natural borders Natural borders are geographical features that present natural obstacles to communication and transport. Existing political borders are often a formalization of such historical, natural obstacles. Some geographical features that often constitute natural borders are: Oceans: oceans create very costly natural borders. Very few countries span more than one continent. Only very large and resource-rich states are able to sustain the costs of governance across oceans for longer periods of time. Rivers: some political borders have been formalized along natural borders formed by rivers. Some examples are: the Niagara River (Canada–U.S), the Rio Grande (Mexico–U.S.), the Rhine (France–Germany), and the Mekong (Thailand–Laos). If a precise line is desired, it is often drawn along the thalweg, the deepest line along the river. In the Hebrew Bible, Moses defined the middle of the river Arnon as the border between Moab and the Israelite tribes settling east of the Jordan (). The United States Supreme Court ruled in 1910 that the boundary between the American states of Maryland and West Virginia is the south bank of the Potomac River. Lakes: larger lakes create natural borders. Examples are Lake Tanganyika, with the Democratic Republic of the Congo and Zambia on its west shore and Tanzania and Burundi on the east; and the Great Lakes which form a substantial part of the border between Canada and the United States. Forests: denser jungles or forests can create strong natural borders. One example of a natural forest border is the Amazon rainforest, separating Brazil and Bolivia from Peru, Colombia, Venezuela and Guyana. Mountain ranges: Many nations have their political borders defined along mountain ranges, often along a drainage divide. Examples are the Pyrenees, which have separated Spain and France or their forerunners for millennia; the Andes separating Chile and Argentina; and the Alps separating Italy, France, Switzerland, Germany and others. Throughout history, technological advances have reduced the costs of transport and communication across the natural borders. That has reduced the significance of natural borders over time. As a result, political borders that have been formalized more recently, such as those in Africa or Americas, typically conform less to natural borders than very old borders, such as those in Europe or Asia, do. Landscape borders A landscape border is a mixture of political and natural