The Gobi desert
The Mississippi river was the border.
The western border of the 13 colonies was formed by the Appalachian Mountains.
Pennslvania's boundries are mostly man made. you can tell that they are if they are perfectly straight. if they are all squiggly then it means they are most likely made by a river. In Pennsylvania's case the eastern border is the Delaware river except for the strange looking manmade border that forms the northern arc on Delaware's northern border. The other strange feature is in the northwest corner of the state near Erie where a point was chosen to draw a manmade line which provided the 51 mile Erie Lake Coastline with access for commerce and the northernmost point in the state is therefore both a natural and manmade border.
Mediterranean Sea
The east was bounded by the Mississippi and the West ( andSouth) by the Missouri River.
The Grand Canyon located in Northern Arizona
the Himalayan mountain and the karakoram mountain
Karakoram and Himalaya mountain ranges.
The Great Lakes were the northern border. The St. Lawrence River was also a border. The War of 1812 had many conflicts on and around the Great Lakes.
delta
Ohio river
Florida's northeastern and eastern borders are formed by the Atlantic Ocean. Its northern border is formed by Georgia and Alabama.