About 360 million years ago, all of the continental land masses came to form a single land mass, which we call Pangaea (meaning "all lands"). Most of North America was still underwater, although the compression of the continents colliding with one another caused the Appalachian Mountains to rise and a great swamp to form across the midsection of the continent. A few highland areas existed in areas of Texas, Nevada, and the Great Lakes. The first reptiles came into existence during this time, while great layers of organic matter in the swamp collected to later become coal. (This is known as the Carboniferous Period.) After some 75 million years of evolution, the reptiles became what we now call dinosaurs and dominated animal life over the entire planet. The trilobites finally became extinct at this same time (225 million years ago).
Geologic evidence indicates that starting about 200 million years ago, the continent of North America changed course and began drifting away from Pangaea. The Appalachian Mountains split, with a portion now on North America and the rest in what is today Scotland and the Scandinavian Peninsula in Europe. The mountains also stopped growing at this time and have been eroding ever since. The first birds came into existence some 160 million years ago. Most of the dinosaurs suddenly died out 70 million years ago. It was at this time that the Rocky Mountains began to be pushed up in the western portion of North America. The cause was probably the rapid speed at which the continent was moving westward.
Current evidence indicates that the first primates were also coming into existence about 65 million years ago, as mammals replaced the dinosaurs as the dominant species on all the continents, except Australia. Elephants appeared 45 million years ago, with the mammoth coming to populate North America. After over 60 million years of evolution, the primates eventually evolved into humans--a mere 2 million years ago (in the form of the Homo habilis species). Modern humans (the Homo sapiens species) evolved 40,000 years ago, at about the same time as what may have been the earliest crossing of humans into North America from Asia. (Most scholars accept that humans crossed into North America 12,000 years ago--evidence of crossing prior to that is somewhat controversial.) The earliest recorded history of any civilization dates back 5,000 years. Our modern calendar dates back 2,000 years. The modern period of European settlement in North America is only 500 years old. If the 110-story Sears Tower* in Chicago were to represent the entire history of the planet earth, the top 12 inches would be that in part which the various human species have been present here.
There are two types of glaciation: continental and alpine. Alpine glaciers are still seen in the high mountains of the western US. Continental glaciers no longer exist but were significant in shaping the landscape of the northern Interior Lowlands. The Pleistocene Epoch (a.k.a. the Great Ice Age), started about 2 million years ago and has been characterized by several periods in which glaciers advanced from the north to blanket large portions of North America and Eurasia. Up to a mile (5,280 feet) thick, these glaciers scoured the land and flattened the hills which they covered. The last of these glacial periods ended about 12,000 years ago. The glaciers have left behind numerous small lakes (kettles) and erratically meandering (or lost) rivers. The Ohio and Missouri Rivers roughly mark the southern boundary of the glacial advances.
US in Carboniferous Period - about 300 million years BP