From up to 10 million years ago, Montana had a tropical climate similar to the present Caribbean. The red soil that lies on top of the Renova Formation has the chemical and mineral composition of modern laterites, or tropical soils. The tropical climate coincided with the huge lava flows that built the Columbia Plateau. Some of the lava flows dammed rivers, forming lakes. Laterites deposited in these lakes are sandwiched between black basalt and contain perfectly preserved fossils of tropical leaves similar to those found in Florida and the Caribbean. Streams began to flow during the tropical period.
About 10 million years ago, the tropical climate changed to a desert even drier than the first dry spell. Montana’s climate from 10 to 2.5 million years ago was similar to today’s Death Valley. The brown gravels with layers of sand and mud that were deposited in the mountain valleys during the second dry spell make up the Six Mile Creek Formation. The Six Mile Creek Formation typically rests on the red tropical laterites or on the Renova Formation. It does not typically contain plant fossils, as the climate was too dry to support much plant life. However, the sparse vegetation in desert regions is very nutritious, so there were some large mammals that grazed in the region during this time. Occasional fossils of early horses and camels are found.
At about 2.5 million years ago, the climate became wetter again. Streams began to flow once more, and green plants returned to Montana. Accompanying this climate change was the last developmental phase of Montana’s landscape: the beginning of the great ice ages. Ice ages are periods in Earth’s history when the global climate was a bit wetter and a few degrees cooler. Each year in the mountains and at the high latitudes, more ice accumulated than melted. The ice eventually advanced to cover all of Canada and parts of the northern United States. Montana’s landscape shows ample evidence of such events, such as glacially carved mountains, glacial sediment and moraines, glacial outwash, etc.