The South Fork country looks the way it does today because of geological events that took place up to several hundred million years ago. Look to the east of the South Fork and you'll see the Snake River Mountain Range, where the tallest peaks east of the river rise more than 9,000 feet. On the south side of the river, the Swan Valley Range rises from Antelope Flats, with mountains topping out at about 7,000 feet. Sedimentary rock, carried by water 400 million years ago, are the building blocks of these mountains.
Antelope Flats, a long, broad bench above the river, formed when volcanoes oozed basalt lava in a series of eruptions about two million years ago. The lava blocked the flow of the South Fork for a time, flooding much of the upper river valley. The river eventually burst through and carved its way across the lava, forming the South Fork canyon.
Glaciers were another geologic force in the South Fork Basin starting about 200,000 years ago, and retreating only 10,000 years ago. Glaciers sculpted features of mountains, carved terraces near the river, and carried milky melted glacial water and sediment farther down the valley.