Oregon is located in the vicinity of the Johnstown moraine, the terminal moraine from the last major glaciation, which started to retreat about 20,000 years ago. The moraine forms a prominent topographic feature running south and west of Madison, and extends north over the Baraboo Hills and up to Antigo.
When the climate warmed about 15,000 years ago, the glaciers started to retreat, leaving distinctive landforms in part of Dane County.
Pollen grains preserved in basins like Hook Lake, near Oregon, and Lakes Wingra, and Mendota record the changes in vegetation that took place as the landscape changed: Sphagnum gave way to sedges and grasses and deciduous forests replaced spruce.
As the glaciers retreated, big game animals like mammoths and mastodons migrated northward. We don’t have any remains of these big animals near Oregon, but they have been found in Wisconsin.