The mountain ranges — the broad, high ridges and peaks that define the Colorado Rockies — were formed by geological processes over millions of years and have not changed their fundamental character in the few thousand years of human prehistory.

The peaks stood roughly where they stand today.

The valleys ran roughly where they run today.

But the ecological communities that occupied that topography were different.

The early Holocene in the Rocky Mountain region was a period of climatic transition — warmer and drier than the preceding glacial period, but not yet fully settled into the modern climate patterns.

Tree lines fluctuated as the climate changed.

Species ranges shifted as temperatures and precipitation patterns evolved.