At Mammoth Hot Springs, hot waters heavily charged with lime have built up tier upon tier of elaborately carved and fretted terraces made of a hard rock called travertine. The springs that created the terraces are constantly moving and shifting — leaving old terraces dormant and forming new ones.

In 1915, automobiles were officially allowed to enter Yellowstone National Park. In 1920, 5000 cars flooded the park — and their drivers had come to autocamp. This postcard shows one of the public autocamps that were built to accommodate the increased visitation to the park.

Approximately two square miles in area, Upper Geyser Basin contains the largest concentrations of geysers in the park — in fact, nearly one-quarter of all the geysers in the world. It includes Old Faithful along with several other well-known geysers. Colorful hot springs and steaming fumaroles also exist here.

Old Faithful is the most famous and celebrated geyser in the park — and indeed the world. Members of the 1870 Washburn Expedition, who camped near this geyser, named it Old Faithful because they discovered that it erupted at frequent and regular intervals (averaging about every ninety minutes).

Old Faithful Inn, a grand hotel built alongside Old Faithful geyser in 1903-4, was the first true rustic-style western resort. Architect Robert Reamer designed it to fit in with nature rather than — like other fancy resorts — to provide an escape from it. The interior continued the rustic look.