Yosemite National Park, located in the central Sierra Nevada in California, is an icon of the U.S. National Park system. It is famous for its many spectacular geologic features, which include the towering cliffs and hanging waterfalls of Yosemite Valley and the rounded granite domes, deep blue lakes, and jagged peaks and spires of the high country. More subtle but just as spectacular are the vast areas of polished granite, linear scratches, and isolated boulders scattered across the landscape. All of these features owe their origin, at least in part, to glaciers. Glaciers originating at the crest of the Sierra Nevada flowed down preexisting river canyons numerous times throughout the Quaternary Period (the past 2.6 million years). Although the field evidence for past glaciations is necessarily incomplete, at least seven distinct glacial periods have been identified in the Sierra Nevada, spanning a minimum of 1.5 million years.

This map shows the extent of alpine icefields and associated valley glaciers in Yosemite National Park and vicinity during the most recent large glaciation, known as the Last Glacial Maximum, a globally recognized cold period characterized by low sea levels and the growth of ice sheets and mountain glaciers. In the Sierra Nevada, the Last Glacial Maximum glaciation is referred to as the Tioga glaciation. By virtue of being the most recent of the large Pleistocene glaciations, the evidence for the Tioga glaciation is abundant and relatively well preserved in the Yosemite landscape. The Tioga glaciation likely involved at least two, and perhaps as many as four, major glacial advances spanning the interval from approximately 27,000 to 15,000 years ago; the largest of these, representing the maximum ice extent shown on the map, occurred from approximately 21,000 to 18,000 years ago. Although it is possible that the various Tioga-age glaciers in the study area attained their maximum extents at slightly different times during the Last Glacial Maximum, for the purposes of this map we assume that they reached their maximum extents simultaneously. The maximum ice extent shown here may have occupied certain areas only briefly.

During the maximum extent of the Tioga glaciation, glaciers and ice fields covered most areas in and around Yosemite National Park above 2,700 meters elevation, having a profound impact on the Yosemite landscape. In addition to sculpting most of the granite monoliths for which the park is famous, glaciation also dictated the distribution of many geological, hydrological, and ecological features. Thus, the lasting effects of Tioga glaciation are still readily observable in Yosemite National Park today.