The 1966 polygons included in this data release represent the main body portion of the 37 named glaciers of Glacier National Park (GNP) and 2 named glaciers on the U.S. Forest Service’s Flathead National Forest land. This is a subset of the original mapping effort derived from 1:24000 scale mapping of named glaciers and permanent snowfields within Glacier National Park, Montana which were digitized by Richard Menicke (Glacier National Park) and Carl Key (U.S. Geological Survey) in 1993. These data are based on USGS 7.5 minute quadrangle mapping published from 1966 through 1968 which were the result of the earliest park-wide aerial surveys of snow and ice features in GNP. Examination of the aerial photographs shows that seasonal snow was present at some of the glaciers, limiting the ability of the 1966-1968 cartographers to see and map the glacier ice margins. This resulted in some smoothed and generalized outlines of the glaciers where the cartographers were likely guessing where the ice margins were under the snow. In addition, some photographs show exposed glacier margin ice with irregular patterns that are not represented by the mapped ice margin. It appeared that the original cartographers used a more generalized outline for the glaciers and were not concerned with small scale ice features even when they were evident in the photographs. Despite the generalized nature of the glacier outlines, which were also limited by mapping technology and standards of the time, the dataset represents the baseline for the glacier margins derived from aerial photography. In several cases, because of the generalized nature of the 1966-1968 mapping, a glacier perimeter did not seem as if it reflected likely location in the basin topography. In these cases the original USGS aerial imagery was referred to for verification and revision if the error seemed significant. Specifics of margin revision are detailed in attribute files for those glaciers that warranted change as part of the time series analysis conducted by Dan Fagre and Lisa McKeon (USGS) in February - August, 2016. For each glacier, determination of what constituted the "main body" was made in accordance with USGS criteria outlined in Supplemental Information section of the xml file and some disconnected patches were eliminated in the interest of keeping this analysis strictly to glacier main bodies.