Glaciers are masses of ice, snow, water, rock and sediment that move under the influence of gravity. They are formed when snow and ice accumulation exceeds summer melting. They "retreat" when melting outpaces snowfall.
U.S. Geological Survey (USGS) Glaciers FAQs - select Glaciers from the dropdown menu of topics
Northern Rocky Mountain Science Center (NOROCK) Glacier Research
Many years of snow accumulation compacts the bottom layers of snow into ice. Under the huge weight of the snowpack (usually 100ft/30.5m thick or more), the ice becomes viscous and allows the mass to slide downhill. The USGS Climate Change in Mountain Ecosystems Program minimum size criterion for a glacier is 0 .1 km² (100,000 m²), or about 25 acres. Below this size, the accumulated ice generally does not move and is stagnant. The appearance of crevasses (cracks) and fracturing attest to a glacier’s movement.
Glaciers don't "bulldoze" landscape as much as they melt and re-freeze, plucking material from areas of snow deposition and moving it to other areas, like downhill conveyor belts.
The current glaciers in the park are estimated to be at least 7,000 years old and peaked in size in the mid-1800s, during the Little Ice Age. Millions of years before that, during a major glacial period known as the Pleistocene Epoch, enough ice covered the Northern Hemisphere to lower sea levels 300 feet. In places near the park, ice was a mile deep. The Pleistocene Epoch ended around 12,000 years ago.