The drive up Fall River Road from Idaho Springs climbs fast, and by the time you reach the trailhead parking for St. Mary's Glacier you're already at altitude — around 10,400 feet — and the air has that thin quality that reminds you your lungs are doing more work than they advertise.
The trail itself is short, about 0.75 miles up to the glacier, but the elevation gain is real and the terrain switches between packed dirt, loose rock, and eventually snow or ice depending on the season. In September I was picking my way across consolidated snow while watching a few determined souls with snowboards work a small pitch on the upper field. It was 65 degrees at the trailhead. It's that kind of place.
St. Mary's Glacier is technically an ice field rather than a true moving glacier — it doesn't flow under its own weight the way glaciers do — but it holds snow year-round at this elevation and has the character of a glacier in every practical sense. The lake below it, St. Mary Lake, is that particular shade of high-altitude blue that photographs never quite capture: deeper than sky, cleaner than anything at lower elevations.
The trailhead is about an hour from Denver, which makes it a plausible day trip even in summer when the city gets hot. The $20 parking fee has been a source of some local controversy but hasn't kept people away. Go early in summer; the lot fills by mid-morning on weekends.