Just as an aside, I got back last week from a trip to the Pacific NW. I saw many glaciers that were below the 10,000 ft level. It's rather amazing how glaciers seem to exist at lower elevations in that part of the country.
Olympic NP has high precipitation and moisture levels. Even though the Olympic Mountains are all below 8K feet, the wet winters replenish them after they melt during the summer.
Mt Rainier is absolutely covered with glaciers that go down to as little as 6K ft elevation. You can get a good look at the Nisqually Glacier just a half mile flat hike from the parking lot.
The most intriguing glacier was the one on Mt St Helens. They lost most of their glaciers after the eruption, but apparently a new one formed on a glass-like insulating layer in the crater that protected the new glacier from the heat coming from the volcano.