From the American Annual of Photography, 1908:

It is a natural snow-cap resting on the stump of a felled tree. The cap is nine feet in diameter and nearly four feet thick. Its weight has caused the rim to bend so that the top becomes a curved dome. The originally horizontal strata of the snow slope steeply downwards near the rim and small pieces break off where the strength is least, hence the edges are rough though the top is smooth. The cap acts as an umbrella sheltering the ground beneath from snowfall. The structure had taken some months to grow and would have been difficult to dislodge, for the snow was firmly welded by its own pressure. The total weight of the snow cap was calculated at about one ton.

See also Mushroom Rocks.