He fell, or was shot, or collapsed, in a rocky gully at the edge of the glacier.

That gully was oriented in such a way that it sheltered his body from the direct forces that typically destroy glacier-preserved material: the shearing movement of the glacier itself.

While the ice above and around him moved and compressed and eventually flowed downslope, the hollow in which he lay was protected.

He was not ground to pieces by glacial movement.

He was simply frozen in place, gradually covered, and left.