I was able to figure it out eventually, but it's really unclear from the actions section that there's an alternate route up to the glaciers. I had to open up the "Close Dam and Cross towards Laurion" combined action, travel to the dam (making sure I remained on the Santorini side), and *then* select Depart Towards Glacier, with no real indication that it was a separate route aside from me figuring it out from the map and some clues at the elevator complex.