The southmost of Chile is a maze of islands separated by long, narrow, twisting fjords, and tucked back on the mainland east of those islands are a couple of really neat glaciers. Between the two we visited, you can learn a lot about the effects of moving ice.
A glacier according to one geology professor is “a helluva lotta ice doing odd things.” We visited two. The more southern Amalia glacier is about a kilometer in width along its leading edge. Its surface and leading face are bright white, jagged, and it plunges directly into the water. It calves frequently; you can tell because the leading vertical surface is clean with sharp edges and corners, and chunks of ice hug the shore on either side.
Amelia glacier
North of it back another fjord, Pio XI (the XI here is eleven, not zhee. I have no idea what came of the other ten) is about six kilometers on its leading edge. Its surface is rounded and not nearly as erose as Amalia’s. You know this glacier is slow-moving because it is building a terminal moraine as we speak, a long, narrow mound of debris in front of the ice. To explain:
Glaciers start out at high elevations as ice fields that get compressed from snow into fern into ice. The ice begins to creep downhill, pushed by the weight of ice and snow on its trailing edge. Eventually, it will continue its slide downhill propelled by its own great weight plus that of the mother icefield uphill of it.
As it moves along, it scrapes off the rocks over which it travels. Some of the scrapings are ground as fine as flour. Some are plucked away in the form of gravel or stones or even large rocks. As the glacier gouges into the walls of its valley, rocks and dirt tumble down onto the ice to become a part of the mass. These are the dark streaks you see along the edges, the lateral moraines. If another glacier joins it from a side channel, it brings its own rocks and dirt, and its lateral moraine may form a dark streak near the middle where they join.
Pio XI glacier
The whole sluggish monster carves its way inexorably downhill from cold into warm until melting stops the ride. Pio XI is melting faster than it is moving, so it is dropping its load of flour, stones, and rocks on its leading edge. Amalia is moving faster than it is melting, so the only moraines it leaves behind are some rocks along the sides. She is overriding whatever debris she drops along the melting snout.
Many factors influence its speed, too many to mention. Amalia is moving fast enough that its sharp, jagged edges have not melted much. Pio XI is sliding downhill so slowly that on the downhill edge; the snout or leading edge; it is melting faster than it is moving. Ice is melting right out from under all its entrained debris; the debris drops as a ridge of unconsolidated rocks and stones and gravel along the front face. And the glacier’s surface is rounded rather than jagged. The jags have melted.
I noted that on the north side of Pio XI (to the left as you face the glacier) there is a huge lateral moraine out on the ice a few hundred meters from the edge. It’s not just some rocks and stones, it is a major ridge of debris. Most likely a landslide on a steep slope crashed down onto the ice.
During the last glaciation, ice crept down out of the Andes to cover the whole of this area, abrading the island hills into round-topped mounds. They are drumlins, rounded on the upstream side where the ice moved up over them, and very steep on the downstream side where the ice plucked away the rock. There is vegetation of a sort, stunted forests of southern beech, but the hills are so steep the trees are struggling.
It’s a nice place to visit, but you wouldn’t want to live there.