Portland Canal
Made by nature
The Portland Canal is a long glacial fiord that forms the border between southeasternmost Alaska and adjacent Canada. Calling it a “canal” is misleading albeit common in the Alaska Panhandle; Spanish explorers called these narrow water bodies canals, which means channels, and English explorers continued the tradition. It gives the same impression as Giovanni Schiaparelli’s name canali for the lines he saw on Mars in 1877, but the Italian word for channels came into English as canals, with an erroneous inference of artificial construction.
But there is nothing man-made (or Martian-made) about the Portland Canal. It is a 150-kilometer-long U-shaped valley carved by glaciers in the past two million years. The average water depth in the fiord is around 300 meters (1,000 feet) and the rounded mountains along it are about 1,500 meters above sea level (4,500-5,000 feet), so the ice was about 1,800 meters thick (somewhat more than a mile) here 25,000 years ago.
The rocks surrounding the fiord seen in the photo at top from 2005 are mostly Eocene granitic rocks that were intruded about 45 to 55 million years ago (Wilson and others, 2015, Geologic map of Alaska: U.S. Geological Survey Scientific Investigations Map 3340, 197 p., 2 sheets, scale 1:584,000). They formed as one of the consequences of various collisions between mostly relatively small, elongate tectonic terranes and North America. Much of far western Canada and much of Alaska are composed of such added (accreted) terranes, and essentially all of the Alaska Panhandle and the Alexander Archipelago are such rocks or related material, complexly attached to each other (Johnson, 2001, The Great Alaskan Terrane Wreck: reconciliation of paleomagnetic and geologic data in the northern Cordillera: Earth & Planetary Science Letters 193, p. 259-272).
Glacial valleys are classically U-shaped. In many fiords the side walls are typically very steep if not quite vertical, enough so that a small boat can essentially touch the walls without fear of grounding on shallow material. In the photo above, the captain had decided to approach a waterfall to get some cold fresh water to accompany our lunch during a Smithsonian Journey for which I got to serve as the house (or ship) geologist.
There are multiple ideas for the origin of the granites in the hills in the photos, but to my mind the most likely is a northeast-directed push by the outboard (on the ocean side of the system) Alexander Terrane toward North America. The collision thickened the crust and probably generated some subduction, with these granites resulting from the melting that came from the subduction. Their timing, 45 to 55 million years ago, makes them very late or even post-accretion, the last magmatic action in a sequence of collisions that began in Early Cretaceous time (120 million years ago) or earlier.
The Alaska Panhandle is underlain by long linear NW-SE-trending belts of igneous and metamorphic rocks. The granites along the Portland Canal have equivalents as far north as Skagway, 600 km (375 miles) away and well into British Columbia to the southeast. They underpin the Coast Mountains here. I wrote about New Eddystone Rock, at left in the image above, in a previous post.
Portland Canal was given its name by George Vancouver in 1793, for William Cavendish-Bentinck, 3rd Duke of Portland (1738-1809). As the international boundary between Alaska and Canada, Portland Canal was disputed somewhat for many years, until the final position of the boundary was settled by arbitration in 1903 under the Hay-Herbert Treaty.