A geological choke point
The entrance to Trollfjord is a dramatic constriction in the landscape of the Vesterålen archipelago. Only 100 meters across, the passage is flanked by near-vertical mountain walls that rise to between 600 and 1,100 meters. The fjord itself extends for about 2 kilometers, widening to a maximum of 400 meters inside the narrow mouth before terminating in a steep-sided basin. The maximum water depth within the fjord reaches 72 meters.
The mountains surrounding the fjord are composed of some of the oldest rocks in Norway, part of a geological province formed between 2.7 and 1.6 billion years ago. These durable Precambrian rocks, primarily granite and gneiss, resisted the full force of glacial erosion, forcing the ice to flow through pre-existing weaknesses in the crust. The entire Lofoten-Vesterålen region is an exposed basement high, a section of deep continental crust pushed upwards and later scoured by successive ice sheets.
The submerged dam
The fjord's most interesting feature is hidden beneath the water at its entrance: a submerged sill. This underwater ridge is a remnant of the last glacial period. It is likely a terminal moraine, a pile of rock and debris deposited at the glacier's snout, or a bedrock threshold that the ice could not fully plane down. This sill acts as a barrier, separating the deeper inner basin of the fjord from the adjacent Raftsundet strait.
During the retreat of the Scandinavian Ice Sheet, this sill would have dammed a meltwater lake within the Trollfjord valley. As the immense weight of the ice disappeared, the land began to rise in a process known as isostatic rebound. This geological uplift, combined with rising global sea levels, eventually caused water to overtop the sill. The resulting flow of water cut the narrow channel through the blockage, creating the 100-meter-wide entrance visible today. This process of a sill breach is a common mechanism for fjord formation, where the entrance is significantly shallower than the inner basin, controlling water exchange and influencing the fjord's ecosystem.
The geology of Trollfjord and its submerged sill shows powerful glacial processes. The narrowness of the entrance, the depth of the basin, and the composition of the ancient rocks all show the history of ice, rock, and monumental shifts in the Earth's crust over thousands of years.