Utah's Green River has puzzled geologists for generations: rather than flowing around the Uinta Mountains, it cuts a canyon roughly 700 metres deep straight through them, including the dramatic Canyon of Lodore. New research led by Dr Adam Smith of the University of Glasgow's School of Geographical and Earth Sciences proposes an answer in a process called lithospheric dripping, in which dense rock at the base of the crust sinks into the mantle, dragging the land surface down above it.
The team estimates that the drip detached between 2 and 5 million years ago. While the land was pulled down, the river established its course and eroded through the rock; once the drip broke away, the surface rebounded, leaving the river apparently flowing uphill through a mountain range about 50 million years old and roughly 4 kilometres high. Seismic imaging revealed a cold, rounded anomaly about 200 kilometres below the surface, between 50 and 100 kilometres across, which the researchers interpret as the detached drip itself.
Supporting evidence lines up neatly: the crust beneath the Uintas is several kilometres thinner than expected for mountains of their height, and the predicted rebound matches an elevation change of over 400 metres inferred independently from river networks. The study, with co-authors at University College London, the University of Utah and the Utah Geological Survey, appears in the Journal of Geophysical Research: Earth Surface, and the authors argue it rules out rival explanations, including that the river predated the mountains.