Overdeepened fjords can plunge more than a thousand meters yet still perch hundreds of meters above the neighboring ocean floor. The apparent contradiction comes from how glaciers, solid Earth, and ocean basins share vertical space on a continental margin.
Glaciers act as high pressure milling machines, driven by basal sliding and abrasion, that exploit preexisting faults and fracture zones in crystalline bedrock. This produces an overdeepened U shaped trough whose floor can sink well below present sea level without ever intersecting the much older and structurally separate abyss of the adjacent continental shelf. After deglaciation, isostatic rebound shifts the lithosphere upward as ice load is removed, a process governed by viscoelastic relaxation and entropy production in the mantle. The whole coastal block, fjord included, lifts relative to the regional geoid while the deeper oceanic crust barely moves.
At the same time, marine sedimentation progressively infills outer shelf and slope basins. Turbidites, contourites, and deltaic deposits build a thick sedimentary pile that raises the effective seabed, while the hard bedrock floor of the fjord remains largely intact beneath a comparatively thin glacial and postglacial cover. The result is a suspended rock valley: extremely deep in a local sense, yet still resting on continental crust that stands hundreds of meters above the nearby ocean basin carved and subsided on a completely different tectonic and isostatic baseline.
