Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the lowest continuous surface line (the thalweg) along a valley or watercourse. This phenomenon is observed under modern-day glaciers, in saltwater fjords and freshwater lakes remaining after glaciers melt, as well as in tunnel valleys which are partially or totally filled with sediment. When the channel produced by a glacier is filled with debris, the subsurface geomorphic structure is found to be erosionally cut into bedrock and subsequently filled by sediments. These overdeepened cuts into bedrock structures can reach a depth of several hundred metres below the valley floor.

Overdeepened fjords and lakes have significant economic value as harbours and fisheries. Overdeepened basins and valleys filled with sediment (termed tunnel valleys) are of particular interest to engineers, petroleum geologists, and hydrologists; engineers apply the information for developing foundations and tunnel construction, petroleum geologists use tunnel valley locations to identify potential oil fields, while hydrologists apply this knowledge for groundwater resource management.

Overdeepening is exhibited across the range of glacially eroded geologic features. It is common to fjords, fjord lakes and cirques formed by glaciers constrained by mountainous terrain as well as tunnel valleys formed on the periphery to the continental glaciers which characterize ice ages.

Fjords are formed when a glacier cuts a U-shaped valley by erosion of the surrounding bedrock. Most fjords are overdeepened (i.e., deeper than the adjacent sea). Fjords generally have a sill or rise at their mouth caused by reduced erosion toward the mouth and added to by the previous glacier's terminal moraine, in some cases causing extreme tidal currents with accompanying saltwater rapids.

The Sognefjord in Norway stretches 205 kilometres (127 mi) inland. It reaches a maximum depth of 1,308 metres (4,291 ft) below sea level, and, as is characteristic of overdeepening, the greatest depths are found in the inland parts of the fjord. Near its mouth, the bottom rises abruptly to a sill about 100 metres (330 ft) below sea level. The average width of the main branch of the Sognefjord is about 4.5 kilometres (2.8 mi). Cliffs surrounding the fjord rise almost sheer from the water to heights of 1,000 metres (3,300 ft) and more. The Skelton Inlet in Antarctica shows similar overdeepening to 1,933 m (6,342 ft), as does the Messier Channel in Chile which deepens to 1,288 m (4,226 ft).

Nesje writes "...glaciers are necessary for fjord formation. The strongest indication for glacial erosion is the overdeepening of fjord floors well below present and past sea level and their outer rock threshold. Measured in volume eroded within a limited time span, an ice stream forming its own clearly defined drainage channel (fjord) is apparently one of the most significant erosive agents in operation on Earth."

Some freshwater lakes which have formed in long glacially-carved valleys with extensive overdeepening and often with terminal moraines blocking the outlet are called fjords or "fjord lakes" (which follows the Norwegian fjord-naming convention). Fjord lakes are commonly formed in mountainous regions which channel ice flows through narrow valleys.

Although they exist in many countries, the fjord lakes found in British Columbia, Canada, are illustrative of their nature. There the interior plateau is dissected by numerous elongated, glacially overdeepened lakes. One such lake is Okanagan Lake, which is 3.5 km wide, 120 km long, and excavated by glacial erosion to over 2,000 m (6,562 ft) below the surrounding plateau (and 600 m (1,969 ft) below sea level), although much of that depth is filled with glacial sediment so that the current maximum lake depth is 232 m (761 ft). Similar fjord lakes in excess of 100 km (62 mi) in length are found elsewhere in British Columbia. Kootenay Lake located between the Selkirk and Purcell mountain ranges in the Kootenay region of British Columbia is approximately 100 km (62 mi) in length and 3–5 km in width formerly discharged through the Purcell Trench into Lake Missoula in Montana. Similarly tunnel channels in the Flathead Valley beneath Flathead Lake were formed by subglacial drainage from multiple sources such as northwest of the valley (the Rocky Mountain trench), north of the valley (the Whitefish Range), and northeast of the valley (the Middle and North Forks of the Flathead River) and funneled into the valley, exiting south eventually into the Mission Valley and glacial Lake Missoula. The bases of the tunnel channels are cut well below the elevation of Flathead Lake, indicating that erosion occurred in hydrostatically pressurized subglacial tunnel channels beneath the ice in British Columbia.