Where a continental ice sheet reaches the shoreline the ice may extend out to sea as an ice shelf. Modern ice shelves around the Antarctic continent extend hundreds of kilometres out to sea forming areas of floating ice which cover several hundred thousand square kilometres. These ice shelves partially act to buffer the seaward flow of continental ice: melting of the floating ice of an ice shelf does not add any volume to the oceans, but if they are removed then more continental ice will flow into the sea and this will cause sea level rise. Ice shelves such as those around the Antarctic contain relatively small amounts of sediment because there is little exposed rock to provide supraglacial detritus, so the main source is basal debris. Ice shelves break up at the edges to form icebergs and melt at the base in contact with seawater. Ice in a marine setting also occurs where temperate or poythermal valley glaciers flow down to sea level: these tidewater glaciers can contain large amounts of both supraglacial and basal debris. Sea ice is frozen seawater and does not contain any sedimentary material except for wind-blown dust.
Erosional features associated with marine glaciers
Where continental ice from an ice sheet or valley glacier reaches the shoreline of a shallow shelf the ice may be grounded on the sea floor. The movement of the ice mass and drifting icebergs may locally scour the sea floor, resulting in grooves in soft sediment that may be metres deep and hundreds of metres long. Meltwater flowing subglacially may be under considerable pressure and can erode channels into the seafloor sediment beneath the ice, forming tunnel valleys that subsequently may be filled with deposits from the flowing water. The tunnel-valley deposits and the glacial scours features are preserved within shallow-marine strata in places such as continental shelves that have been covered with ice.
Marine glacial deposits
The terms till and tillite are also used to describe unconsolidated and lithifed marine glacial, glaciomarine, deposits. The primary characteristics of the material are the same as the glacial sediment associated with continental glaciation. The detritus released from the bottom of an ice shelf forms till sheets, which may be thick and extensive beneath a long-lived shelf. These deposits may be divided into those deposited close to the ice front (ice-proximal glaciomarine sediments), which are typically poorly sorted diamictons with little or no stratification or other sedimentary structures, and ice-distal glaciomarine sediments, which are composed mainly of sediment released from icebergs. The more distal glaciomarine deposits are subject to reworking by shallow marine processes: waves and currents produce a grainsize sorting of material, sand may be reworked to form wave and current bedforms and the finer-grained material may be transported in suspension to be deposited as laminated mud. Mixing of the glacially derived material with other sediment, such as biogenic material, can also occur. The edges of ice shelves break up to form icebergs that can travel many hundreds of kilometres out into the open sea, driven by wind and ocean currents, but they often carry relatively little detritus. Icebergs formed at the front of tidewater glaciers are generally small, but may be laden with sediment. As an iceberg melts, this debris will gradually be released and deposited as dropstones in open marine sediments. Dropstones can be anything up to boulder size and their size is in marked contrast to surrounding fine-grained, pelagic deposits. Although rarely found in deep marine strata, dropstones are important indicators of the presence of ice shelves and hence provide evidence of past global climate conditions. However, similar deposits can also result from sediment released from floating vegetation.
Distribution of glacial deposits
Extensive ice sheets are today confined to the polar regions within the Arctic and Antarctic circles. During the glacial episodes of the Quaternary the polar ice caps extended further into lower latitudes. The sea level was lower during glacial periods and many parts of the continental shelves were under ice. Upland glacial regions were also more extensive, with ice reaching beyond the immediate vicinity of the mountain glaciers. The growth of polar ice caps is known to be related to global changes in climate, with the ice at its most extensive when the globe was several degrees cooler. Other glacial episodes are known from the stratigraphic record to have occurred in the late Carboniferous and Permian (the Gondwana glaciation in the southern hemisphere), in the early Palaeozoic and in the Proterozoic.
