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Sea level is used as datum in geodesy and geology.

Contemporary sea level is changed locally by climatic influences and universally especially by variation in glacier volume. These eustatic changes should now be ascertainable with sufficient accuracy by averaging world mareographs. At present a rise is going on. This should be allowed for in the interpretation of data from each individual station. There then remain climatic influences, to be evaluated meteorologically; local settling and crustal movements, to be distinguished by precise levelling.

Sea level as datum for the geological past is involved in many problems.

Nick just below sea level in the hypsographic curve: this is evidently due to the action of external processes.

Cyclothems: it is not yet known whether these are of eustatic nature.

Guyots: the flat tops are probably sinking 20 meters per million years.

Glacial low levels: roughly 100 meters minus. The river bed on the Sunda shelf has not been warped since the peak of the last ice age.

Raised beaches: reef terraces prove the recent rise of island arcs facing deep-sea trenches and the intermittent nature of the movement, each jump adding about 1 milligal or less to the local value of gravity. There is a deplorable lack of data on tilting, warping, and dating of these terraces, which could teach so much of crustal movements.

Daly terraces: slightly raised young terraces are of common occurrence, and Daly ascribed them to eustatism. The amount and dating of the supposed movement is still debated. In some areas, however, there is a singular absence of evidence on a recent sinking of sea level.

Isostatic recoil. The post-Glacial updoming of the Canadian and Fennoscandian shields is recorded in countless raised beaches. Field work appears to point toward the establishment of hinge lines, with updoming on the inside and permanent stability on the outside. The hinge line migrated from south to north in several jumps.

Inter-Glacial terraces have been widely studied outside glaciated areas, but not all can agree to their eustatic nature. The origin of the older terraces that lie far above sea level for an ice-free world is a vexing problem.

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