Terrace formations of fluvial, coastal, and lacustrine origins may be found in physiographic regions and hydrologic systems throughout the conterminous United States. Coastal and lacustrine terraces form by deposition of sediments along a shoreline, followed by lowering of the water level or elimination of the lake. The earlier depositional surface is then exposed at a elevated topographic position. Depositional terraces along river valleys form in a similar manner, by aggradation of the river valley followed by incision of the river into its valley, which leaves the former depositional surface at an elevation above the new level of the river and floodplain.
Not every stretch of river valley, coastline, or ancient lakeshore has terrace formations. Steep-walled canyons or coastal cliffs, for example, may lack any depositional record of past river levels. In other instances, water levels may rise after a terrace is formed and exposed, thereby covering the terrace with more recent deposits or the contemporary body of water. Risking overstatement, however, it may be said that all major river systems contain terrace formations in some if not all reaches of the trunk and tributaries, and any lakes or oceans which had higher levels in the past will have left behind some depositional record in the form of terrace formations.
River aggradation and incision are caused by changes in sediment supply, discharge, and gradient, which in turn may be influenced by a combination of factors: changes in climate and vegetation, glaciation of the river basin, uplift or subsidence, intra-plate tectonic activity, volcanic activity, karst development, river piracy, and change of basin areas. Because of variations within drainage basins, different portions of river systems often develop terrace formations with distinct sedimentological profiles or different base and surface elevations, thicknesses, stratigraphic positions, or gradients.
Lakes and lake terraces are generally influenced by of the same climatological and geologic factors which influence fluvial systems. Quaternary lacustrine terraces in non-glaciated regions record fluctuating lake levels due to glacial cycles of climatic change, especially in the Basin and Range province. Lake terraces in glaciated regions provide evidence of widespread damming of rivers by ice sheets, piedmont glaciers, or glacial sediment. Many of the tributaries of the Columbia, Mississippi, and Susquehanna rivers contained Quaternary ice-dammed lakes. Lake levels are recorded by shoreline deposits such as deltas and bars, and erosional features such as strandlines and bedrock platforms.
Coastal terrace development is strongly influenced by tectonic uplift or subsidence of coastal zones, as well as glacioeustatic sea level change. As with lakes, past marine shorelines are often marked by erosional terraces and strandlines. Quaternary terraces are abundant along the coasts of the Pacific Ocean, the Gulf of Mexico, and the southern and mid-Atlantic Ocean. Researchers face a formidable task in attempting to interpret the relative importance of tectonic adjustments and glacioeustatic fluctuations upon the formation of coastal terraces.
Terrace formations are subject to rapid destruction. They usually consist of loose sediment which is easily eroded, and the agents which cause their formation commonly consume them later like savings in a sedimentary bank. The topographic positions of terraces along or above shorelines and rivers exposes them to erosion by precipitation, streams, waves, and wastage on over-steepened slopes. It should be no surprise that the vast majority of terraces which still exist today were formed during the Quaternary period.
Much of our knowledge of the Quaternary has been taken from storage in terrace formations. Terraces often contain fossils, archaeological artifacts, volcanic ash beds, loess deposits, and paleosols, all of which may serve for dating or correlating geological events. The stratigraphic succession among terrace formations can also offers insight to chronological problems. Terrace formations record episodes of glaciation, climatic changes, variations in base level, tectonic adjustments, distributions of flora and fauna, and human occupation of river basins.
Deciphering the geologic record of terrace formations is no easy task. In most cases of research, important questions remain unanswered or new problems arise. Terraces are usually found in some stage of degradation, perhaps only as isolated remnants of former surfaces. Quite often it is not clear how to correlate the terrace remnants from different locations, especially when terraces vary in lithology and position within a study area. Despite the abundance of dateable materials in terraces, the ages of terrace formations often cannot be determined with certainty. Furthermore, the development of terraces is commonly influenced by multiple factors, including tectonic adjustment, and the relative importance of each factor can not be easily weighed.