Whether they formed under shallow or deep water, thick geosynclinal sediments were regarded until recently as essential precursors to mountains. Every orogenic belt presumably had evolved stage by stage from geosyncline to mountain, ultimately producing a peripheral accretion to some evergrowing continental craton. Conversely, by implication, thick sediment prisms along any present continental margin inevitably should lead to mountains. These long-standing deterministic generalizations were hardly justified, however, for modern orogenic belts are not consistently located at continental margins, nor do they all contain thick sediments. Moreover, it is impossible to designate any uniquely geosynclinal sediment type. Most geosynclinal sediments are results more than causes of orogenesis; an orthogeosyncline is simply a sediment-filled orogenic belt.
The early, strictly uniformitarian sea-floor spreading model for geosynclines was unacceptable because it regarded continental terrace sediment prisms (miogeoclines) formed on passive or nonorogenic continental margins as essential, evolutionary precursors to mountain building—a holdover from the venerable tectonic cycle. But most existing continental terraces are almost 200 million years old and still show practically no tectonic mobility. Genetically, these miogeoclines belong to a different genus than accumulations formed in active orogenic zones. Miogeoclines form on passive trailing edges of diverging continents, whereas orthogeo- synclines form near active leading edges of converging lithosphere plates. Plate tectonics shows how these two genetically distinct sediment prisms may become coincidently crushed together in orogenic belts. Rather than being simple concentric accretions of successive orogenic belts, continents are mosaics of very complexly truncated, overprinted, and even rifted tectonic elements containing haphazard relics of former plate margins.
The geosynclinal concept has suffered from an analogue syndrome. Dogmatic generalizations were applied to all cases from an incomplete list of supposed modern analogues, and tectonic environments were confused with sedimentary ones. But intensive marine research over the past two decades has provided many more well- documented possible analogues for the testing of truly actualistic models. Armed with these, as well as with new tectonic insights and new vocabulary, geosynclinal studies can advance from a long descriptive phase to a more genetic one.