Mesozoic eugeosynclinal sequences of turbidites with radiolarian cherts, pillowed basalts, and ultramafic rocks, appear to characterize much of the exposed Pacific continental margins and much of the Tethyan tectonic belts. Extremely great stratigraphic thicknesses have been reported for many of these deep-ocean sequences. These eugeosynclinal rocks have had a complex history of penecontemporaneous deformation and subsequent tectonic displacements, and have been uplifted and added to the margins of the continents. Based on studies of the southern continental margin of Alaska, the apparently great thicknesses and the subsequent uplift of these eugeosynclinal sequences seem best explained by deposition in oceanward-migrating trenches and the repeated landward uplift of the sedimentary fill in these successive trenches.