The sediments would become finer toward the dam. Nature is attempting to grind down topographic highs and move the resulting sediments to fill the ocean basins. However, when rivers reach the oceans they dump their sediment loads along the coasts, because of the decrease in current velocity. The later ocean currents do not have the sustained velocities and capacities to move the sediments out into the open ocean, and thus sediments get 'trapped' on the continental margins. Dikes are thin tabular bodies that cool rapidly as they intrude the country rock, and the rapid cooling results in fine crystals.
Rhyolites are high and silica and therefore very viscous, in contrast to low viscosity basalts.
Therefore, unlike basalts, rhyolites do not flow and spread out over long distances, and hence do not form shield volcanoes with gentle slopes. Bowen suggested that all igneous rocks could be derived through the crystal fractionation of ultramafic or mafic melts. The first forming crystals would be mafic, and if they separated out, the remaining melt would be higher in silica, perhaps to the point of forming a granitic composition. These pre-existing rocks may have become more felsic through previous igneous, sedimentary, and metamorphic processes. .