John Southard's MIT lecture on cross-stratification examines how sedimentary structures record the history of flowing water and waves. He explains how bedforms such as ripples and dunes migrate and deposit inclined laminae separated by erosion surfaces, letting geologists reconstruct past flow directions from the rock record. The lecture covers distinct styles including climbing ripples, trough cross-stratification, and large dune deposits, showing how their geometry depends on the balance between sediment supply and bedform migration. It also addresses oscillatory and combined flows, introducing hummocky cross-stratification and the practical difficulty of telling flow types apart once structures are preserved in rock. Part of MIT's RES.12-003 course on fluid motions and current-generated sedimentary structures, the talk is aimed at students learning to interpret ancient depositional environments from physical evidence rather than assumption.