Shoulders and terraces commonly separating the steep walls of glacial troughs from the gentle upper valley slopes originate in many ways--as remnants of trough floors of earlier glaciations, as valley forms made by interglacial rivers, as cirque floor remnants, as structural features developed on resistant horizontal strata, and as remains of lateral moraines. Opinions vary greatly as to the amount of alteration of preglacial landscapes by glacial action.