At a geological conference at Harvard I showed blueprints made directly from glass plates covered with artificial ripplemarks. At the same time I exhibited rock slabs of fossil ripplemarks and photographs of others shaped like horseshoes. These were variants of the rippledrift process seen on sandy beds of running streams. I also showed photographs of swash marks running along the upper steep slopes of beaches. And of the wind-formed rippledrift of dry sand dunes. From the deserts of Peru come photographs of _medaños_, or crescent dunes, hills of sand tapering to curved points at both ends. The points are downwind, the high horseshoe toe of the hill is upwind, and like a coral atoll the edifice is current-formed. Ripplemarks can form in hundreds of fathoms of ocean water if the storm waves on the surface of the sea are big enough. A particle of water on the crest of a wave is lifted up and down in a long vertical ellipse. A particle deep down under the wave is lifted fore and aft in a long horizontal ellipse. Under a three-hundred-foot length of wave in the English Channel in deep water the bottom particles of water are shoving sand back and forth, and making packed ripplemarks. A big sand grain becomes a lump for small sand grains to bump against. They make a heap which piles up and lengthens out. The heaps merge and we get a tightly packed and ridged sandy bottom. Each ridge has an eddy first on one side, then on the other, as the water particles reverse in direction. Oscillation builds first flocculence, then alignment, then even spacing. The opposite sides of a ridge have equal slopes. Rippledrift is made by a current in one direction. It is usually not so regular or in such straight ridges as ripplemarks. If a stream of water is jetted over sand round and round in a ringshaped tank, ridges will migrate along the bottom, but they are smeary. The regular ripples in dry sand on dunes have flatter slopes upwind, steep scarps downwind. They are regular, probably because wind blowing is intermittent and back currents occur. So they become more like ripplemarks. On the bottoms of water streams, the horseshoe rippledrift requires a nice adjustment of lumps and side points migrating downstream. All rippling requires a sand of mixed sizes of grains. If they were all alike they would not ripple, for the larger grains have to obstruct the smaller ones in order to produce the ripple pattern. Rippledrifting as a whole is a building mechanism. Mixed with wave currents which move beaches along, including beach pebbles, it can be compounded into building oceanic islands. The crescent dunes of the desert are dependent on the prevailing winds being loaded with a sand supply at a windward erosion source.