If you leave salad dressing for a while, the water and oil separate, and a clear boundary is formed between the two. If you then slowly rock the bottle, waves appear on the boundary. In the same fashion as the waves on the surface, these waves are caused by the restoring force of gravity. The waves on the water's surface occur at the boundary of air and water, however no matter what the two substances are, the principle behind waves formed at the boundary of two substances with differing densities is the same. When the gap in density between the two substances gets smaller, restitutive force decreases, and the period of the wave increases. If you get a cycle as long as the rotation of the earth, a wave totally different to that of the salad dressing bottle emerges.