Ripple marks in sandstone. These ripples in the sand have hardened over time to sandstone. This rock sample was part of an old stream bed, where the stream provided continuous deposits of sediment. Over time, pressure on the sediment causes it to slowly lose its original moisture content, a process known as compaction. As more sediment is deposited on top of this layer, more pressure is exerted, thus squeezing more moisture out of the original layer. Photographed in Pichincha, Ecuador.