Imbrication is the response of sediment particles to strong, sustained water currents. In this case it is large cobbles in Nevada's Truckee River, running right to left, that have become imbricated. In rocks, the imbrication of pebbles is a good indicator of ancient current directions.
Often imbrication can be subtle, but it's always worth looking for when studying sandstones and conglomerates. Imbrication is also called shingling, as the particles line up overlapping like shingles.