"Imbrication of gravel (lower one-third in right half of picture) in north wall of gravel pit, Salt River bed and 27th Avenue, Phoenix, Arizona. Stream flow at time of deposit was from right to left. Person is pointing to a layer of black, manganese-oxide-coated gravel indicative of past water-table position." Source: Groundwater Hydrology - Bouwer (page 57) This picture shows a gravel pit containing a mixture of coarse and fine grained materials. The lower portion of the above picture shows a deposition of very coarse grained material. On top of this material is a thinner layer of finer grained material.