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e e e 3-3 Identification of Stratigraphic Units The origin of the defined stratigraphic units as alluvium dictates that certain relationships existed between the deposits and the carrying capacity of the stream at the time. Further, the relationships that exist today are not necessarily the same as at the time the sediment was deposited. For example, during portions of the Pleistocene, runoff and sediment transport were on an appreciably larger scale than at present. The primary identifying characteristics of a stratigraphic unit are: color, thickness, grain size distribution, texture, structure, plasticity, consistency, density, dispersive characteristics and cementation. Slight variations that may occur do not necessarily signify the presence of a new stratigraphic unit, particularly if the deposits are conform- able with units above and below the one being described. The location of stratigraphic units areally within the alluvial valley is as important as their position and identification within the vertical profile. Because most alluvial channels meander about within a rela- tively short geologic time period, it is probable that stratigraphic units in a valley confined by side slopes will be at about the same elevation across the valley. This requires field checking before assuming a uniform valley-wide distribution of stratigraphic units exists. Geomorphic History as a Determinant of Erosion Resistance The history of the stratigraphic unit from the moment of deposition determines its erosion resistance. The grain size distribution and texture of the deposit is initially the most imvortant characteristic and it may remain so if the environment following deposition is not conducive to modifications tending to change the arrangement or bonding of particles. These modifications can be produced by one or a number of post depositional changes such as the movement downward of particles from the overlying deposits, by weathering, by consolidation or chemical action that increases or decreases the bond between particles. The result of this geomorphic history is that the stratigraphic unit responds in characteristic manner to the hydraulic forces that may be exerted against its exposed profile. The unit may remain as individual particles free of any bond with adjacent ones, or it may resist as a mass due to coherence between particles. The bond between particles that may be reflective of a long history in a particular environment may be destroyed when the profile is disturbed.