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3-4 The following is an example of the depositional and environmental influ- ~ ences that can affect the erosion resistance of a stratigraphic unit. ,..., A sequence of stratigraphic units may indicate that in this instance they are alluvial deposits consisting of (I) light colored, finely stratified noncoherent silt and fine sand about two feet thick. Below this unit there is (2) a compact dark gray clayey silt horizon several feet in thickness. Then there is (3) a layer of gravel in a matrix of compact brown silty clay that starts a foot above the stream bed in the exposed banks and extends below the streambed and the proposed invert grade to unknown depths. The reasons for a uniqueness in character of stratigraphic units within a profile are several fold. Taking the profile cited above as an example, the number I stratigraphic unit of this hypothetical profile is an accumu- lation of recent sediment. Soil forming processes have not had time to modify the sediment's characteristics over that contributed by the texture and grain size distribution of the transported sediment and its mode of deposition. The bulk of unit 2 accumul~ted either as a mixture of, or dominantly' of, lateral accumulation as the stream meandered or vertical accumulation during overbank flooding. The developed soil in this unit indicates a following period relatively free of deposition or erosion. Time was available for limited weathering of the sediment, and for humus to accumulate. The gravel layer identified as unit 3.was deposited at a time of relatively high velocity flow. The high discharges that brought down the gravel could have formed a braided wash that extended from one side of the valley to the other. During flows of lower velocities fine sediment was deposited and migrated into the gravels. With age and e weathering of clay forming materials this stratigraphic unit, with its characteristic erosion resistance properties, evolved. The sediment source, texture, and mode of deposition of units I and 2 could be very similar, yet their appearance including color, degree of consolidation, and erosion characteristics can be quite different. These differences created by changes occurring since deposition can vary widely depending upon the interaction of the average annual climatic environment: humid or arid, cold or hot. The climate following deposition can create a wide change in erosion characteristics for different reasons. In a humid hot climate, textural changes can occur by weathering of clay form- ing minerals, leaching of soluble salts, carbonates, etc. or by movement of fine clay-size particles from the upper to the lower profiles. Struc- tural changes can be caused by alternate wetting and drying, and consequent swelling and shrinking, as well as by growth and decay of vegetation. .. In an area affected by a permanently high water table, little or no post- depositional changes in the deposits may occur. This is especially the case when there is little oxygen in water in which the stratigraphic unit is submerged. The reason for this is that oxidation essential to e