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color of the sand grades upward from gray through red and brown, to orange-brown or light grayish brown at the top In GT-4 and several other bonngs, the base of the channel deposit forms a sharp contact with an underlying bed of gray or brown silt This bed is dry and considerably more consolidated than the sand of the overlying channel deposit, although this is not evident in the blow counts Root holes, clay- coated polygons, and other weathering features were occasionally observed in the silt bed The thickness of this bed depends on the depth of incision of the Modesto channel (Figure 5) In boring GT-5, this bed is more than 15 feet thick and includes a clay bed or clayey silt interval in its middle portion In GT-6, the bed is approximately 10 feet thick and the clay interval is near the top In GT-1 and TB-2, the bed is less than 10 feet thick and the clay interval is absent, and in GT-3, the entire bed appears to be eroded and the Modesto channel lies directly on the Riverbank Formation(Figure 6) 4.1.2 Riverbank Formation The contact between the Modesto and Riverbank formations is marked by red, oxidized, clayey or silty sand This zone is unusually consolidated (stiff, and 30 to 40 blows of the dnll hammer are needed to collect a 6-inch sample Locally, particularly in the vicinity of GT-3, the sand is cemented, forming a "hardpan", and as many as 80 blows may be needed to collect a sample It is unknown whether calcite, silica, or iron oxide is the primary cement, but white calcareous veins and blebs have been observed in some samples This weathered and cemented interval is several feet thick and is interpreted as an ancient soil horizon formed during a period of landscape stability following deposition of the Riverbank Formation This soil is well known and widespread at the top of the Riverbank Formation, soil scientists refer to it as the San Joaquin or Snelling Soil (Figure 4) The degree of weathering and soil formation decreases downward in the borings, and most of the formation consists of relatively unweathered brown or gray sand or gravely sand In the deepest logged boring (GT-3), silty to fine-grained sand coarsens downward from 54 feet to fine-to-medium-grained sand between 75 and 80 feet and gravely coarse-grained sand between 80 and 90 feet The sand apparently becomes finer grained to the bottom of the boring at 100 feet This sequence was not recognized in GT-I, where the interval between 67 and 88 feet consists of light brown or tan clayey silt Below this, coarse-grained gravely sand was penetrated to the bottom of the boring The presence of this thick silt bed in GT-1 suggests that the upward-fining sand bed in the other borings fills a large channel whose margin hes between GT-I and the other borings(Figure 6, panel A C) The upper portion of the Riverbank Formation is dry in all borings, moisture content increases downward, and at the present time, the formation is saturated below a depth of 65 feet The Riverbank aquifer forms a continuous, extensive shallow aquifer at the site(Figure 5) 4.1.3 Facies Maps The lateral stratigraphic variations discussed above are illustrated in map view in a series of generalized facies maps in Appendix A The maps show the dominant hthology in two intervals of the Modesto 6