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0 , boring to the others (Figures 3-5) The upper one, shown as the Modesto Formation in Figure 3, includes five distinct units that are more than 5 feet thick The lower one, shown as the Riverbank Formation in Figure 3, is composed primarily of a thick, oxidized zone that represents an ancient soil horizon 4.1.1 Modesto Formation Unit 1--Within the Modesto Formation, three or four separate sand Keds could be distinguished on the basis of color and grain size The uppermost bed consists of brown to grayish-brown sand that fines upward from medium-to-coarse-grained at the base to fine-to-medium-grained at the top In TT-2, this bed was somewhat finer grained and more yellow in color The bed was sampled only at a depth of 15 feet in TT-3, where it exhibited a strikingly bimodal grain size distribution consisting of fine and very coarse grains, with little or no intervening particle sizes Unit 2--The lower contact of unit 1 occurred at a depth of between 15 and 20 feet in all three borings Below this, TT-1 and TT-2 penetrated an orange-brown to brown, slightly oxidized sand bed that was fine grained to very fine grained This bed is more consolidated than unit 1 It is apparently lens-shaped in cross section(Figure 3), and was not observed in TT-3 Unit 3--Underlying unit 2 in TT-1 and TT-2 and unit 1 m TT-3 is a relatively thick silt bed that ranges from gray through brown to orange in color In TT-HP-1, this bed is slightly sandy A moderate diesel odor and blue-gray discoloration was noted in the sample at 25 feet, and hydrocarbons were detected by the PID (Appendix A) and the laboratory at this depth (Figure 4) This bed ranges from 5 to nearly 10 feet thick, and as is usually the case with silt beds in the subsurface, unit 3 is relatively consolidated Unit 4--Underlying the silt bed is a less consolidated, yellow to gray well-sorted sand bed that is generally coarse to very coarse grained Unit 5--In most of the borings, yellow,fine-grained, slightly consolidated silty sand underlies unit 4 at a depth of approximately 45 feet In TT-2, thin veins of caliche cement were noted in this bed In the core from TT-1, this bed appears to coarsen upward from very fine grained to medium grained 4.1.2 Riverbank Formation Underlying unit 5 of the Modesto Formation is a thick, strongly oxidized paleosol that probably represents the weathered upper surface of the Riverbank Formation TT-HP-1 and TT-HP-2 penetrated only the upper part of this soil horizon, but the monitonng wells appear to have penetrated its entire thickness At Ieast two other units could be recognized below it I Unit 6--Approxunately 20 feet of orange-brown to red-brown sand underlies the Modesto Formation In TT-1, it fines upward from coarse-grained, well-sorted red-brown sand at 60 feet to clayey, poorly sorted silty sand or brown silt at 46 feet The sand is finer grained in TT-2 but shows a similar upward- 5