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The only physical constraint to development The Altamont Hills are formed of marine in the major soils group is the expansiveness sedimentary rocks 75 million to 95 million of the clay minerals. This tendency to swell years old. These rocks,which underlie the and shrink with repeated cycles of wetting project site at depths of hundreds of feet, are and drying is a fairly common feature of organic shales,limestones and siltstones fine-grained soils. The condition is rela- that have been folded and faulted. Local tively easily remedied by removing the fault zones that are associated with this expansive soils at the locations where activity include the Midway,the Midland, foundations will rest and replacing them the Tracy-Stockton, and a number of branch with engineered backfill. It also is possible faults in the Altamont Hills. With the to recompact the expansive soils sufficiently exception of the Midway fault, these to allow their use as support material. features are pre-Quatemary, i.e.more than 2 million years old, and do not show clear The recent basin deposits of the Great evidence of displacement during that time. Valley are less than 10,000 years old and The Midway fault, southwest of the site, underlie the flatland soils along the Old shows evidence of Quaternary movement, River. These sediments were deposited but has not fractured for at least 10,000 during flood stages of the major rivers that years. None of the faults are in Alquist- lie between the natural levees of the valley Priolo Special Study Zones and none has floor and the alluvial fans of the valley been mapped on the project site. The walls. They usually consist of silt and clay Midland fault trends toward the site from containing peat,mulch and other bits of the north, and may have been responsible organic debris that account for their agricul- for the Vacaville earthquake of 1892. tural value. Because the activity of the Midland fault is The high terraces and hills are underlain by not clearly known, an investigation of its non-marine sediments that are 2 million to 5 probable location and level of activity will million years old. These represent the be included in the geotechnical studies to be ancient deposits of rivers that flowed from incorporated in the early phases of project the Altamont Hills, west of the site. They design. If an active portion of the fault also have been compacted by their own occurs on the site, setbacks and open space weight,but are not solid rock. Because the will be incorporated into the project design substrata in the project area are unconsoli- to ensure the safety of people and structures dated deposits,it is expected that consider- in the communities. able variation occurs in their geological character. Site-specific geotechnical studies Air Quality: will be used to establish the existence of stable construction conditions in each phase The site mostly lies within the San Joaquin of development. Valley Air Basin, under the jurisdiction of Site Description 3.4