 By contrast, rocks of the Pliocene-age Sonoma Volcanics, which are mainly pyroclastic breccias and tuffs, with some interbedded volcanic flows, make up the Howell mountains that border the west side of the valley.Suisun and Green Valley Creeks drain the northern uplands and create an alluvial fan that covers most of the valley floor, and builds out to the south into the Suisun Marsh.Soils developed on the alluvial fan, where most of the vineyards are located, range from a very thick layer of Sycamore silty loam that covers most of the fan on the valley floor, to thinner Rincon and Brentwood clay loams that develop where the fan laps onto the valley margins, and drainage is somewhat better.