Geological Survey Water-Supply Paper Volume 1494-1495 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. Excerpt: ... clay, sand, and gravel composed the bulk of the material (unit 2). During periods of relative quiescence, drainage in the area was integrated in the normal processes of erosion. Low places were soon filled with wash or "mudflow" from the high places, low divides were eroded, and some of the lakes and swamps were drained. When the periods of quiescence were relatively long, larger streams heading outside the area of volcanic eruptions brought nonvolcanic debris into the area. In the final stages of volcanic activity extrusions or ejections of silicic material formed deposits of pumice, welded tuff, and flows of primarily rhyolitic composition mapped as the St. Helena rhyolite member (unit 3). LITHOLOGY AND WATER-BEARING PROPERTIES SONOMA VOLCANICS, UNDIFFEBENTIATED The Sonoma volcanics, undifferentiated, consist mainly of andesite tuffs and interbedded flows of andesite and basalt. In some areas flows predominate, as in the steep canyons at the head of Green Valley just east of the southern part of Napa Valley. These flows make a dense body of massive flows several hundred feet thick, which may correspond with the Mark West andesite of Osmont (1905). They are poorly water bearing but are generally outside the lowland areas of Napa and Sonoma Valleys. Elsewhere in the mapped area the undifferentiated volcanic rocks comprise tuffs and interbedded flows. These flows are of massive to vesicular basalt, andesite, and some rhyolite. The flows form the steep cliffs along parts of both sides of Napa and Sonoma Valleys, which are separated by reaches of gentler slope underlain by tuff. In general, the flows are discontinuous, and they range in thickness from a few feet to a few tens of feet. Although they contain water in fractures and doubtless can...