Bulletin - United States Geological Survey (No. 603) - nouveau livre

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: posed of medium to fine grained massive sandstone, blue-gray when fresh but weathering to a light tawny yellow on exposure, formed very largely of angular or subangular grains of quartz and feldspar. It contains beds of clay shale and shaly sandstone alternating with the prominent sandstone beds. In the lower part the sandstone is massive, but the alternation of zones of hard and soft strata from tens to hundreds of feet in thickness permits certain of the harder beds to weather out as thick slabs, exposing in places smooth inclined planes several hundred feet in length along the dip slope. (See Pis. II and IV, A.) These heavy slabs form the face of the picturesque castellated buttes which extend northwestward from Joaquin Ridge toward New Idria and which constitute the eastern edge of the mountainous region. Here and there the sandstone is conglomeratic, but the beds of coarse sediments are neither thick nor persistent along the strike and were apparently formed under very local conditions. They are usually only a few feet in thickness and are nowhere as thick as the similar conglomerate zones north of Panoche Pass. (See PL IV, B.) Reddish-brown to tawny-yellow sandstone concretions are abundant. These concretions range from nodules a few inches in diameter to spherical masses with a diameter of 8 or 10 feet. They are not, however, typical of this part of the stratigraphic column alone but appear higher in the Panoche formation as well as in the overlying Moreno formation and in the Cantua sandstone member of the Martinez (?) formation. Cavernous-weathering concretionary sandstone typical of the Panoche formation is shown in Plate XIII, A. On Joaquin Ridge the concretionary sandstone has a thickness of 4,400 feet. It gradually decreases in thickness toward New Idria, and at that ...