This article is an excerpt from the book called “Geologic Guidebook of the San Francisco Bay Counties” This blog takes us back to the prehistoric era and talks about the forests of the San Francisco bay. thanks to the author.
There is, perhaps, no single feature of the natural environment which tells so much about local conditions as its vegetation. One need not be a botanist to recognize marked changes in the plant population as he travels from the Pacific shore eastward across the Coast Ranges into the Great Valley; the oaks and pines of the Sierra foothills, with their understory of Manzanita, are different both from the trees of the grassy savannas below and the forests of the mountains beyond; and the Sierra summit trees disappear altogether, to be replaced by low-growing shrubs and herbs which can survive the snow and wind of winter at high altitudes. The forests, chaparral, and grasslands of today reflect many characteristics of the landscape and climate where they occur.
When a student of earth history turns to the record of the past, he surveys evidence provided by the rocks of which the earth is made; he searches for indications of ancient land surfaces-the topography of other ages; from layers of ancient sediments he digs out fossils representing successive populations which have spread over sea and land, filling each environmental niche as do the animals and plants of today. With all this evidence collected and wisely interpreted, pictures rise up from remote antiquity like images developing on a photographic film. To understand the world of today and its background, these pictures of yesterday must be clearly drawn and understood.
A fossil is the remains of a once-living thing preserved in the rocks. Although the individual which it represents may have been dead for millions of years, there is still residual in a petrified bone, a shell of a calm, an imprint of a leaf, some evidence of the organic processes which carried it through its life during an earlier geologic period. Studied from the standpoint of these vestiges of its former existence, a fossil becomes more than an object long dead-it comes to represent a symbol of life and living which has fortunately been preserved from earlier ages to offer its testimony in solving the problems of earth history. The fossils of land plants tell much about the past, for their growth and distribution have always been controlled by temperature and rainfall, and by the hills and valleys of former landscapes. Therefore the record of ancient forests is one of the most accurate sources of information regarding the history of the bay area during the late chapters of geologic time.
This discussion will not go back to the beginnings of plant history because there is no record in the bay region of the great forests of ferns, horsetails, and clubmosses which lived in the eastern part of North America during the Carboniferous period. Nor are there any-where in the bay area fossil examples of the cycads, conifers, and ginkgos, which were so widely distributed during the Mesozoic time. No one region has rocks of all ages exposed, and it is necessary, therefore, to piece together the incomplete fragments of earth history from many places. In the vicinity of San Francisco is found one of the most continuous sequences of Pleistocene is also well represented. These are chapters just preceding the day in which we live when forests were made up of plants closely resembling those of today. Before turning to this well-known section of plant history, however, the less complete records 75 million years, will be considered.
The most ancient plant fossils known to occur in the bay area were collected many years ago during coal-mining operations near Tesla in Alameda County. Within recent years additional material has been collected, including leaves of the fan palm (Sabalities), magnolia, avocado (Persea), Sebastian (Cordia), and huanchal (Cupania). All of these leaves are large and thick as compared with foliage of trees growing in temperature regions, and with few exceptions, their modern equivalents are found on trees and shrubs growing in the tropics. The close association of the leaf-bearing beds with shales containing marine fossils suggests that the habitat of this forest was on the borders of swamps and lagoons along the shore of the sea. A similar forest, but of a river flood plain type, has been described from Chalk Bluffs and vicinity, on the west slope of the Sierra Nevada. The modern distribution of these forests in Mexico and Central America, in areas without frost, emphasizes the extent of climatic change in the bay area since the Eocene epoch.
During the remainder of the Eocene, and in the Oligocene and Miocene epochs which followed, shallow seas continued to cover wide areas in west-central California. The sediments deposited in the bays and along the shores of these seas contain the remains of animals of sorts that today live only in saltwater. The presence of a coral in Oligocene deposits near Walnut Creek, and of mollusks like those now living as far south as the Gulf of California, suggests that the sea of this epoch was much warmer than the present waters of San Francisco Bay. During the Miocene epoch which followed, there is evidence of a gradual reduction in water temperatures, though even at this time the life of the sea was like that now found in warmer waters along the coast of southern California. In these marine deposits, there are included only fragments of plants that lived along adjacent shores. A few pieces of driftwood have been found, now petrified. And in the Kriker formation north of Pittsburg there are incomplete leaves which may have been blown into the sea from nearby headlands. Although the forests of the bay counties can scarcely be reconstructed from this meager record, a brief survey of the fossil plants found elsewhere in California during Oligocene and Miocene time will suggest the probable aspect of the vegetation during these epochs.
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