The study of the geologic and hydrologic aspects of Sonoma County provide a rational basis for the evaluation of the various ground water parameters. Before decisions affecting the mode, occurrence, quality, or use of ground water can be made, a knowledge of these two basic sciences is necessary.
Ground water exists at many places beneath the surface of the earth. It is normally hidden from view but, on occasion, can be seen flowing from springs and wells or seeping into tunnels. Certain properties of the various geologic materials control the ability of ground water to enter into, move through, be stored in, or be extracted from the ground. Therefore, an understanding of geology is necessary in order to gain an understanding of ground water found within a particular area of investigation. A basic part of the study of the geology of an area includes the preparation of a map showing surficial exposures of each rock type. Interpretation of the map facilitates the study of the underground configuration, characteristics, and physical properties of these materials. This, in turn, enables the evaluation of ground water storage, movement, and yield, as well as the identification of areas of ground water quality problems, recharge, and extraction.
The fundamental unit of a geologic study is the geologic formation. A formation has been defined as any assemblage of rocks which have some character in common, whether of origin, age, or composition. In this sense, the term "rock" is defined as any naturally occurring part of the earth's crust, and includes hard, dense granite, lava ash, clay, sand, and even loose, uncohesive soil.
The rocks in Sonoma County have been divided into three major rock groups, viz. igneous, sedimentary, and metamorphic. Igneous rocks were formed from the cooling, solidification, and crystallization of molten magma or lava and are of two types, intrusive and extrusive. When formed at depths of several miles, they are spoken of as intrusive. These rocks contain mineral crystals that can be seen by the unaided eye and are characterized by the granitic rocks exposed at Bodega Head. When formed on the surface, such as a lava flow, igneous rocks are spoken of as being extrusive. -Most crystals in this latter rock type are
minute; only a few are visible to the unaided eye. A typical extrusive rock type is the basalt found in the Sonoma Mountains. Certain groups of intrusive and extrusive rocks are chemically identical and have inherent characteristics which are not readily apparent. This is because a single mass of molten magma can produce three strikingly different rocks when solidification takes place under differing conditions. For example, when formed at great depth, a fairly coarse-grained granitic rock will result. This same magma, if formed near the ground surface under conditions of lower temperature and pressure, will solidify as a fine grained rhyolite. When temperatures and pressures are still lower, the molten mass will chill and obsidian, or volcanic glass will result. This latter rock type contains no crystals.
The second major rock group is the sedimentary rocks. These rocks are formed from the products of erosion of older rock materials. As weathering progresses, erosional products wash downslope and become deposited as gravel, sand, and clay in adjacent valleys or on the bottom of the sea. In time, because of burial and compaction, clay turns to shale, sand to sandstone, and gravel to conglomerate. These are the most common sedimentary rock types; they occur in the Coast Ranges west of Healdsburg Sedimentary rocks also include limestone, a deposit of calcium carbonate, and chert, a deposit of silica.
The third group of rocks is the metamorphic rocks. The term "metamorphic" means changed or altered. This rock type is formed whenever igneous or sedimentary rocks become buried deep within the earth and where pressures and temperatures are extremely high. In such an environment, the minerals which make up the original rocks undergo alteration and mixing to form entirely new minerals. Metamorphosed igneous rocks become greenstone and schist, sandstone becomes quartzite,, limestone becomes marble, and shale becomes slate. Metamorphic rocks in Sonoma County occur north of Guerneville and in the Mayacmas Mountains.
To the average person, the rocks making up the crust of the earth appear to be a solid and nearly unbreakable mass. In reality, however, this crust is very plastic. This is shown by the forces which slowly bend and squeeze the rock upward into folds called anticlines and downward into folds called synclines.. If these bending conditions continue over a great length of time, and if folding cannot relieve all of the strain, then rupture and movement, accompanied by an earthquake, occur along a plane of slippage called a fault. This movement may be either in a horizontal or in a vertical direction, or a combination of both.
It is the continued tectonic force of folding and faulting over tens of millions of years which moves rock masses from the surface to zones deep within the earth where the temperatures and pressures of metamorphism come into play. This same force then brings the metamorphosed rocks back up to the surface, where they become exposed by erosion.
One of the initial phases of the study of ground water geology is the preparation of a geologic map showing the surficial exposures of the various formations. Through the combined use of the geologic map and geologic sections, a three-dimensional picture of a region can be visualized.
The various formations are designated on the map by symbols Each symbol contains letters representing the age and either the rock type or the formational name. For example, on Plate 1, the Merced Formation, found west of Sebastopol,, is designated on the map by the symbol ""Tm"". The first letter stands for the age of the material, in this case Tertiary, and the last for the name ""Merced"". Using the geologic map included in this bulletin, one can readily view the areas of outcrop of each geologic formation exposed in Sonoma County.
One of the most important aspects of geology is the concept of geologic time. Because geologic time encompasses hundreds of millions of years, it is not practical to think of time in numbers of years. Instead, geologic time has been divided into major and minor time units, each bearing a particular name and each representing a specific identifiable time span.
The major divisions of geologic time are the eras. The very earliest division is the Precambrian. During this era, which ended 570 million years ago, life was in its most primitive form. Following the Precambrian was the Paleozoic Era, whose name means ancient life. This era extended over a time span of 345 million years, during which all forms of plant and animal life became well established. The Mesozoic, or middle life, era extended from the close of the Paleozoic Era to 65 million years ago and was the time of the dinosaurs. The most recent era, that in which we live, is the Cenozoic, or modern life, era; it extends back 65 millions years.
The Paleozoic Era has been divided into seven periods, and the Mesozoic Era into three periods. The Cenozoic Era has been divided into two periods, the Tertiary Period, which covers the time span from 65 to 2 million years ago, and the Quaternary Period, which includes the last 2 million years. Both of these
periods have been further subdivided into epochs. Present-day time is included in the Holocene Epoch, which goes back 10,500 years.
Another way of looking at geologic time since the beginning of the Paleozoic Era is to compress it into an imaginary one year of time. Were this the case, the Paleozoic Era would have lasted eight months and the Mesozoic Era another three and one-half months. The Ceozoic Era would have begun during the next-to- week of the year. The entire history of man, stretching back over 2,000 years, would have been allowed only about 15 minutes at the close of the year.
Figure 2 presents a stylized view of geologic time showing the significant events and creatures of geologic history. Figure 3 is a geologic time scale showing the relative time position of the various geologic formations in Sonoma County.
The geologic formations which underlie Sonoma County can be divided into two basic groups, viz. water-bearing and nonwater-. A water-bearing formation is one that readily absorbs, transmits, and yields usable quantities of ground water to wells. Conversely, a nonwater-bearing formation is one that yields only limited quantities of water to wells. In some cases, nonwater-bearing formations can yield mineralized, unpotable water. With but one exception, all of the water-bearing formations of Sonoma County are of Cenezoic age. Furthermore, nonwater-bearing rocks are all "hard rocks", that is, they are consolidated and massive. Conversely, most of the water-bearing rocks include soft sandstone, clay, alluvial soils, and river gravels.
Nearly all of the materials that make up the water-bearing formations have open spaces containing ground water. The size of these openings ranges from minute pores in clays to intergranular openings in deposits of sand and gravel. The porosity, or per- of the total volume of the openings, is not necessarily indicative of the ease with which ground water moves through the material. If the openings are very small, or if they are not connected, the material has a low permeability. Materials of low permeability, such as clay, transmit very little water. In contrast, materials of high permeability, such as river gravel, yield large amounts of ground water. A geologic bed, or stratum, which readily transmits ground water (i.e., has a high permeability) is called an aquifer. In contrast, materials which contain ground water but cannot transmit extractable quantities (i.e., have low permeabilities) are called confining beds. Certain strata can act as aquifers in one area and as confining beds in another area because of lateral changes in permeability resulting from changes in the physical characteristics of the materials.
click here for a diagram called looking back in geologic time
Ground water exists in two zones beneath the ground surface, as shown on Figures 4 and 5. The upper zone is the zone of aeration. Here, most openings in the geologic materials are partly filled with air and partly with water. Wells do not produce ground water from the zone of aeration because the molecules of water adhere tightly to the various geologic materials. If perched ground water occurs in the zone of aeration, it is contained in an isolated saturated zone which is separated from the main ground water body by an underlying impermeable stratum. Well ""B"" on Figure 4 represents a well producing from a perched aquifer.
In the lower zone, or zone of saturation, all of the interconnected openings in the geologic materials are filled with ground water; little or no air is present. Ground water exists in this lower zone under either unconfined or confined conditions. An aquifer containing unconfined ground water is one that is not overlain by a confining bed. The upper surface of an unconfined body of ground water is called the water table. It is represented by the level of water in a well tapping unconfined ground water. Well "D" on Figure 4 represents a well producing from an unconfined aquifer. Unconfined ground water moves very slowly in the direction of the downward slope of the water table.
A confined aquifer is one that is overlain by relatively impermeable material and is isolated from overlying aquifers except in areas of recharge. Ground water contained in confined aquifers is under pressure, and the level to which this confined ground water will rise in a nonpumping well is the potentiometric surface of the ground water. This latter is an imaginary surface that represents the upward pressure exerted by the confined ground water on the materials overlying it. Where the potentiometric surface is below ground, water will rise in the well to some point above the top of the aquifer, as represented by Well "A" on Figure 5. If the potentiometric surface is above ground, the well will flow as represented by Well "C".
The stratification of aquifers and confining beds is the result of deposition under continually changing environments. Coarsedeposits, sand and gravel, are laid down along stream channels. They are coarsest at the apex of alluvial fans and become finer- grained the farther removed they are from the mountains Silts and clays are deposited by slow-moving streams, in flood areas adjacent to active channels, and in lakes, swamps, and bays.
The earth's water circulatory system is known as the hydrologic cycle. In this cycle, shown on Figure 6, water evaporates from the ocean and other bodies of water; it is also given off into the atmosphere by plants. This water collects as clouds and then returns to earth as precipitation. The precipitation either forms
click here for a diagram of confined and unconfined aquifers
click here for a diagram of the hydrologic cycle
surface runoff or infiltrates the ground water body. In time, runoff and ground water collect in bodies of water and the cycle begins anew.
The pattern of movement of a particle of water from the time it enters the ground to the time it emerges, either naturally or from a well, is controlled by the subsurface conditions encountered. Upon entering the ground, the particle of water moves downward through the zone of aeration and into the zone of saturation. This happens whenever water from precipitation, streamflow, applied irrigation, and all of the other various sources moves into the ground through the open spaces in permeable materials. The area over which this is accomplished is called a ground water recharge area. These areas may be found on mountains, along foothill slopes, and on valley floors. In Sonoma County, important recharge areas occur along the channel area of the Russian River. Here, the deposits are very permeable, allowing for the rapid infiltration of water down to the ground water body. Water flows over these recharge areas during the entire year, affording a continual replenishment to the ground water body.
Water which infiltrates the permeable material eventually reaches a zone of saturation. The water then moves under a hydraulic gradient into confined aquifers. Ground water under pressure moves laterally toward areas of lower pressure, such as pumping depressions. In cases where the pressure relief area is along a stream channel, springs form and help to maintain streamflow during periods of low precipitation.
The general ground water movement pattern of a valley can be interpreted from maps which show lines of equal elevation of the ground water surface. From such a map, the direction of ground water movement is interpreted as being perpendicular to the contour lines and moving from the higher elevation contour to the lower. The relative spacing between the contour lines indicates the hydraulic gradient of the ground water, which is an index of the resistance encountered as the water moves through the various permeable materials. Other physical barriers which may impede the movement of ground water are also indicated by the patterns or spacings of the ground water contours. The effect of faults on the movement of ground water can often be interpreted from the contour maps. Where faults have positioned a particular water-bearing stratum opposite an impermeable stratum, ground water may rise along the fault zone and appear at the ground surface as springs.
The water well numbering system used in this bulletin is based on the rectangular system of subdivision of public land. When Sonoma County was first settled, most valley lands became parts of 25 land-grant ranchos. Rancho names ranged from those of Miwok and Pomo Indian derivation, such as Petaluma (from ""peta"", place, and ""yome"", flat), to surnames such as Bodega (named for Spanish explorer Juan Francisco de la Bodega)) and Spanish descriptive names, such as Agua Caliente (hot spring). Lands outside of the rancho land grants later became identified as public lands. These were later surveyed into townships of 36 square-mile area and referenced to the Mount Diablo Base and Meridian. Each township was divided into 36 sections of roughly one square-mile area. Because land-grant areas do not have township and section lines, these have been projected across for the purpose of numbering water wells.
A state well number has two basic parts, its township location and its section location. For example, Well No. 7N/8W-17G1 is located in Township 7 North, Range 8 West, and Section 17; this places the well west of Santa Rosa. Each section is subdivided into 16 quarter-quarter sections (40-acre tracts); these are identified by a letter designation. This particular well is in Tract ""G"", which also can be described as the southwest-quarter of the northeast-quarter of Section 17. The final number is the sequential number within that particular tract.
Sonoma County is situated in the Coast Ranges Geomorphic Province (refer to Appendix B for definitions of geologic, hydrologic, and related terms), which is characterized by northwest-trending mountains and valleys. The topography of the county ranges from low-lying mud flats along the north shore of San Pablo Bay, through which Petaluma Creek and Sonoma Creek move sluggishly, to Mount St. Helena, which frequently is mantled by snow in winter and has a crest elevation of 4,343 feet (1,324 meters). (Refer to Appendix C for English-Metric conversion tables.) Major mountain areas include the Mayacmas Mountains-Mount St. Helena-Sugarloaf Ridge zone, which forms the eastern boundary with Napa County, the Sonoma Mountains which separate the Santa Rosa Plain and Sonoma Valley, the rugged mountainous area between Healdsburg and the coast, and the low, rolling hills west of Sebastopol..
Six mayor valley areas contain the principal population centers of the county. In the south part of the county are located Sonoma Valley, drained by Sonoma Creek, and Petaluma Valley, drained by Petaluma Creek; both of these streams are tributary to San Pablo Bay. To the north are the valleys of the Russian River watershed. These include Cloverdale Valley, Alexander Valley, Dry Creek Valley, and the Santa Rosa Plain. Smaller valley areas include Rincon Valley, Kenwood Valley, and Bennett Valley, all tributary to Santa Rosa Plain, and Knights Valley and Franz Valley, which are tributary to the Russian River.
The known geologic record in Sonoma County begins with rocks which date back to the middle of the Jurassic Period, or about 150 million years ((m.y.).) ago. Events which took place during the hundreds of millions of years preceding this period have been obliterated by later geologic conditions. During the some 125 million years from the Jurassic Period to the middle of the Tertiary Period, the area now known as Sonoma County was a part of the sea floor. At times this floor was covered by many thousands of feet of water teeming with all varieties of marine life. This sea floor continually received sediments from neighboring lands to the east; all the while, at depths of several miles, vast masses of molten magma slowly migrated upward to solidify as crystalline rock.
Near the end of the Miocene Epoch (12 m.y.. ago) forces deep within the earth began forming what are now the mountains of northwestern California. These forces brought the ocean floor above sea level and exposed the sediments to wind and rain, where erosion shaped mountains and valleys. Continued mountain-building caused periods
of widespread faulting which resulted in the displacement of several thousands of feet of formerly adjacent sediments. During the Pliocene Epoch (2 to 12 m.y.. ago), much of what is now Sonoma County was above sea level. The topography was one of low, rolling hills with many lakes occupying valley areas. The lakes abounded with life, particularly microscopic plants called diatoms.
Near the close of the Pliocene Epoch (2 m.y.. ago), volcanic activity broke out to the east. Many volcanic vents and cinder cones spewed out volumes of lava and ash as eruption after eruption took place. By the close of this activity, a broad volcanic highland had been formed along what is now the eastern boundary of the county. To the northwest, near what is now the town of Annapolis, there was a long, low, northwesterly-trending valley which was slowly being filled with soft, sandy sediments. Similar materials also were being deposited farther south on the plains which sloped gently from the volcanic highlands to the sea.
Once again, mountain building forces came into play, and the long valley near Annapolis was slowly elevated until only its remnants are seen today along the ridgetops.. Meanwhile to the south, geologic forces were shaping Sonoma\ and Kenwood Valleys. To the west, however, were still rolling hills, and no evidence of the Santa Rosa Plain could be seen. About one m.y.. ago, during the Pleistocene Epoch, a syncline began to develop west of Sonoma and Kenwood Valleys. Continued arching of the sediments eventually created the Santa Rosa Plain. Toward the close of this epoch, sea level stood about 300 feet (91 meters) lower than it does today because of the vast volumes of water locked in the glaciers of the great ice ages. Drainage from the Santa Rosa Plain originally flowed south toward San Pablo Bay, but uplift near Penngrove\ blocked this southward drainage. Subsequently, drainage turned toward the Russian River, which continues to carve a westward course across the Coast Ranges.
Nearly all of the geologic formations of Sonoma County yield water to wells. Well yields range from 1,000 gallons per minute (gpm), or 3,800 liters per minute (1/m), in wells completed in coarsealluvial materials to less than 1 8pm (3.8 1/m) in wells completed in consolidated rocks. Wells with high yields usually produce water of good to excellent quality; those of markedly lower yields may produce water containing significant quantities of undesirable mineral constituents.
Appendix D to this bulletin presents a detailed discussion of the ground water geology of Sonoma County. Included in the appendix is a discussion of the geologic history of the area as it affects
ground water, a discussion of each geologic formation along with its water quality and water yielding characteristics, and finally a discussion of the geologic structure of Sonoma County and its effect on ground water quality and movement.
Data on the physical and water-bearing characteristics of each geologic unit in Sonoma County is summarized on Table 1. The surficial extent of each geologic unit is presented on Plate 1; their subsurface extent is shown in the geologic sections which appear in Figure 7. Water quality data for the various geologic units appear on Table 2, and well yield data appear on Table 3.
click here for the index to the geologic cross-sections
click here for the west half of geologic section A - A'
click here for the east half of geologic section A - A'
click here for the west half of geologic section B - B'
click here for the east half of geologic section B - B'
click here for the west half of geologic section C - C'
click here for the east half of geologic section C - C'
click here for the west half of geologic section D - D'
click here for the east half of geologic section D - D'
click here for geologic section E - E'
click here for the geologic section F - F'
click here for geologic section G - G'
click here for the west half of geologic section H - H'
click here for the east half of geologic section H - H'
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