Two-day Soils Field Trip to Central Valley, Sierra and Coast Range

We'll start our trip by leaving San Francisco via the Bay Bridge.  We won't make any stops along this corridor, but on I-80 in the east bay, typical soils include a mixture of haploxerolls and haploxeralfs on hills of more consolidated sedimentary and igneous rocks (granitic and metabasic) with occasional xererts on clay-rich sedimentary rocks, aquolls developed in alluvium on the "flats" of the Oakland-Richmond corridor, and natrixeralfs, fluvaquents and fill along the bay front.

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Order Alfisols "alf"

  • typical forest soils; in eastern US, found just E (wet side) of mollisols
  • from "pedalfers" -- aluminum and iron 
  • argillic horizon
  • BS > 35%, but probably < 50%

Suborder Xeralfs

  • xeric moisture regime
  • one of the most common suborders in California, often associate with oak woodland sites

Take Hwy 12 E near Fairfield, then right at Grizzly Island Road. 

Stop : Delta Histosols

The Sacramento-San Joaquin Delta is of course where these two rivers have slowed significantly and thus deposited sediments eroded from the Sierra Nevada and Coast Ranges. Because of the subsiding nature of the Central Valley, the original extent of the delta might be considered to be much greater than its size today -- certainly there were significant areas of riparian wetlands extending through much of both parts of the valley. The result has been extensive soils formed on fine alluvium, and frequent wetlands. Wetlands soils are usually of two types, the most common being muck -- composed of a mixture of mud and organic material and mapped as haplaquolls and fluvaquents. Peat, mapped as Histosols, is composed primarily of organic material, and is somewhat less common, but is here we have an example. The soils here are clearly an accumulation of peat formed from imperfectly decomposed tidal marsh vegetation. Unfortunately, they are only mapped as "histosols" -- let's see if we can identify whether it's fibrists, hemists, or saprists.

Return to Hwy 12, and continue eastward towards Rio Vista.  Though we'll pass some areas of higher land and typical xeralfs, we are still predominantly in an area of delta soils, composed of a scattering of more alluvial sediments producing Columbia Aquic Xerofluvents, more peaty mucks classified as haplaquolls, and a few true histosols.  Many of the islands in the delta have experienced considerable subsidence due to draining of wetlands and the resulting oxidation of organic deposits.

Order Entisols "ent"aquents57.jpg (7504 bytes)

Suborder Aquents

Great Group Fluvaquents

Suborder Fluvents



East of Rio Vista, cross the bridge and then turn left on the levee road Hwy 160.  Cross the river at Walnut Grove, then turn right (south) to pick up J11 east to Thornton.  Turn left on J8 (Franklin Blvd.) and go north past the Mokelumne River crossing until you get to The Nature Conservancy's Cosumnes River Preserve Visitor Center.  We'll take a walk here along the river to look at alluvial "basin" soils such as the Columbia Xerofluvents.  Associated xerofluvents are the Cosumnes (on lower floodplains) and Sailboat (on natural levees, with a buried soil) series. 

Coarse-loamy, mixed, superactive, nonacid, thermic Oxyaquic Xerofluvents
very deep, moderately well drained soils formed in alluvium Flood plains with bar and channel topography in some natural areas, and natural levees and have slopes of 0 to 8 percent.  Vegetation consists of a fairly dense cover of oaks, cottonwoods, willows, vines, shrubs and grasses near stream channels, but more open away from the channels. These soils occur in the central valley of California. The soils are moderately extensive

Used for irrigated hay, small grain, orchard and row crops.

Head back south towards Four Corners and Thornton (driver follow the map), and find New Hope Road which crosses the Mokelumne River into Sacramento County again.  Turn right at an intersection to get to Kost Road and follow it east to its end at J10.  Turn right (S) on J10 and cross back into San Joaquin County.  Turn right (W) at Forest Lake Road, and follow it west a little more than half a mile, where you should see evidence of San Joaquin Durixeralfs on a lower terrace.  If it's not obvious, we may have to look around for a good exposure.  According to the Series description, the type location is on an"east facing complex slope of 1% in a vineyard at elevation of 40 feet....3700 feet west of intersection of Southern Pacific Railroad and Forest Lake Road; 1,100 feet south of Forest Lake Road and about 100 feet west into vineyard; 1,700 feet north and 2,600 feet east of the southwest corner of sec. 3, T. 4 N., R. 6 E."

Stop. San Joaquin Durixeralf -- Iron Hard Pan Soils.

Alluvial terraces formed during the Pleistocene as Sierra Nevada alluvial fans are found along the entire eastern margin of the Central Valley, except where interrupted by recent river channels and floodplains. An iron-silica hardpan has developed as shallow groundwater seepage and evaporation has concentrated these minerals within the soil. These soils have relatively low agricultural value, though some are planted with grain and clover, and are usually used for rangeland & pasture. We'll try to find a good spot to check these out.

The San Joaquin Durixeralf is the California State Soil, established in the 1990's due to the efforts of high school students in San Joaquin Valley.  The State Soil is known for its inability to be cultivated, tendency to strand tractors in quagmires, and general weirdness.  Associated soils are Glann Aeric Haplaquepts and dark-gray Alamo adobe clay Typic Duraquolls in flat depressions.

Now turn around and head east again, taking a short jog north on J10 to Liberty Road, where we'll turn right (east) and continue to Hwy 88 where we'll turn left (NE).  Along the way, we'll cross somewhat higher terraces and likely see Redding gravelly loam Abruptic Durixeralfs, with an iron-silica hardpan, characterized by "hogwallow" microrelief, and fairly acid (pH < 5.3). On higher hills are Pentz gravelly loam Vitrands on rhyolitic tuff, with a more neutral reaction.

After crossing into Amador County, start looking for Jackson Valley Road, approximately four miles into the county.  Turn right on Jackson Valley Road and follow it in a wide circle to the south and east until we stop at a DFG reserve at Apricum Hill.

Stop. Ione Formation.

Brief hike gets us to Apricum Hill, where a fascinating tropical paleosol has been exhumed. Not officially an Oxisol, since it is not a current soil development, it still is a highly acidic soil with abundant kaolinite with a lateritic layer; soils developed on it seem to be classified as Amador Xerepts. Ione Manzanita (Arctostaphylos myrtifolia) and Ione Wild Buckwheat (Eriogonum apricum) are endemic to this formation. This is believed to be a Tertiary soil development, developed under a tropical climate.

Continue north to Ione, then take Hwy 24 NE towards Plymouth and then Hwy 49 to Placerville, then on to Auburn.  Hwy 49 tends to follow the gold country Sierra foothills, where the soils are some of the "oldest" forest soils in the state, with alfisols (xeralfs) and ultisols (xerults and humults) dominating the picture, except where serpentinite produces a mollisol tendency due to its tendency toward herbaceous vegetation -- and thus not the forest-associated alfisols and ultisols.  A typical serpentine soil is the Henneke Lithic Argixeroll. 

From Auburn to our campsite, there are several routes to take, dependent on time available.  Eventually we'll make our way to our campsite, at the UC Foothills Field Campus on the Yuba River.

Order Ultisols "ult"

Suborder Humults and other Ultisols suborders

Suborder Xerults occur in the same area, but have less OM

Suborder Aquults have an aquic moisture regime



Possible Side Trip to hydraulic mine.

Hydraulic mining of deposits of Tertiary gravels along the ancestral Yuba River are well illustrated here. The tailings and severely eroded hillsides have obviously little if any soil developed on them in the relatively short period of time since hydraulic mining ceased, partly due to the resulting instability of the sediments left behind. Interestingly the soils being mined are quite well developed -- Horseshoe Haplohumults can be found in unmined patches. We'll have a look.

Possible Side Trip into Sierra Nevada: on Hwy 20, continue up to Truckee. Increasing frequency of Sierra batholith granitic rocks, with scattered volcanic deposits (e.g. lahar deposits) and some peaks. Thin xerepts on granite, frequent rounded boulders and domes ­result of dilatation & exfoliation. Vitrands on volcanic materials. Near crest, colder climates create Cryochrepts, Cryumbrepts, Cryands. NE-facing escarpments have evidence of recent glacial activity. Stony soils developed on glacial moraine deposits, gravelly alluvium (proglacial sediments), bare rock and rubble characterize this area.
series properties geographic setting distribution & extent
Xeric Haplohumults
on mafic volcanic rock, especially tuff breccia 1200 to 5000 feet; ponderosa pine. extensive in Sierra & Cascades


Soils of the Sierra Foothills Field Camp

series properties geographic setting distribution & extent
Auburn xerepts shallow soil on eroded sites, with A & Bw horizons on lithic contact at 20", on metabasic or metasedimentary rock such as amphibolite schist, greenstone schist, or diabase. foothills with slopes of 2 to 75 percent.  Elev: 125 to 3,000 feet. Climate: subhumid with hot dry summers and cool moist winters. Precip: 20 to 40 inches.   Rock outcrops are common. Lower foothills of the Sierra Nevada. Extensive.
Sobrante Mollic Haploxeralfs moderately deep soils with A & Bt, on basic igneous or metamorphic rocks: amphibolite schist, diabase, andesite, or basalt. foothills with slopes of 2 to 75 percent. Elev:  125 to 3,500 feet. Climate: subhumid with hot dry summers and cool moist winters. Precip: 20 to 40 inches. Coast Range & Sierra Nevada foothills

Day 2

We'll drop out of the foothills and head for Marysville, crossing the Yuba along the way where widespread mine tailings can be seen.  In Marysville, continue W on Hwy 20.  Where we cross the Feather River, Columbia fine sandy loam Aquic Xerofluvents are seen again on recent alluvium.

Outside of Yuba City, we see evidence of the orchard crops (walnuts, peaches, prunes, almonds, kiwis) typically associated with the well-drained alluvial terrace soils: haploxerolls of the Conejo-Tisdale Association.  Many orchards have actually benefited from gravelly hydraulic mine tailings that filled many areas of the valley, though at the time of the major tailings-related floods of the late 19th century, the farmers certainly didn't like getting their towns buried in sediment.

We then descend to much more poorly drained soils of the Oswald-Gridley-Subaco Association -- xererts and argixerolls widely used for rice and prunes.

We'll either take a right at Acacia Avenue and take the Pass Road across the southern slopes of Sutter Buttes, or continue simply W on Hwy 20. 

In either case, near the Sacramento River are a variety of alluvial soils, including the Columbia fluvents we've seen before.  Before ascending terraces on the west side, we'll cross through a variety of "basin soils" associated with alkaline or saline soils.  The origin of these soils is perhaps a combination of (1) a semiarid rain-shadow climate; and (2) a salt- and alkali-rich parent material derived from shallow marine sediments deposited in this former ocean basin.  Colusa clay loam Natrixeralfs are found 2 miles south of Colusa, and are characterized by black alkali in profiles. Near the intersection with Hwy 45, older, imperfectly drained soils with alkali and gypsum salts in the profiles indicate Marvin clay loam Aquic Haploxeralfs, and another soil used for rice cultivation due to nearly impermeable clay in lower horizons.

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Stay on Hwy 20 through Colusa and continue through Williams.

series properties geographic setting distribution & extent
Coarse-loamy, mixed, superactive, thermic Typic Natrixeralfs
very deep, somewhat poorly drained soils that formed in alluvium on nearly level flood plains with slopes of 0 to 2 percent; mean annual precipitation is 16 inches

Pasture, both dryland and irrigated saltgrass; Natural vegetation is salt grass and other halophytic plants

Flood plain of the Sacramento River in the lower Sacramento Valley. The soils are not extensive.
Fine, smectitic, thermic Aquic Haploxeralfs
grayish brown, mottled, slightly acid, silty clay loam A horizons, mildly alkaline transition horizons and dark grayish brown, mildly alkaline, silty clay Bt horizons and light olive brown, calcareous B3 and C horizons. on nearly level flood plains at elevations of 10 to 100 feet under annual grasses and forbs.

Natural vegetation is annual grasses and forbs, and open stands of valley oaks . Saline-alkali tolerant plants are in alkali areas.

Central Sacramento Valley, California. The soils are inextensive.

Use: Irrigated and dry cropland and pasture. Main crops are grain, field crops, sugar beets, alfalfa and rice.

Fine-silty, mixed, nonacid, thermic Mollic Endoaquepts
grayish brown, slightly acid, slightly clay loam A horizons; grayish brown and light brownish gray, distinctly mottled, mildly to moderately alkaline, silt loam B horizons; and stratified light brownish gray and pale brown mottled loam, fine sandy loam and loamy fine sand calcareous C horizons. on nearly level flood plains at elevations of 10 to 100 feet. The soils formed in mixed sedimentary alluvium; 15 to 20 inches precip.

Natural vegetation consists of annual grasses and oak.

The flood plain of the Sacramento River and its tributaries in central California. The series is of moderate extent.

used for orchard, row, truck, and field crops excluding rice.

Stay on 20 through Williams. 3.5 miles past Williams, we'll see a sequence of vertisols, including various endoaquerts (e.g. Willows clay) on the "basin" (lower) soil areas, and Myers clay Aridic Haploxererts on alluvial fans.  Most rice cultivation is on the basin soils.  A variety of soils develop on alluvial fans, and the various series are distinguished on the basis of texture:  the clay-rich Myers and the more silty Yolo loam, widely used for non-rice intensive agriculture (note the Yolo is an entisol, and has no B horizon at all).  The Brentwood is more of a clay loam, and has developed a Bt horizon.
series properties geographic setting distribution & extent
Fine, smectitic, thermic Aridic Haploxererts
grayish brown and dark brown clay A horizons, and brown calcareous C horizons. nearly level alluvial fans. Average annual precipitation is 15 to 30 inches. Used for dry farmed grain, irrigated row, field crops and rice This soil occurs on the west side of the Sacramento Valley and in the interior valleys of the Coast Range of California. The series is of moderate extent.
Fine, smectitic, thermic Sodic Endoaquerts
very deep, poorly to very poorly drained sodic soils formed in alluvium in nearly level basins in intermountain valleys and large valleys. rice, sugar beets and safflower; original vegetation was saline-sodic tolerant plants On the west side of the Sacramento and San Joaquin Valleys and intermountain valleys of the Coast Range, California. The soils are moderately extensive
Fine, smectitic, thermic Typic Haploxerepts
A & minimal Bt horizons, in valley fill from sedimentary rocks on nearly level to gently sloping fans. They occur at elevations of 40 to 400 feet. The climate is dry
subhumid mesothermal, with hot dry summers, and cool moist winters. Mean annual rainfall is 12 to 20 inche
Valleys of the Coast Range and west side of the Sacramento and San Joaquin Valleys. The soils are moderately extensive.
Typic Xerorthents
thick grayish brown, neutral silt loam A horizons and brown and pale brown mildly alkaline silt loam C horizons.  Yolo soils have less than 35 percent clay and less than 15 percent fine sand or coarser. nearly level to moderately sloping alluvial fans. The soils formed in fine-loamy alluvium derived from sedimentary formations West side of Sacramento Valley, central California, and in the valleys of the California Coast Range. The series is extensive.

Suborder Orthents


Ascending up terraces and alluvial fans into the Coast Ranges, we pass Hillgate loam & clay loam Typic Palexeralfs developed in association with an original oak woodlands community.  After passing a gap in a ridge formed of the Great Valley Sequence of sedimentary rocks, we enter Salt Creek Valley, on either side of which are residual soils such as Altamont Typic Chromoxererts, Los Osos Typic Argixerolls, and Contra Costa Mollic Haploxeralfs, developed on sandstones and shales.  The Great Valley Sequence extends under the sediments deposited more recently in the Central Valley. When the road crosses over to the Bear Creek drainage, we find extensive Henneke stony clay loam Lithic Argixerolls developed on serpentine. As we ascend the Coast Ranges, we expect the precipitation to increase somewhat.

series properties geographic setting distribution & extent
Clayey-skeletal, magnesic, thermic Lithic Argixerolls
serpentinitic; depth to serpentinite is 10-20". Rock outcrops, stones, and cobbles are common and occupy 5 to 60 percent of the surface. mountainous uplands; Used mostly for wildlife and watershed. The principal native plants are scattered oaks, Foothill pine, Coulter pine, cypress, and shrubs such as leather oak, whiteleaf manzanita, muskbrush, and toyon. Grasses are sparse, usually squirreltail and a few annuals. Coast Ranges, foothills of the Sierra Nevada and Klamath Mountains of California. The soils are extensive
Fine, smectitic, thermic Aridic Haploxererts
deep, well drained soils that formed in material weathered from fine-grained sandstone and shale; slickensides, cracks. on gently rolling to very steep uplands; Used for grazing and dry farmed grains, mainly barley. The principal plants are annual grasses, forbs, and scattered oak trees In the Coast Range in central and southern California and the Sutter Buttes. The soils are extensive.
Contra Costa
Fine, mixed, superactive, thermic Mollic Haploxeralfs
moderately deep, well drained residual soils foothills & mountains; Vegetation is mostly trees and annual grasses Foothills and mountainous ranges of the east-central and northern coast ranges, California. The soils are moderately extensive.
Fine, smectitic, thermic Typic Palexeralfs
very deep, well to moderately well drained soils that formed in alluvium on nearly level to moderately sloping old terraces at elevations of 15-2,000 feet; precipitation varies from 14 to 30 inches; grasses and valley/blue oaks West side of Sacramento Valley and Coast Range valleys, soils are of moderate extent.

At the summit, we pass into Lake County. Steep slopes produce high erosion rates and thinner soils. Millsholm Lithic Haploxerepts is one example, though good old haploxeralfs dominate. This area is primarily used for grazing, wildlife habitat, and watershed.  At Long Valley, note the Still-Lupoyama Association of haploxerolls developed on the flood plain and xerofluvents and riverwash are found on recent deposits -- note gravel pits.

As we approach Clear Lake, look for the left turn at Hwy 53 where we'll leave Hwy 20 behind.

Possible Side Stop: Vitrands - Cinder Land Complex

This would be a side trip, by going west on Hwy 20. In just a few miles look for a small sign for St. Anthony's church on the left, and stop near the quarry

Pleistocene and Holocene volcanism has been significant to the Clear Lake area. Mt. Konocti, a composite cone which produced both rhyolitic tephra and basaltic lava flows as recently as the last 5000 years, dominates the landscape. These two tephra cones are probably younger, but their apparent age is younger than their actual age due to the high permeability of the loose basaltic cinders. Note the red color from iron oxides (remember that basalt is composed of iron-rich minerals).

At Lower Lake, we get to Hwy 29.  We'll be taking this into Napa County, but first we'll do a brief side trip in the opposite direction, towards Mt. Konocti,  a 450,000 Ma composite cone on the right. On it have developed a series of haploxeralfs.  We'll turn before we get to it, to have a look at soils with significant obsidian in their makeup -- the Glenview-Arrowhead-Bottlerock complex.  Then we'll return to Lower Lake and head toward Calistoga and Napa.

series properties geographic setting distribution & extent
Fine, halloysitic, mesic Ultic Palexeralfs
very deep, well drained soils formed in material derived from obsidian and pyroclastic materials hills with slopes of 2 to 50 percent; used mainly for cropland producing walnuts, timber production, watershed and wildlife habitat Obsidian flows in the Clear Lake volcanic field in Lake County, California. The soils are not extensive.
Clayey-skeletal, mixed, mesic Ultic Haploxeralfs
moderately deep, well drained soils formed in material weathered from obsidian. Mineralogy is influenced by amorphous material; lithic contact on volcanic flows; Used mainly for watershed and wildlife habitat. Natural vegetation consists of chamise, manzanita, and scrub oak Clear Lake volcanic field, Lake County, California. The series is not extensive.
Loamy-skeletal, mixed, mesic Ultic Palexeralfs
deep, well drained soils formed in material derived from obsidian on hills and dissected volcanic flows "

When we cross into Napa County, on the slopes of Mt. St. Helena, we'll see more volcanic soils of the Kidd (rhyolite pm) and Hambright (basalt pm) group of "very stony loams and loams on uplands."  Note the contrast between rhyolitic and basaltic parent materials.

Medial, mixed, mesic Lithic Haploxerands
shallow, somewhat excessively drained soils formed in material weathered from rhyolitic tuff.  A & Bw on R. Natural vegetation is hoary manzanita, chamise, ceanothus, scrub oak, knobcone pine, grasses, forbs and scattered ponderosa pine. Northern Coast Range Mountains and Cascade Range
Loamy-skeletal, mixed, thermic Lithic
shallow, well drained soils formed in material weathered from basic igneous rocks -- basalt on plateaus, basalt flows, and hillslopes Coast Ranges

Somewhat more developed residual soils of the Forward-Boomer-Felta and Forward-Aiken groups will be found as we descend to Napa Valley.  Near Calistoga, we'll take Tubbs Lane to further contrast the rhyolitic and basaltic parent material sources.  Then we get to increasing alluvial soils in Napa Valley.  Past Calistoga, these residual soils are found mainly to the right of the road in the forested areas, developed above volcanic rocks and uplifted alluvial deposits, noted by a plant cover of Doug fir, ponderosa pine, black oak, and chaparral communities, with land uses tending toward timber, wildlife habitat and recreation, at least where undeveloped by new homes.  The Aiken soils are reddish ultisols similar to those we saw in the Sierra foothills.

Medial, mixed, mesic Typic Vitrixerands
moderately deep, well drained soils formed in material weathered from rhyolithic tuff. on rhyolitic tuff, on hills and mountains. Slopes are 2 to 75 percent Occurs at moderate elevations in the Cascade and Coast Ranges of northern California
Fine-loamy, mixed, mesic Ultic Haploxeralfs
deep and very deep, well drained soils that formed in material weathered from metavolcanic rock on greenstone (metavolcanic), in hills and mountains. Mountainous areas of Siskiyou County in northern California south to San Diego County.  extensive.
Loamy-skeletal, mixed, thermic Pachic
grayish brown, slightly acid, very gravelly heavy loam A horizons and grayish brown, slightly acid, very gravelly clay loam B2t horizons underlain by gravelly old alluvium On dissected terraces at elevations of 100 to 2,000 feet. Slopes are 5 to 75 percent. The soils formed in mixed gravelly alluvium from mixed igneous rocks Used for grazing. Native vegetation is white oak, manzanita, annual grasses and shrubs.
Sonoma and Napa Counties
Fine, parasesquic, mesic Xeric Haplohumults
very deep, well drained soils formed in material weathered from basic volcanic rocks.  Surface layer reddish brown loam, subsoil yellowish red clay. on broad gently sloping tabular ridges with moderately steep to steep sideslopes of 2 to 70 percent at elevations of about 1,200 to 5,000 feet. They formed in material weathered from basic volcanic rocks, principally tuff breccia. The Sierra Nevada and Cascade mountains of California and Oregon

As we progress down Napa Valley, we'll find the soils increasingly dominated by the alluvial soils of the Bale-Cole-Yolo group, with a few terrace areas of the Haire-Coombs group. The Bale-Cole-Yolo soils are "nearly level to gently sloping, well drained and somewhat poorly drained loams, silt loams, and clay loams on flood plains, alluvial fans, and terraces" formed in deep alluvium derived from igneous and rhyolitic bedrock, with plant cover of annual grasses, forbs, willows, blackberry, and scattered oak.  The Haire-Coombs soils are more gravelly. 

Fine-loamy, mixed, superactive, thermic Cumulic Ultic Haploxerolls
very deep, somewhat poorly drained, gravelly-sandy alluvial soils on nearly level to gently sloping alluvial fans and terraces at 100 to 300 feet. They formed in stratified, gravelly and sandy alluvium from mixed sources. The climate is subhumid mesothermal with hot dry summers and cool moist winters. Used mostly for wine grape production, Napa County
Fine, mixed, thermic Pachic Argixerolls
Bt horizon; very deep, somewhat poorly drained soils that formed in alluvium on river terraces, basins, flood plains, or on alluvial fans with slopes of 0 to 5 percent. Uncultivated areas have oak-grass vegetation with some shrubs and forbs. Used mostly for production of orchards, vineyards, truck crops, and irrigated pasture. North coastal counties, California. The soils are moderately extensive
Typic Xerorthents
thick grayish brown, neutral silt loam A horizons and brown and pale brown mildly alkaline silt loam C horizons. nearly level to moderately sloping alluvial fans. The soils formed in fine-loamy alluvium derived from sedimentary formations West site of Sacramento Valley, central California, and in the valleys of the California Coast Range. The series is extensive.
Fine, mixed, thermic Typic Haploxerults
gray and grayish brown, neutral or slightly acid, light clay loam A horizons, pale brown, strongly acid, clay B2t horizons, and pale yellow, strongly acid, gravelly clay loam C horizons nearly level to moderately steep hills at 20 to 2,400 feet. Formed in terrace deposits and in part in residuum weathered from arkosic sandstone and granodiorite. Pasture, in Sonoma, Napa and Monterey Counties, California
Fine-loamy, mixed, thermic Ultic Haploxeralfs
well drained, moderately slowly permeable soils on gravelly terraces on gravelly terraces at 100 to 500 feet. formed in gravelly alluvium from mixed sources. Subhumid mesothermal climate with warm dry summers and cool moist winters. Orchards, vineyards, irrigated pasture, in Napa County, California (limited extent)


Look for the turn onto Hwy 12.  A short ways to the west, the alluvial/terrace soils are briefly left in favor of residual soils on sandstone and shale.  See if we can see where these alfisols of the soils of the Bressa-Dibble-Sobrante group can be found.

Fine-loamy, mixed, thermic Typic Haploxeralfs
well drained, moderately deep soils over weathered sandstone/shale foothills with slopes of 5 to 75% Coast Ranges of Central CA,  moderately extensive
Fine, smectitic, thermic Typic Haploxeralfs
moderately deep, well drained soils that formed in material weathered from shale and sandstone:  similar to Bressa but more clay undulating gentle to very steep uplands West side of Sacramento Valley southward in the Coast Range
Sobrante Mollic Haploxeralfs moderately deep soils with A & Bt, on basic igneous or metamorphic rocks: amphibolite schist, diabase, andesite, or basalt. foothills with slopes of 2 to 75 percent. Elev:  125 to 3,500 feet. Climate: subhumid with hot dry summers and cool moist winters. Precip: 20 to 40 inches. Coast Range & Sierra Nevada foothills

Crossing Carneros Creek, the soil types briefly change to the Bale-Cole-Yolo group we saw in Napa Valley.

Near the Sonoma County border, we may detect some rockier inceptisols of the Toomes series of haploxerepts, with more oak and scrub vegetation, and developed on weathered volcanics.  While classified as an inceptisol, the profile described in the county survey has no B horizon.   

In Sonoma County, we'll see more of the same variations, with some residual alfisols, but mostly alluvial and terrace soils similar to what we've seen before.  Eventually we'll get to some of the bay-margin muck and clay-rich soils.  These are typical salt-marsh wetland soils, rich in organic matter, but not histosols like are found in the more freshwater areas.  A typical soil is the Reyes fluvaquent, with Clear Lake endoaquert found in the more clay-rich sites, typically slightly inland from the tidal areas.  Other bay margin soils common in this area are aquepts


Series properties geographic setting distribution & extent
Fine, mixed, acid, thermic Sulfic Fluvaquents
deep, somewhat poorly drained soils that formed in alluvium from mixed sources. Reyes soils are in reclaimed and protected marsh areas and have slopes of 0 to 2 percent Mainly around the edges of Suisun and San Pablo Bays and scattered in the Sacramento Delta in California. The series is of moderate extent.
Clear Lake
Fine, smectitic, thermic Xeric Endoaquerts
very deep, poorly drained soils that formed in fine textured alluvium derived from sandstone and shale basins and in swales of level drainageways In small valleys of the Coast Range and along the San Joaquin and Sacramento Valleys. The soils are moderately extensive

aquepts.jpg (28395 bytes)
We'll end our trip coming into Marin County and heading across the Golden Gate Bridge.  On the south side of the Waldo tunnel fog drip reduces the tendency for the more seasonally extreme xeric moisture regime, and the ustic regime predominates: argiustolls are common on the hillsides. In Marin County, hillside soils north of the tunnel are typically xerolls and xerepts.

Xeric vs. Ustic moisture regimes

To be xeric, the entire moisture control section of a soil profile must be dry in the summer for 45 consecutive days in 6 out of 10 years.  The ustic moisture regime includes summer-dry climates, but not those that do not meet this requirement.  Fog is the likely reason why the more coastal soils don't experience this.

Order Mollisols "oll"
  • mollic epipedon
  • dark color (< 3.5 moist, < 5.5 dry)
  • base saturation > 50%
  • not too hard, not too dry

Suborder Ustolls ustolls.jpg (6733 bytes)xerolls.jpg (42687 bytes)

  • ustic soil moisture regime
  • US Great Plains wheat belt and Dust Bowl,
    developed on prairie grasses

Great Group Argiustolls

  • argillic horizon

Suborder Xerolls

  • driest of the mollisols
  • widespread in Idaho, Utah, Washington, and Oregon
  • central Turkey