Established Series


The Contra Costa series consists of moderately deep, well drained soils formed in material weathered from shale and sandstone. These soils are on foothills and mountains that have slopes of 2 to 75 percent. The mean annual precipitation is 10 to 35 inches and the mean annual temperature is about 60 degrees F.

TAXONOMIC CLASS: Fine, mixed, superactive, thermic Mollic Haploxeralfs

TYPICAL PEDON: Contra Costa clay loam - annual grass range. (Colors are for dry soil unless otherwise noted).

01--1/2 to 0 inches; partially decomposed layer of leaves and twigs; abrupt smooth boundary. (0 to 1 inches thick)

A--0 to 5 inches; brown (7.5YR 5/4) clay loam, dark brown (7.5YR 3/4) moist; moderate coarse granular structure; hard, friable, sticky, plastic; many fine roots; about 12 percent coarse fragments; neutral (pH 6.9); abrupt wavy boundary. (4 to 10 inches thick)

Bt1-- 5 to 12 inches; brown (7.5YR 5/4) clay loam, dark reddish brown (5YR 3/4) moist; moderate very coarse blocky structure; very hard, firm very sticky, plastic; few fine and medium roots; continuous thin clay films on peds; neutral (pH 6.7); clear wavy boundary. (4 to 10 inches thick)

Bt2--12 to 21 inches; reddish brown (5YR 4/3) clay, dark reddish brown (5YR 3/4) moist; weak coarse prismatic parting to strong very coarse blocky structure; very hard, firm, very sticky, plastic; few medium and large roots; about 5 percent angular rock fragments of gravel size; few thin continuous clay films on peds; neutral (pH 6.7); clear wavy boundary. (6 to 12 inches thick)

BC--21 to 34 inches; brown (7.5YR 5/4) shaly clay, dark reddish brown (5YR 3/4) moist; moderate coarse subangular blocky structure; very hard, firm, very sticky and plastic; few large roots; about 20 percent angular fragments of shale and sandstone up to 1 1/2 inches in diameter; slightly acid (pH 6.5); gradual irregular boundary. (10 to 15 inches thick)

R--34; fractured and shattered massive light yellowish brown and light olive brown shale and fine-textured sandstone; few large roots penetrate along fracture planes more than 4 inches apart.

TYPE LOCATION: Glenn County, California; 6 1/2 miles NE of Stonyford, California on the west side of Needham Grade in the NE 1/4 section 1, T.18 N., R.6 W.; on a NNW facing slope of 55 percent at 1,600 feet elevation with annual grass and blue oak vegetation with a few scattered shrubs.

RANGE IN CHARACTERISTICS: Depth to sandstone and shale is 20 to 40 inches. The mean annual soil temperature is about 57 to 68 degrees F and the soil temperature usually is not below 47 degrees F at any time. The soil between depths of 4 to 12 inches is usually dry from June until October or early November and is moist in some or all parts the rest of the year. Rock fragments make up 2 to 45 percent of the soil volume. Fragments are mostly rounded gravels in the surface and mostly sandstone or shale in the horizon above the bedrock. Reaction is neutral to moderately acid (pH 6.0) and either remains constant or is slightly more acid with increasing depth. The surface 5 inches has 1 to 3 percent organic matter.

The A horizon has 10YR and 7.5YR hue, with value of 4, 5 or 6 dry and 3 or 4 moist and chroma of 2 through 4 dry or moist. Textures are loam or clay loam. The A horizon has granular structure or the soil is massive and hard when dry.

The Bt horizon is reddish brown to brown in 5YR, 7.5YR or 10YR hue with value of 4 through 6, 3, 4 or 5 moist and chroma of 4 through 6 dry and 2, 3, 4 to 6 moist. Textures are clay loam or clay with 35 to 45 percent clay.

A transitional horizon commonly occurs between the Bt horizon and bedrock with texture similar to the Bt horizon except sandstone and shale fragments increase up to about 35 percent. In some pedons the sandstone and shale bedrock is strongly weathered in the upper 4 inches of the rock.

COMPETING SERIES: These are the Argonaut (CA), Conosta (CA), Fancher (T CA), Kilaga (CA), Phipps (CA), Trabuco and Yeguas (T CA) series. Argonaut soils have a paralithic contact at depths of 20 to 40 inches. Fancher soils formed in residuum from serpentine and have Ca/Mg ratios of 2:1 or less. Trabuco soils lack a lithic or paralithic contact within depth of 40 inches.

GEOGRAPHICALLY SETTING: Contra Costa soils are 2 to 75 percent slopes at elevations of 275 to 3,800 feet. They formed in residuum from fine-grained sandstone and shale. The climate is subhumid mesothermal with hot dry summers and cool moist winters. Mean annual precipitation is 10 to 35 inches. Average January temperature is 42 degrees F; average July temperature is 75 degrees F; average annual temperature is 57 to 62 degrees F. The frost-free season is 185 to 290 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Altamount, Bressa, Dibble, Gaviota, Millsholm, Montara, and Sehorn soils. Altamont and Sehorn soils are clayey throughout and have intersecting slickensides. Bressa soils have a paralithic contact at depths of 20 to 40 inches and have fine-loamy control sections. Dibble soils have a paralithic contact at depths of 20 to 40 inches and have B2t horizons with hue of 10YR or 2.5Y. Gaviota, Millsholm and Montara soils have lithic contacts at depths of less than 20 inches.

DRAINAGE AND PERMEABILITY: Well-drained; low to very high runoff; slow permeability.

USE AND VEGETATION: Used mostly for range and as habitat for wildlife. Vegetation is mostly trees and annual grasses; some eroded areas are heavily covered with chamise on south-facing slopes; mixed shrubs and grass are on some northern exposures.

DISTRIBUTION AND EXTENT: Foothills and mountainous ranges of the east-central and northern coast ranges, California. The soils are moderately extensive in MLRA 15.


SERIES ESTABLISHED: Contra Costa County, California, 1935.

REMARKS: The University of California laboratory data do not show enough clay increase to qualify for an argillic horizon. However, those who have observed the official site or have mapped the soils think most pedons have enough clay increase for an argillic horizon.

Runoff terminology adjusted 5/96 to the adjective criteria of the Soil Survey Manual, 10/93.

National Cooperative Soil Survey