Established Series


The Mindego series comprises dark-colored, fine-textured, well-drained soils developing under timber in weathered basic igneous rocks. They appear to be grumusolic in some respects, but have textural B horizons. They form in Coast Range valleys in California where the climate is mild and humid. Summers are warm, rainless and foggy; winters are cool and wet. Average annual rainfall is from 40 to 50 inches, mean annual temperature from 53 to 57 degrees F. Parent rocks are mainly basalt and diabase, probably with some mantle of slope wash from Mindego and related soils. Sweeney soils form in the same kind of parent material, but they are browner colored, generally shallower to bedrock, develop under grass with scattered trees, and have more distinct horizonation.

TAXONOMIC CLASS: Fine, smectitic, mesic Pachic Argixerolls

TYPICAL PEDON: Mindego clay loam (virgin).

A1--0 to 2 inches; very dark gray (10YR 3/1.5) dry; very dark brown, (10YR 2/1.5) moist; clay loam that appears to be high in organic matter. Moderate coarse and medium granular structure. Very hard, friable, plastic, sticky. Many worm casts, (pH 7.3) clear, smooth lower boundary. (1/2 to 3 inches thick)

B1--2 to 10 inches; very dark gray (10YR 3/1) dry; very dark brown, (10YR 2/2) moist; heavy clay loam; moderate fine and very fine subangular blocky structure; very hard, friable, plastic, sticky; numerous roots and insect burrows; thin, patchy clay skins, especially on vertical ped surfaces; (pH 7.3) gradual lower boundary. (4 to 11 inches thick)

B2t--10 to 30 inches; very dark grayish-brown (10YR 3/2) dry; very dark brown, (10YR 2.5/2) moist; clay; moderate fine and very fine subangular blocky structure; extremely hard, firm, very plastic, sticky; nearly continuous thin clay skins; many roots; (pH 6.5) gradual lower boundary. (10 to 30 inches thick)

B3--30 to 46 inches; very dark grayish-brown (10YR 3/2) dry; very dark brown, (10YR 3/3) moist; stony clay loam; weak very fine subangular blocky structure; hard, friable, very plastic, very sticky; moderate continuous clay skins; numerous large roots; contains numerous soft basalt cobbles and stones; (pH 6.5); clear, irregular lower boundary. (8 to 20 inches thick)

Dr--46 inches; weathered, fractured basalt bedrock that becomes harder and more massive with increasing depth; upper parts are penetrated by large tree roots; soft, weathered rock in upper part has pH 7.3.

TYPE LOCATION: About 50 feet northeast of bridge across La Honda Creek, 1 mile northeast of village of La Honda, San Mateo County, California.

RANGE IN CHARACTERISTICS: These soils range in depth to bedrock as indicated in the profile description. Color of A1 varies from very dark gray to dark reddish brown. On steeper slopes the Bt is less clayey, approaching a clay loam. Types recognized to date are loam, clay loam, and stony clay loam. Phases recognized to date include the following: moderately steep, steep, very steep; slightly eroded, moderately eroded.

GEOGRAPHIC SETTING: Deep, moist ravines and valleys in the Coast Range. Hilly to very steep (mountainous) topography.

DRAINAGE AND PERMEABILITY: Runoff is medium to rapid. Permeability moderate to moderately slow. Well drained.

USE AND VEGETATION: Well suited to timber production, especially for redwood. Dominantly coniferous forest. Principal species are coast redwood and Douglas fir; hardwood species include madrone, California black oak and poison oak. Sparse understory of shrubs and herbs.

DISTRIBUTION AND EXTENT: Coast Range of California. Possible also in Oregon.


SERIES ESTABLISHED: San Mateo County SCD, California, survey, 1956. The name is taken from Mindego Hill.

OSED scanned by SSQA. Last revised by state on 2/56.

National Cooperative Soil Survey