No detailed mapping of the soils on the Quail Ridge peninsula has been pursued, and only three soil map units, Bressa-Dibble complex 30-50% and 50-75% slope and Maymen-Millsholm-Lodo association, appear in the soil survey of the area (Lambert and Kashiwagi 1978). The Bressa-Dibble complexes cover approximately 2/3 of the Reserve. All of the soils within the Reserve are derived from lower Cretaceous-Upper Jurassic marine mudstone, siltstone, sandstone, and conglomerate that was uplifted to form the Inner Coast Range about 5 million years ago. These soils are found at elevations of ca. 150 to 700 meters (492 to 2297 feet).

Bressa-Dibble Complexes

The Bressa and Dibble soil series are the dominant soils in these map units and are so finely intermingled that they could not be separated at the scale of the survey map. The sedimentary rocks from which these soils formed generally are thinly interbedded. At Quail Ridge, the rocks are tilted to near vertical, and the soils are quite shallow, giving rise to a fine-scale variation of soils on the landscape. At the soil family level, the Bressa soils are identified as fine-loamy, mixed, active, thermic Typic Haploxeralfs; the Dibble soils are fine, smectitic, thermic Typic Haploxeralfs, and have a higher clay content than the Bressa soils.

Bressa soils are mapped only in complex with Dibble soils; the proportions vary as a function of slope. Two slope phases of the Bressa-Dibble complex are found at Quail Ridge Reserve, 30-50% slope and 50-75% slope. Bressa-Dibble complex 30-50% slope occurs on uplands at around 350-700 m elevation and consists of about 65% Bressa silt loam, 20% Dibble silty clay loam, 15% Lodo, Maymen, Millsholm, and Sobrante loams, and other small areas of clayey soils. These steep soils are characterized by rapid runoff and moderate to severe erosion.

Slump; photo by Shane Waddell
The soils on the Quail Ridge peninsula are all very slide prone. Slumps, like the small one above, cut deeply into unweathered materials and can rapidly displace large volumes of material.

Bressa-Dibble complex 50-75% slope consists of very steep soils comprising 70% Bressa silt loam, 15% Dibble silty clay loam, and 15% Lodo, Los Gatos, Maymen, Millsholm, and Sobrante loams, and other small areas of clayey soils. Runoff tends to be very rapid on these slopes, which also erode readily. The steepness limits the usefulness of these soils for cattle grazing, which may explain in part the unusually well preserved native grasslands on the Reserve.

A representative profile of the Bressa soil described in the Napa County survey includes a pale brown, slightly acidic surface silt loam 25 cm (10 inches) thick. Subsoil is a light yellowish brown, slightly acidic, silty clay loam about 60 cm (24 inches) thick. These layers are underlain by soft weathered sandstone at about 85 cm (34 inches). A representative profile of the Dibble soil includes a surface layer of pale brown and brown, slightly acidic silty clay loam 22 cm (9 inches) thick. The subsoil is brown and yellowish brown, slightly acidic silty clay and clay 63 cm (25 inches) thick. Weathered sandstone is found at a depth of 85 cm (34 inches). All of the areas of grassland, oak woodland, and the cooler and moister north-slope forests on the Reserve occur on Bressa-Dibble soils. In general they are thicker and have more water holding capacity than the other soil type on the Reserve.

vegetation transition; photo by Shane Waddell
An indication of soil change is often visible in a dramatic shift in vegetation. Above, the dense chaparral vegetation in the foreground is on the 30-50% Bressa-Dibble complex and the open grassy area in the distance shifts to the 50-75% Bressa-Dibble complex

Maymen-Millsholm-Lodo association

The remainder of the Reserve, comprising the steepest, driest, chaparral areas, is mapped as Maymen-Millsholm-Lodo association. An association is a group of soils that could be separated at the mapping scale (in contrast with a complex, which cannot be separated at this scale), but that have such similar behavior that there is no advantage in doing so. This map unit is characterized by the soil survey as “somewhat excessively drained” and shallow – no doubt important reasons that it supports only drought-tolerant chaparral vegetation at Quail Ridge.

The Maymen-Millsholm-Lodo association is found on slopes from 30-75%. Maymen gravelly loam is found in convex areas on north-facing slopes from 30-75% slope. Convex areas on steep, south-facing slopes from 50-60% on ridge tops harbor Millsholm loam. Lodo loam is in convex areas on south-facing slopes from 30-75%. In general the association is composed of about 50% Maymen soils, 20% Millsholm soils, 20% Lodo soils, and 10% rock outcrop. Maymen soils are loamy, mixed, active, mesic Lithic Dystroxerepts. Millsholm soils are loamy, mixed, superactive, thermic Lithic Haploxerepts. Lodo soils are loamy, mixed, thermic Lithic Haploxerolls.

Soils in this association have very rapid runoff and are highly prone to erosion. The majority of the large landslides on the Reserve occur on these soils.

Maymen soils typically have a pale brown, moderately acidic, gravelly loam surface layer 15 cm (6 inches) thick. The subsoil is light yellowish brown, strongly acidic gravelly loam 15 cm (6 inches) thick, underlain by fractured sandstone. Millsholm soils have a surface layer of pale brown, moderately acidic loam 10 cm (4 inches) thick. The subsoil is a yellowish brown, moderately acidic clay loam 20 cm (8 inches) thick. Sandstone occurs at a depth of 30 cm (12 inches). Lodo soils have a surface layer of brown, neutral loam 10 cm (4 inches) thick. The subsoil is a brown, neutral loam that borders on a clay loam, 8 cm (3 inches) thick. Fractured sandstone is at a depth of just 18 cm (7 inches).

Photo Credits: Title, Slump, other Slump, and Vegetation transition (Shane Waddell).

This page last updated: July 5, 2005  

Contact: Dr. Virginia Boucher
John Muir Institute of the Environment
109 The Barn, University of California, Davis, CA 95616
Phone: 530-752-6949; email:

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