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).
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.
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
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
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
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).