Marin County, California
The most common soil here is Tocaloma, on about 8% of the land: a loam, a balanced mix that crumbles in your hand. Tomales and Saurin come next.
About 54% of the land drains well, and 11% stays wet for long stretches; groundwater comes within two feet of the surface under 8% of it.
USDA rates 78% of the land very limited for a standard septic drain field, and 71% very limited for homes with basements. That means extra cost or special design, not that it can't be done.
Worth knowing: 3% of the land floods occasionally or more, and bedrock lies within 40 inches under 52% of it.
Soils change over short distances, so look up an address below for the soil at that exact spot and the reasons behind each rating.
The soils you're most likely standing on
The biggest named soils in Marin County, California, each drawn five feet deep from its official USDA description, in its recorded colors.
Dots mark gravel; white flecks mark lime. Shares are by acres across the survey area, and each mapped patch usually mixes several of these soils.
The biggest mapped areas
The dominant drainage class is Well drained and the dominant hydrologic group is D (very slow infiltration, high runoff). The most common soil order is Mollisols — grassland soils with a thick, dark, fertile topsoil rich in organic matter. This report summarizes the major soil map units across the survey area to help you understand what to expect when buying, building, or gardening in Marin County, California. Search your address to find the exact soil composition, drainage, and series details at your specific location.
| Soil Map Unit | Acres | Drainage | Hydro Group | Dwellings | Septic |
|---|---|---|---|---|---|
| Water | 59K | D | Not rated | Not rated | |
| Tocaloma-McMullin complex, 50 to 75 slopes | 25K | Well drained | B | Very limited | Very limited |
| Tocaloma-Saurin association, extremely steep | 25K | Well drained | B | Very limited | Very limited |
| Tocaloma-Saurin association, very steep | 18K | Well drained | B | Very limited | Very limited |
| Los Osos-Bonnydoon complex, 30 to 50 percent slopes | 13K | Well drained | D | Very limited | Very limited |
| Xerorthents-Urban land complex, 0 to 9 percent slopes | 10K | Not rated | Not rated | ||
| Blucher-Cole complex, 2 to 5 percent slopes | 9K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Dipsea-Barnabe very gravelly loams, 50 to 75 percent slopes | 9K | Well drained | B | Very limited | Very limited |
| Maymen-Maymen variant gravelly loams, 30 to 75 percent slopes | 8K | Somewhat excessively drained | D | Very limited | Very limited |
| Reyes clay | 8K | Somewhat poorly drained | C/D | Very limited | Very limited |
| Reyes clay, 0 to 2 percent slopes | 8K | Somewhat poorly drained | C | Very limited | Very limited |
| Tocaloma-McMullin complex, 30 to 50 percent slopes | 8K | Well drained | B | Very limited | Very limited |
| Palomarin-Wittenberg complex, 50 to 75 percent slopes | 7K | Well drained | B | Very limited | Very limited |
| Tomales-Steinbeck loams, 5 to 15 percent slopes | 6K | Moderately well drained | D | Somewhat limited | Very limited |
| Saurin-Bonnydoon complex, 30 to 50 percent slopes | 6K | Well drained | C | Very limited | Very limited |
| Yorkville clay loam, 30 to 50 percent slopes | 6K | Moderately well drained | D | Very limited | Very limited |
| Los Osos-Bonnydoon complex, 15 to 30 percent slopes | 6K | Well drained | D | Very limited | Very limited |
| Tomales loam, 9 to 15 percent slopes | 5K | Moderately well drained | D | Somewhat limited | Very limited |
| Centissima-Barnabe complex, 50 to 75 percent slopes | 5K | Well drained | C | Very limited | Very limited |
| Bonnydoon gravelly loam, 30 to 75 percent slopes | 5K | Somewhat excessively drained | D | Very limited | Very limited |
Soil Orders in This Area
The USDA classifies every soil into one of 12 soil orders. Here are the dominant orders found in this survey area.
What This Means
Building & Foundations
About 71% of soils are rated "very limited" for dwellings with basements. Specific challenges include shallow bedrock, steep slopes, poor drainage in some areas, flood-prone areas. A geotechnical assessment is recommended before building.
Septic Systems
About 78% of soils are rated "very limited" for septic absorption fields. Despite good drainage overall, limitations may stem from shallow bedrock, steep slopes, or seasonal conditions. Engineered or alternative systems are frequently required. Always get a professional perc test before purchasing land that needs septic.
Gardening & Agriculture
Well-drained soils on sloped terrain — good for most plants but watch for erosion. Terracing, contour planting, and mulching help retain moisture and topsoil. Drip irrigation is more effective than sprinklers on slopes.