12 - Clallam gravelly sandy loam, 0 to 15 percent slopes. (CmC)
This moderately deep, moderately well drained soil is on hills. It formed in compact glacial till. The
native vegetation is mainly conifers and shrubs. Elevation is 40 to 1,800 feet. The average annual
precipitation is 16 to 30 inches, the average annual air temperature is about 48 degrees F, the average
frost-free period is 160 to 200 days, and the average growing season (at 28 degrees) is 210 to 260 days.
Typically, the surface is covered with a mat of organic material 2.5 inches thick. The surface
layer, where mixed to a depth of 6 inches, is dark brown gravelly sandy loam. The upper part of the
subsoil is brown gravelly sandy loam about 4 inches thick, and the lower part is brown very gravelly sandy
loam about 18 inches thick. Compact glacial till is at a depth of 28 inches. Depth to impact glacial till
ranges from 20 to 40 inches. In some areas of similar included soils, the surface layer is sandy loam or
very gravelly sandy loam.
Included in this unit are small areas of Catla, Dick, Elwha, Hoypus, McKenna, Mukilteo, and
Yeary soils. Also included are small areas of Clallam soils that have slopes of more than 15 percent and
soils that are more than 40 inches deep to compact glacial till. Included areas make up about 25 percent
of the total acreage. The percentage varies from one area to another.
Permeability of this Clallam soil is moderate to the compact glacial till and very slow through it.
Available water capacity is low. Effective rooting depth is 20 to 40 inches. Runoff is medium, and the
hazard of water erosion is slight. Water is perched above the compact glacial till from January through
April. The effect of the layer of compact glacial till on use and management is similar to that of a hardpan.
This unit is used mainly as woodland. It is also used as hayland, pastureland, cropland, and
homesites.
This unit is suited to year-round harvesting operations. Logging roads require suitable surfacing.
Rock for road construction is not available in areas of this unit.
Seedling establishment and the hazard of windthrow are the main concerns in the production of
timber. The low precipitation during the growing season and low available water capacity contribute to
seedling mortality. Reforestation can be accomplished by planting Douglasfir seedlings. If seed trees are
present, natural reforestation of cutover areas by Douglas-fir occurs periodically. When openings are
made in the canopy, invading brushy plants can delay the establishment of seedlings. Because the
rooting depth is restricted by the compact glacial till, trees occasionally are subject to windthrow.
If this unit is used for homesite development, the main limitation is wetness. Excavation for roads
and buildings increases the risk of erosion.
Use of this unit for septic tank absorption fields is limited by wetness and depth to the compact
glacial till. Because of the depth to glacial till, onsite sewage disposal systems often fail to function
properly during periods of high rainfall. Use of heavy equipment during construction compacts the soil and
reduces permeability, particularly during periods when the soil moisture content is high. Absorption lines
should be installed on the contour to prevent surfacing of effluent in downslope areas.
12 - Clallam gravelly sandy loam, 0 to 15 percent slopes. (CmC) (con’t)
Table 10 – Sanitary Facilities
Soil name and map symbol
Clallam gravelly sandy
loam, 0 to 15 percent
slopes
Septic tank absorption
fields
Severe: cement pan,
wetness
Sewage lagoon
Severe: cemented pan,
slope, wetness
Table 14 – Physical and Chemical Properties of Soils
Soil name and map
symbol
Clallam gravelly
sandy loam, 0 to
15 percent slopes
Depth (In)
Clay (%)
Permeability (In/hr)
0-10
3-8
0.6-2.0
10-28
3-12
0.6-2.0
28
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Table 15 – Water Features
Soil name and map symbol
Hydrologic
group
Clallam gravelly sandy
loam, 0 to 15 percent
slopes
C
High Water Table
Depth (ft)
Kind
Months
1.5-3.0
Perched
Jan-Apr
Clallam County Critical Area Code Application
Geologic Hazard
Soil Type
Landslide (1.a.ii)
Compact Glacial Till
SCS Erosion
Hazard Potential
ESA Trigger
No
Landslide (1.a.iv)
<Moderate
No
Erosion (1.b.v)
<Moderate
No
Seismic (1.c.iii)
Note:
1. Landslide hazard (1.a.ii) is defined as glaciolacustrine silt and clays on terraces.
2. Seismic soils 1.c.iii) are defined as beaches, Mukilteo muck, Lummi silt loam,
Sequim- McKenna-Mukilteo complex, and Tealwhit silt loam
No