LOCATION GEOHROCK MT
Established Series
Rev. DES-KTS-RJS
04/2011
GEOHROCK SERIES
The Geohrock series consists of very deep, well drained soils that formed in gravelly alluvium or colluvium. These soils are on dissected alluvial fans, fan remnants, stream terraces, hills and escarpments. Slopes are 1 to 60 percent. Mean annual precipitation is about 11 inches, and mean annual air temperature is about 43 degrees F.
TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive, frigid Aridic Haplustalfs
TYPICAL PEDON: Geohrock gravelly loam, rangeland (colors are for dry soil unless otherwise noted).
A--0 to 4 inches; pinkish gray (7.5YR 6/2) gravelly loam, brown (7.5YR 4/2) moist; moderate very thin platy structure parting to moderate very fine granular; slightly hard, very friable, moderately sticky and slightly plastic; many very fine roots; 20 percent angular gravel; slightly alkaline (pH 7.4); clear smooth boundary. (3 to 5 inches thick)
Bt--4 to 10 inches; brown (7.5YR 5/3) gravelly clay loam, brown (7.5YR 4/3) moist; moderate medium prismatic structure parting to moderate medium subangular blocky; hard, firm, moderately sticky and moderately plastic; common very fine roots; many very fine tubular and interstitial pores; continuous distinct clay film on faces of peds and lining pores; 30 percent angular gravel; slightly alkaline (pH 7.4); clear smooth boundary. (3 to 7 inches thick)
Btk--10 to 18 inches; brown (7.5YR 5/3) very gravelly loam, brown (7.5YR 4/3) moist; weak medium subangular blocky structure; slightly hard, friable, moderately sticky and slightly plastic; common very fine roots; many very fine tubular and interstitial pores; few faint clay films on faces of peds; 40 percent angular gravel; many medium masses of calcium carbonate and continuous faint coatings of calcium carbonate on underside of gravel; strongly effervescent; moderately alkaline (pH 8.1); gradual wavy boundary. (0 to 10 inches thick)
Bk1--18 to 24 inches; light brown (7.5YR 6/4) extremely gravelly loam, brown (7.5YR 5/4) moist; weak fine and medium subangular blocky structure; slightly hard, friable, moderately sticky and slightly plastic; common very fine roots; many very fine tubular and interstitial pores; 60 percent angular gravel; disseminated calcium carbonate and continuous distinct calcium carbonate casts on undersides of gravel; violently effervescent; moderately alkaline (pH 8.4); gradual smooth boundary. (4 to 10 inches thick)
Bk2--24 to 40 inches; pale brown (10YR 6/3) extremely gravelly loam, brown (10YR 5/3) moist; weak fine and medium subangular blocky structure; slightly hard, friable, moderately sticky and slightly plastic; common very fine roots; 70 percent angular gravel; disseminated calcium carbonate and continuous distinct calcium carbonate casts on undersides of gravel; violently effervescent; moderately alkaline (pH 8.4); gradual smooth boundary. (combined thickness of Bk1 and Bk2 horizons - 14 to 30 inches thick)
C1--40 to 45 inches; pinkish gray (7.5YR 6/2) extremely gravelly loam, brown (7.5YR 5/2) moist; massive; slightly hard, friable, moderately sticky and slightly plastic; few very fine roots; 80 percent angular gravel; disseminated calcium carbonate; slightly effervescent; slightly alkaline (pH 7.8); clear smooth boundary. (5 to 28 inches thick)
C2--45 to 60 inches; pinkish gray (7.5YR 6/2) extremely gravelly loam, brown (7.5YR 5/3) moist; massive; slightly hard, friable, moderately sticky and slightly plastic; 60 percent angular gravel; disseminated calcium carbonate; strongly effervescent; moderately alkaline (pH 8.0).
TYPE LOCATION: Lewis and Clark County, Montana; 1,200 feet south and 1,000 feet east of the NW corner of sec. 8, T. 11 N., R. 4 W.
RANGE IN CHARACTERISTICS:
Soil temperature - 40 to 47 degrees F.
Depth to top of argillic horizon - 3 to 5 inches
Depth to secondary calcium carbonate accumulation - 6 to 12 inches
A horizon
Hue: 5YR, 7.5YR, 10YR or 2.5Y; dry or moist
Value: 4 to 7 dry; 3 to 5 moist
Chroma: 2 or 3; dry or moist
Texture: sandy loam, loam or clay loam
Clay content: 10 to 35 percent
Rock fragments: 15 to 60 percent--10 to 45 percent gravel, 0 to 30 percent cobbles
Reaction: pH 6.6 to 7.8
Bt horizon
Hue: 5YR, 7.5YR, 10YR or 2.5Y; dry or moist
Value: 4 to 6 dry; 3 to 5 moist
Chroma: 2 to 4; dry or moist
Texture: clay loam or sandy clay loam
Clay content: 25 to 35 percent
Rock fragments: 20 to 60 percent--20 to 50 percent gravel, 0 to 30 percent cobbles
Calcium carbonate equivalent: 0 to 5 percent
Reaction: pH 6.6 to 7.8
Btk horizon
Hue: 5YR, 7.5YR, 10YR or 2.5Y; dry or moist
Value: 5 or 6 dry; 4 or 5 moist
Chroma: 2 or 3; dry or moist
Texture: loam, sandy clay loam or clay loam
Clay content: 15 to 35 percent
Rock fragments: 25 to 60 percent--25 to 50 percent gravel, 0 to 10 percent cobbles
Calcium carbonate equivalent: 5 to 15 percent
Reaction: pH 7.4 to 8.4
Bk horizons
Hue: 5YR, 7.5YR, 10YR or 2.5Y; dry or moist
Value: 6 or 7 dry; 4 to 6 moist
Chroma: 2 to 6; dry or moist
Texture: loam, sandy loam or sandy clay loam
Clay content: 10 to 25 percent
Rock fragments: 35 to 80 percent--20 to 75 percent gravel, 0 to 25 percent cobbles
Calcium carbonate equivalent: 3 to 15 percent
Reaction: pH 7.9 to 9.0
C horizons
Hue: 5YR, 7.5YR or 10YR; dry or moist
Value: 6 or 7 dry; 4 or 5 moist
Chroma: 2 to 6; dry or moist
Texture: sandy loam, sandy clay loam or loam
Clay content: 10 to 25 percent
Rock fragments: 45 to 80 percent--20 to 70 percent gravel, 0 to 25 percent cobbles
Calcium carbonate equivalent: 3 to 15 percent
Reaction: pH 7.4 to 8.4
COMPETING SERIES: There are no competing series.
GEOGRAPHIC SETTING:
Landform - dissected alluvial fans, fan remnants, stream terraces, hills and escarpments
Elevation - 3,600 to 6,500 feet
Slope - 1 to 60 percent
Parent material - gravelly alluvium or colluvium
Climate - long, cold winters; moist springs; warm summers
Mean annual precipitation - 10 to 14 inches
Mean annual air temperature - 38 to 45 degrees F.
Frost-free period - 70 to 120 days
GEOGRAPHICALLY ASSOCIATED SOILS: None listed.
DRAINAGE AND PERMEABILITY: Well drained; moderate permeability.
USE AND VEGETATION: Geohrock soils are used mainly for rangeland. Potential native vegetation is mainly bluebunch wheatgrass, western wheatgrass, prairie junegrass, Sandberg bluegrass, needleandthread, forbs, and shrubs.
DISTRIBUTION AND EXTENT: Geohrock soils are moderately extensive in Southwestern Montana. MLRAs - 43B, 44B.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana.
SERIES ESTABLISHED: Lewis and Clark County Area, Montana, 1991; proposed in Lewis and Clark County, Montana, 1979.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon - from 0 to 4 inches (A horizon)
Argillic horizon - from 4 to 18 inches (Bt, Btk horizons)
Secondary calcium carbonate accumulation - from 18 to 40 inches (Btk, Bk1, Bk2 horizons)
Geohrock soils have a frigid temperature regime and an ustic moisture regime that borders on aridic.
ADDITIONAL DATA: Soil interpretation records - MT0612, MT0504.
Classification changed from Borollic Haplargids to Typic Eutroboralfs in 1994; changed to Aridic Haplustalfs in 1997.
Additional Information: Because the calcium carbonate equivalent in the Bk horizons ranges from 3 to 15% and clay ranges from 10 to 25%, this soil requires further investigation as to whether it has a calcic horizon.
National Cooperative Soil Survey
U.S.A.