Origin of Terraced Hillslopes on Active Folds in the Southern San Joaquin Valley, California

Geomorphology 42 (2002) 131–152 www.elsevier.com/locate/geomorph

Origin of terraced hillslopes on active folds in the southern San Joaquin Valley, California

Adam E. Bielecki, Karl J. Mueller *

Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USA

Received 11 July 2000; received in revised form 20 April 2001; accepted 24 April 2001

Abstract

The northern-facing forelimbs of the active San Emigdio and Wheeler Ridge anticlines in the southern San Joaquin Valley (SSJV) are marked by numerous, sharply defined terracettes. Terracette treads and risers are 3–5 and 7–11 m wide, respectively. The terracettes are discontinuous along their length, with the longest being ~300 m, although most terminate more abruptly forming lobate mounds. The terracettes form on 10–25° slopes underlain by loosely consolidated coarse conglomerate beds of the Pleistocene Tulare Formation that dip subparallel to terracette risers. Soil developed on the slopes is comprised of a 1- to 2-m-thick calcic Bk horizon, overlain by a 0.5-m-thick argillic claypan Bt horizon, followed by 1- to 3-m-thick AB and A horizons that have been extensively bioturbated. A number of hypotheses have been proposed for the genesis of similar terracettes. The two most viable tectonic models for our study area are based on growth of active fault- bend folds and include (i) sediment onlap with seismic folding and (ii) flexural slip faulting. We produced a very high- resolution digital elevation model (DEM) (1.67-m spatial resolution and F5-cm vertical accuracy) using NASA’s RASCAL (RAster SCanning Airborne Lidar) instrument and completed numerical modeling that we interpret to disprove both of the active fold growth models. The discovery and mapping of similar terracettes on NW- to NE-facing hillslopes on the Kettleman Hills anticline and Comanche Point in the northern Tehachapi Mountains suggest that a northern-facing aspect is an important factor contributing to terracette development. The terracettes are also present only on hillslopes that overlie blind or emergent thrust faults. We propose that a soil creep model based on flow-dominant mass wasting is the most likely cause of terracette development. Creep is confined to the thick A and AB cumulic soil horizons that delaminate above the rigid Bt claypan and occurs predominantly on northern-facing slopes due to increased soil moisture retention. The barrier to creep caused by the decrease in slope at the base of fold limbs leads to the development of terracettes that become more sharply defined in a downslope direction. We interpret the overthickened A and AB soil horizons to form preferentially on northern-facing slopes due to increased bioturbation and deposition of eolian silt. Strong ground motions produced by earthquakes may facilitate soil creep, and thereby contribute to the overall development of the terracettes. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Terracettes; Soil creep; Solifluction; Fault-related folds; Flexural slip; Mima-like mounds

1. Introduction

* Corresponding author. Fax: +1-303-492-2606. The forelimbs of active folds along the leading E-mail address: [email protected] (K.J. Mueller). edge of the active fold belt in the northern Transverse

Fig. 1. Topographic map of the northern Transverse Ranges marked with locations of terraces found on the northern-facing front limbs of active folds (A). Oblique aerial photographs of forelimbs on the Wheeler Ridge (B) and San Emigdio (C) anticlines. Slope images of RASCAL DEM data for the eastern (D) and western (E) key areas selected for detailed analysis in this project. A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 133

Table 1 mited detailed mapping of terrace morphology to List of viable hypotheses applicable to the terrace steps found in two key areas where terracettes are best developed SSJV (Fig. 1). a Hypothesis Reference Laser altimetry data was provided by the National Coseismic folding Mueller and Suppe (1997) Aeronautics and Space Administration under the (fold growth terraces) Topography and Surface Change component of the Flexural slip faulting Mueller and Suppe (1997), Bielecki (1998) Mission to Planet Earth Program. The RASCAL Mima-like mound: Dalquest and Scheffer (1942) instrument was a prototype scanning laser altimeter fossorial-rodent (LIDAR) mounted on a T-39 jet aircraft platform. Mima-like mound: Hilgard (1884) Using a nominal straight-and-level flight pattern, gilgai RASCAL mapped the ground surface in 100-m-wide Mima-like mound: Aten and Bollich (1981) anthropogenic swaths by firing a laser 5000 times a second and Livestock grazing Howard and Higgins (1987) recording the time it takes for the laser pulses to Slumping (slab slides) Sharpe (1938) bounce off the ground and return to the aircraft. This Solifluction Costin (1950), Embleton (1975) time-of-flight data is then corrected for aircraft ori- Soil creep Clayton (1966) entation and a global positioning system (GPS)- a References are for publication(s) that define the hypothesis as derived trajectory for the T-39 platform in order to applied to terrace development. derive accurately georeferenced surface elevation values. With flight speed nominally maintained at 100 m/s, this topographic data has 1.67-m spatial resolution and F5-cm vertical accuracy (Bielecki, 1998). Ranges are consistently covered by flights of closely The end result is longitude, latitude, and elevation (x, spaced terracettes (Fig. 1). These landforms are pre- y, z) data in a geographic lat/lon coordinate system served on northern-facing slopes of Comanche Point using the WGS84 datum (Rabine et al., 1996). The and the Wheeler Ridge, San Emigdio, and Kettleman strips of flight-corrected data were provided by the Hills anticlines. Numerous hypotheses exist for the Goddard Space Flight Center in structured binary origin of the terracettes (Table 1). The motivation for files, which were then parsed and gridded into 14 our work was to test the possible origin of the land- separate digital elevation models (DEMs) using the forms by either tectonic processes related to growth of Interactive Data Language (IDLk) software package. active folds (Mueller and Suppe, 1997), or mass A number of graphical products were derived from wasting on actively rising hillslopes. these DEMs including bytescale, contour, ortho- graphic, aspect, slope, and shaded relief images using the Environment for Visualizing Images (ENVIk). 2. Methods However, slope maps provide the most accurate representation of terracette morphology and distribu- We mapped terraced hillslopes with an airborne tion due to the sharp distinction between flat treads laser altimeter and surveyed topographic profiles and sloping risers (Fig. 2) and are the principal using an electronic distance meter. Soils were des- graphical representation of the RASCAL data illus- cribed in trench excavations (Mueller and Suppe, trated in this paper. 1997) and natural exposures along channel walls Hillslope profiles were acquired with a Total Sta- incised into terraced hillslopes. Numerical analysis tion distance meter at azimuths perpendicular to the of structural models were based on fault-related fold terracette treads (Fig. 1). Spacing of data points varied theory (e.g., Suppe, 1983) and the empirical regres- with a denser accumulation across terracette treads, sion of moment magnitude vs. surface displacement but overall average resolution is one measurement per for recent historic earthquakes in southern California 1.54 m. The profiles were used to measure terracette developed by Dolan et al. (1995). Although the DEM tread widths (w) and riser slope angle (a) (Fig. 3) and covered the entire length of the front limbs of the to test the accuracy of the DEM. In addition, numer- Wheeler Ridge and San Emigdio anticlines, we li- ous measurements related to the tectonic models of 134 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152

Fig. 2. Slope image of RASCAL data for eastern Wheeler Ridge produced in ENVIk. Bright pixels represent steep slopes, whereas dark pixels coincide with flatter ground.

landform development, including likely thickness of internal properties of the terracettes, including shrink– bedding between hypothetical flexural slip faults and swell capacity and rheology. spacing of terracettes, were derived from these profiles (Fig. 3). Two trench excavations were completed prior to 3. Terracette morphology laser altimetry data acquisition as part of a prelimi- nary study (Mueller and Suppe, 1997) (Fig. 1). The Terracette treads are typically 3–5 m in width as two excavations were approximately 5 m deep and measured from the DEMs and surveyed profiles (Fig. 60 m long. Soil profiles consistent with USDA stan- 3). Terracette risers range from 7 to 11 m in width. The dards (Soil Survey Staff, 1981) were described at four length of terracettes varies markedly; the longest points in one of the excavations (Table 2) and were exceeds 300 m, but most cannot be traced beyond used to complete a trench log (Fig. 4) and establish the 100 m (Fig. 2). Terracette terminations on uneroded A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 135

Fig. 3. Total station profiles for eastern Wheeler Ridge anticline (A) and San Emigdio anticline (B) key areas. Lower and upper segments of Wheeler Ridge data (A) projected into one profile. Terrace treads and risers defined by straight-line approximation intersections, and all measurements are referenced from those points. Hypothetical flexural slip faults drawn parallel to adjacent risers, creating a dip slightly steeper than average overlying topography.

surfaces are typically abrupt where most taper and The terracettes are only found along the lower disappear over a distance of a few meters. Terracettes sections of the front limbs of Wheeler Ridge and terminate at the walls of incised drainages, at merging San Emigdio anticlines (Fig. 1). They are not present risers, or at small displacement tear faults. A number of on any other surfaces in the region with similar slope, terracettes on both folds narrow abruptly and curve age, and lithology. This includes their absence on the sharply where they encounter the top of incised chan- south limb of Wheeler Ridge, the walls of major nel walls, forming southward-pointing v’s (Figs. 1 and canyons in the core of the folds, and the very young 2). Terracettes on the front limb of San Emigdio alluvial fans that are actively forming at the base of anticline are horizontal and strike parallel to local the front limb. Furthermore, attempts to locate these elevation contours (N78°E). In contrast, terracettes landforms on similar slopes in the Tejon Embayment on the eastern forelimb of Wheeler Ridge anticline, using large-scale (1:20,000) aerial photographs have which strikes roughly N65°W, have treads that are failed. slightly inclined and plunge to the NE. This inclination increases with elevation as the terracette strike varies from N80°W at the base of the slope to N80°E at the 4. Soil stratigraphy higher reaches of the front limb. Hillslopes on which terracettes form slopes of 10–25°. The landforms Soils on Wheeler Ridge systematically vary in become more widely spaced with smaller amplitude age depending on their location relative to syntec- at higher positions on the limb of both folds. tonic deposits that decrease in age towards its east- 136 ..Beek,KJ ule emrhlg 2(02 131–152 (2002) 42 Geomorphology / Mueller K.J. Bielecki, A.E.

Table 2 Soil profile descriptions from the trench excavated at eastern Wheeler Ridgea Horizon Depth (cm) Thickness Colorb Texturec Structured Consistency Clay filmse Carbonatee Lower (cm) boundaryf Dry Moist Dry g Wet h Trench profiles Profile 1 A1 0–11 11 10YR5/3 10YR3/3 L 2msbk sh ss, ps – – c, s A2 11–63 52 10YR5/5 10YR3.5/3 L 1-2msbk sh ss, ps – – c, w AB1 63–105 42 10YR5/5.5 10YR4/3 L 2msbk sh s, p v1ncl 1npo, 1ncl c, s 2AB 105–134 29 10YR5.5/5 10YR4.5/4 L 1msbk sh s, p 1ncl 1ncl c, s 3AB 134–223 89 10YR6/4 10YR5/3.5 SL 1-msbk sh s, ps v1ncl 2kcl (patchy) c, s 4Bt 223–243.5 20.5 10YR6/6 10YR5/6 SCL 3abk vh-h vs, vp 2mkcl, 3ncl 3mkpo, 1ncl c, w-i 5Bt 243.5–268 24.5 10YR5.5/6 10YR5/4 SCL 3-mabk h vs, p v1npo, 1ncl, v1mkcl 3mkpo, 1ncl c-g, s 6AB 268–289 21 10YR6/4 10YR5/4 SL 1msbk lo-so s, ps – 1ncl, v1mkcl a, w-i 7C 289–321 32 10YR6/3 10YR5/3 S sg lo ss, po – 1mkcl (patchy) c, w 8C 321–338+ 17+ 10YR6/3 10YR5/3 S sg lo so, po – 1ncl, 3mkcl (patchy) no

Profile 2 A1 0–19 19 10YR5/4 10YR3/3 L 2m-csbk sh s, p – – c, s A2 19–67 47 10YR5/5 10YR4/3 L 1m-csbk sh s, p – – c, i AB1 67–95 28 10YR5/6 10YR4/3.5 SCL-L 1-2csbk h-sh vs, p+ v1npo, v1ncl – g, w AB2 95–149 54 10YR6/5 10YR5/4 SCL-L 2csbk h–sh vs, p+ 1npo, v1ncl 1ncl, 2npo c, w-i Bt 149–171 22 10YR5/7 10YR5/4 SCL 2cabk-sbk h vs, vp 1mkcl, v1npf 2mkpo, 1ncl c, w 2Bt 171–202 31 10YR6/5 10YR5/4 SCL 1msbk h-sh s, vp 1npo, 2ncl 1kcl, 2mkcl (patchy) c, w 3Ck 202–213 11 10YR8/2 10YR6.5/2 LS 1msbk lo ss, po – 4kcl a, w 4C 213–252 39 10YR7/2.5 10YR5/3 S sg lo so, po – 4kcl (patchy) a, w 5C 252–323 71 10YR6/2.5 10YR5/3 S-LS sg lo so, po – 2mkcl (patchy) a, w 6C 323–349+ 26+ 10YR6/3 10YR5/3 S-LS sg lo so, po – 3mkcl no Profile 3 A1 0–11.5 11.5 10YR5/3 10YR3/3 L 1msbk sh s, ps – – c, s A2 11.5–69 57 10YR5/4.5 10YR3/3 L 1csbk sh so, ps – – c, i AB1 69–84 15 10YR5/5 10YR4/4 SCL 1msbk sh s, p – – g, w AB2 84–99 15 10YR5/5 10YR4/3 SCL 1+ msbk sh+ s, p v1ncl – g, w Bt 99–143 44 10YR5/5.5 10YR5/4 SCL 2csbk sh-h s, p+ 1ncl 2kpo, 3npo, v1ncl c, w-i 2Bt 143–167 24 10YR5.5/6 10YR5/4 SCL 1m-csbk h s, vp 2npo, 1ncl 1npo, 2mkcl, v1npf a, w 3K 167–182 15 10YR7/2.5 10YR6/4 SL 3msbk eh s, po – 4mkcl, 3npf a, i 3Ck 182–250 68 10YR6/4 10YR5/5 plS sg lo so, po – 2kcl (patchy) a, w 4Bt 250–296 46 10YR6.5/4 10YR5/4 LS 3csbk h ss, ps – 4npo a, i 5C 296–348+ 52+ 10YR6.5/4 10YR5/4 LS sg lo ss, po – 4kcl, 3mkcl (patchy) no ..Beek,KJ ule emrhlg 2(02 131–152 (2002) 42 Geomorphology / Mueller K.J. Bielecki, A.E. Profile 4 A1 0–14.5 14.5 10YR5/3 10YR3/3 L 1m-csbk sh-h ss, p – – c, w A2 14.5–37 22.5 10YR5/4 10YR3/3 L 2msbk sh ss, p – – c, i AB 37–64 27 10YR5/6 10YR5/4 SCL 1msbk h s, p+ 3mkpo, 1mkcl – c, w Bt 64–91 27 10YR6/5 10YR5/4 SCL 1msbk eh s, p 2mkpo, 2mkcl, 1npf 3kcl, 2ncl c, w-i 2AB 91–110 19 10YR5/5 10YR4.5/4 SCL 1–msbk vh s, p 1mkpo 2mkcl c, w 3Bt 110–147.5 37.5 10YR6/6 10YR5/4 SCL 2msbk eh s, p v1mkpo, 1npo, 2ncl, 1mkcl – c, w 4Bt 147.5–167 19.5 10YR7/4 10YR5/3.5 SL 2msbk h-sh ss, p 3mkcl 1npf a, w 5BC 167–223.5 56.5 10YR6/4.5 10YR5/4 LS sg lo so, po – 2kcl a, w 6Bt 223.5–254 30.5 2.5Y6/4 2.5Y4/4 SiL 2m-csbk sh so, po 1ncl 1npf a-c, w 7C 254–336 82 10YR6.5/3 10YR5/3.5 plS sg lo so, po – 3kcl, 4mkcl c, w 8C 336–361+ 25+ 10YR6.5/2.5 10YR5/3 cS sg lo so, po – 3kcl, 4mkcl no

a Descriptions and abbreviations follow criteria in Soil Survey Staff (1981). b From Munsell (1975). c L, loam; LS, loamy sand; c, cobbly; pl, platy; S, sand; SL, sandy loam; SCL, sandy clay loam; SiL, silt loam. d 1, weak; 2, moderate; 3, strong; m, medium; c, coarse; vc, very coarse; sg, single grain; abk; angular blocky; sbk, subangular blocky. e v1, very few; 1, few; 2, common; 3, many; 4, abundant; n, thin; mk, moderately thick; k, thick; pf, ped faces; po, pore linings; cl, coating on clasts. f a, abrupt; c, clear; g, gradual; i, irregular; s, smooth; w, wavy. g lo, loose; so, soft; sh, slightly hard; h, hard; vh, very hard; eh, extremely hard. h so, non-sticky; ss, slightly sticky; s, sticky; po, non-plastic; ps, slightly plastic; p, plastic. 137 138 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152

Fig. 4. Log of west wall of 5-m-deep trench excavated at the base of the front limb of Wheeler Ridge marked with soil profile locations. Geographic location of trench marked in Fig. 2. ern end (Mueller and Talling, 1997; Keller et al., Mapping of bedding attitudes along the front limbs 1998). Soils formed in the key area we studied on of San Emigdio and Wheeler Ridge anticlines shows the forelimb of Wheeler Ridge are classified as a that strata immediately underlying terracettes are pachic argixeroll with cumulic mollic epipedon A oriented N78°E; 20°NW and N65°W; 20°NE in the and deep calcic horizons (Soil Survey Staff, 1994; two respective key areas. These measurements agree Birkeland, University of Colorado, personal commu- well with subsurface bedding attitudes defined by oil nication, 1998). This soil is comprised of a 1- to 2- well data (Medwedeff, 1992) and the orientation of m-thick calcic Bk horizon, overlain by a 0.5-m thick the two respective range fronts. We observed numer- hard argillic Bt horizon, followed by 1.5–3 m of A ous burrow openings ranging from 2 to 30 cm in and AB horizons that have been extensively bio- diameter on the terraced slopes. Occasionally, a fresh turbated (Table 2). Terracettes are developed com- rodent heap was observed near the openings. No pletely within the A horizons while underlying soil evidence of bedding was observed that could be horizons (Bt and Bk) have relatively consistent correlated to the terracettes at the surface. thickness and planar contacts which do not mimic or appear otherwise related to overlying terracette morphology (Fig. 4). Texture, structure, and color of 5. Numerical and geometric modeling A and AB horizons do not vary across adjacent terracette treads or risers. Clays are found predom- 5.1. Coseismic folding hypothesis inantly within the AB and Bt horizons where they usually occur as pore linings or coats on clasts. The fold growth terracette model assumes a prism Carbonate stage and other characteristics of soil of strata that onlaps the base of the fold limb is horizons at the trench site are consistent with the translated upward during earthquakes. This translated Q3 to Q4 soils of Keller et al. (1998) that have an prism corresponds to the terracette tread (Mueller and age of ~60–125 ka. In contrast, the detailed soil Suppe, 1997). Trench excavations indicated that profile descriptions acquired on the crest and back- extensive bioturbation destroyed alluvial bedding in limb of Wheeler Ridge between the Wind and Water the upper 1 to 3 m of A and AB horizons (Mueller and Gap by Zepeda (1993) have A and AB horizons Suppe, 1997). We further tested the folding hypothesis with a total thickness of only 40–75 cm which is by comparing models of terracette morphology with less than half the thickness of similarly aged hori- the DEM and total station mapping. zons at the same elevation on the front limb where Measurements from total station profiles show that the terracettes are located. the northward displacement of the active axial surface A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 139

(s), which is similar to fault slip in the folding proposes that the trace of the axial surface should hypothesis, averages 13 m (Fig. 3). Comparison of correspond to the outer edges of terracette treads this slip value to a regression of earthquake magnitude (Mueller and Suppe, 1997). We created maps of the vs. fault slip in southern California (Dolan et al., projected contact between hypothetical axial planes 1995) implies an average magnitude (Mw) of 8.4 for and the ground surface for both key areas to test events at Wheeler Ridge (Mueller and Suppe, 1997), whether they correspond with the terracettes (Figs. 5 an extremely high and unlikely value given the length and 6). Axial surfaces are defined as the interlimb of the fold. The coseismic folding hypothesis also bisector between bedding (20°) and undeformed allu-

Fig. 5. Contour map of eastern Wheeler Ridge anticline key area (A) with rotated cross-section of hypothetical paleo-axial planes used to map their contacts with the ground surface. Axial planes drawn to match spacing of real terraces, with blue axial plane corresponding to blue marker terrace. Slope map of eastern Wheeler Ridge key area (B) with surface traces transferred from contour map. Terrace treads are highlighted in green. 140 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152

Fig. 6. Contour map of San Emigdio anticline key area (A) with rotated cross-section of hypothetical paleo-axial planes used to map their contacts with the ground surface. Axial planes drawn to match spacing of actual terraces. Slope map of San Emigdio key area (B) with surface traces transferred from contour map. Terrace treads are highlighted in green. vial fans to the north (1.6° for eastern Wheeler Ridge 5.2. Flexural slip faulting hypothesis key area; 1.2° for San Emigdio key area). The surface trace of the axial planes were mapped across the DEM An additional tectonic hypothesis for terracette and spaced to coincide with actual terrace spacing development is based on coseismic flexural slip fault- (Figs. 5 and 6). ing, a common deformation mechanism in active A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 141 fault-related folds (Ramsay, 1974; Klinger and Rock- to the model, terracette tread width is equivalent to well, 1989; Tanner, 1989; Becker, 1994; Treiman, slip on each flexural slip fault (Fig. 7). The average 1995; Cooke and Pollard, 1997). According to our flexural slip required to occur on hypothetical bedding model, coseismic flexural slip occurs within growth plane slip surfaces, as measured from total station strata, causing small-scale bedding plane backthrusts profiles, is 4.4 m (Fig. 3). that rupture the ground surface on the forelimb of the The surface traces of bedding plane contacts on fold. Collapse of the hanging walls of the flexural slip which flexural slip might occur should correspond to faults produces flatter hillslopes that degrade into the the center of terracette treads. The contacts of hypo- terracette treads (Fig. 7) (Bielecki, 1998). According thetical flexural slip surfaces were thus mapped for

Fig. 7. Schematic illustrating the development of terraces according to the flexural slip faulting hypothesis. Bedding contact between two units with disparate lithologies provides slip surface dips slightly steeper than overlying topography (A). Coseismic flexural slip (B) causes uplift and surface rupture of the hanging wall unit. Flexural collapse and degradation of the hanging wall unit (C) into the terrace tread observed today, along with incipient sedimentation in the valley. Note that the uplifted wedge of material is similar to an isosceles triangle, making the width of the terrace tread approximately equal to the amount of flexural slip. Diagram illustrating the geometry of wedge–thrust structure and front limb of the folds (D). Length of valley strata incorporated into front limb during any event is approximately equal to the amount of slip on the basal thrust. 142 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 both key areas using bedding attitudes measured in 6. Geomorphic models the field (Figs. 8 and 9). The terracette treads should obey the ‘‘rule of v’s’’ when observed in plan view The remaining hypotheses proposed for terracette and ‘‘v’’ to the north (downstream) in the gullies development along the front limbs of San Emigdio incised on the front limb of these folds. and Wheeler Ridge anticlines can be subdivided into

Fig. 8. Contour map of eastern Wheeler Ridge anticline key area (A) with rotated cross-section of hypothetical flexural slip faults used to map their contacts with the ground surface. Faults drawn to match spacing of real terraces, with the red fault aligned with red marker terrace along the right-hand edge of diagram. Slope map of eastern Wheeler Ridge key area (B) with surface traces transferred over from contour map above. Terrace treads are highlighted in green. A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 143

Fig. 9. Contour map of San Emigdio anticline key area (A) with rotated cross-section of hypothetical flexural slip faults used to map their contacts with the ground surface. Faults drawn to match spacing of real terraces. Slope map of San Emigdio key area (B) with surface traces transferred over from contour map above. Terrace treads are highlighted in green. two categories: (i) those which rely on the presence of genesis of these landforms: (i) a single mechanism is a physical agent (e.g., fossorial rodent, livestock responsible for terracette development, or (ii) the grazing, etc.) and (ii) those based on geologic pro- terracettes may be polygenetic in origin where more cesses (e.g., soil creep, slumping, etc.) (Vincent and than one mechanism may have contributed to their Clarke, 1976). Two possible scenarios exist for the formation. Moreover, two possibilities exist for the 144 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 timing of development: (i) the terracettes may be relict smectite clays (Hilgard, 1884). Most gilgai hypoth- structures created by processes or agents no longer eses involve desiccation cracking of expandable clays, occurring or present in the region or (ii) the terracettes infilling of cracks, and expansion of the soil after are produced by processes or agents still occurring or wetting. These processes, along with differential soil present in the region. The possibility that the terrac- settling or plastic flow, are interpreted to produce ettes are fossil landforms is plausible based on the areas with mounded topography (Washburn, 1988). slow (0.037 m/ka) erosion rate calculated for the front The gilgai hypothesis has already been suggested for limb of these folds (Medwedeff, 1992). the cause of the landforms found along the San Emigdio range front (Seaver, 1986). Seaver (1986) 6.1. Mima-like mound: fossorial-rodent hypothesis noted mounds on slopes of 3–10° and suggests that the shrink–swell capacities of the argillic B horizon ‘‘Mima-like’’ is a general term used to describe are intimately related to the initiation and develop- mound-like landforms of enigmatic origin that have ment of the landforms. In addition, runoff, freeze– varying dimensions and a circular to elliptical shape thaw action, and creep on steep hillslopes are also and are found predominantly in prairies, valleys, and recognized as processes that may serve to amplify the high plains (Washburn, 1988). The fossorial-rodent mounds. hypothesis is the most widely accepted candidate for the cause of Mima-like mounds in most regions. This 6.3. Mima-like mound: anthropogenic hypothesis model states that solitary fossorial rodents in regions with a shallow soil mantle above a dense substratum The primitive farming techniques of Native Amer- will glean soil and detritus in an effort to preserve icans have been proposed for the origin of terraced habitat (Dalquest and Scheffer, 1942). Where topsoil is landforms (Aten and Bollich, 1981). In particular, deep and friable, there is no downward barrier to terracettes in SSJV are thought to be prehistoric strip burrowing activity, so the rodents do not construct lynchets. The limitation of terracettes to the base mounds (Scheffer, 1984). However, where the soil is northern-facing hillslopes is attributed to deliberate thin, the animals build ‘‘nests’’ located at a fixed selection of agricultural areas that are not too steep to distance from one another due to territorial behavior. navigate and provide the necessary soil moisture The fossorial-rodent hypothesis thus implies direct retention during the dry season. transport of material by burrowing animals from inter- mound to mound regions. The animals are strongly 6.4. Livestock grazing hypothesis motivated by an instinct for security, so they build mounds at the center of their feeding range with a The northern foothills of the San Emigdio Moun- minimum thickness of material that protects them from tains have been extensively grazed for the past 150 predators, ground water perched above a clay-rich years. Consequently, the landforms found along the hardpan, and extreme temperatures. These mounds front limb of these folds may be erosive trails made by may also serve as food caches, hibernation or estivation livestock. These trails, known as ‘‘grazing-step terrac- chambers, or as dens to gestate and foster offspring ettes,’’ are found throughout sloping grasslands and (Butler, 1995). The rodent most commonly associated are present in the San Emigdio Mountains (Howard with Mima-like mound development is the geomyidae and Higgins, 1987). These terracettes have been pocket gopher (Thomomys bottae) (Washburn, 1988); identified on the steep gully walls and in the major but for SSJV, the California (Beechey) ground squirrel canyons of both San Emigdio and Wheeler Ridge (Spermophilus beecheyi) is the animal most commonly anticlines. Grazing-step terracettes develop as live- linked to mound construction (Wallace, 1991). stock traverse steep slopes while grazing. The terracette treads are typically barren of vegetation and have 6.2. Mima-like mound: gilgai hypothesis a maximum width of 1 m, but most are 0.2–0.5 m in width. Grazing-step terracettes typically have an anas- Gilgai are low mounds and shallow depressions tomosing pattern with interconnecting ramps (Howard that have been ascribed to the shrink–swell action of and Higgins, 1987). A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 145

6.5. Mass wasting hypotheses ment of these slabs would cause the opening of fissures and crevasses, which would later become Mass wasting can be subdivided into two main infilled with sediment and diffuse into the terracette categories: (i) flow-dominant movements and (ii) slip- tread observed today (Fig. 10). dominant slope failures. Mass movements are facili- Solifluction is defined as the slow downslope tated when processes act to either increase the shear movement (1–10 cm/year) of waterlogged soil that stress on the material or decrease its internal shear is analogous to plastic mud (Young and Saunders, strength (Easterbrook, 1993). In SSJV, soil moisture 1986; Gerrard, 1992). The word solifluction literally has the most influential effect on the rate and distri- means ‘‘soil flow’’ (Embleton, 1975). Solifluction is bution of mass movements (Harris, 1972; Young and most prominent in high-altitude or high-latitude Saunders, 1986; Easterbrook, 1993). In general, soil regions where permafrost and frequent freeze–thaw texture, vegetation, and slope angle play secondary cycles exist. Consequently, solifluction is not wide- roles in determining rates of flow. Types of mass spread where mean annual temperatures are above 0 wasting attributed to terracette and/or lobe develop- °C (Carson and Kirkby, 1972; Embleton, 1975; Pewe, ment include flow-dominant soil creep and solifluc- 1983). Models of landform development involving tion, along with slip-dominant slumping. solifluction are based on the downslope flow of Slumping is a form of slip-dominant mass wasting supersaturated soil that accumulates in ridges oriented that has been proposed as a mode of origin for parallel to contours. In general, solifluction lobes have terraced landforms (Sharpe, 1938). The model an oversteepened toe, soils that are thickest in the involves either the planar translation or rotational riser, and an accumulation of coarser material at the slippage of ‘‘slab slides’’ on a slip surface. This leading edge. In addition, solifluction lobes generally surface can develop on layers of weakness within override one another, causing a shingled stacking of the soil, at the colluvium–bedrock boundary, or at the the features (Embleton, 1975). boundary between soil and a rigid substratum like a Soil creep is defined as the slow, continuous hardpan unit. For SSJV, slip would most likely occur downslope movement of superficial colluvium at rates during coseismic shaking on top of the rigid argillic Bt that range from a few millimeters per year to a soil horizon found throughout the region. Seismic maximum of a few centimeters per year (Sharpe, ground motions would help to initiate downslope 1938; Young and Saunders, 1986). Although the rate movement, while fold limb lengthening acts to uplift of movement is an order of magnitude slower than the material and displace it from the basal support of solifluction, it can be facilitated by bioturbation, SSJV. According to the model, the downslope move- drying–wetting, and strong ground motions produced

Fig. 10. Schematic illustrating two possible modes for development of terraces according to the slumping (slab slide) hypothesis. Coseismic slippage along one plane (A) causes the downslope movement of colluvium, followed by degradation (collapse and infill with sediment) into the terrace tread observed today. Coseismic rotational slippage (B) along a curved plane leads to the development of the terraces observed today. 146 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152

Fig. 11. Schematic illustrating the development of terraces according to the soil creep hypothesis. Profile of slow downslope colluvium movement (A) within the range typically observed (top 1 m). Creep rates taper off with depth and is limited to A and AB horizons that delaminate above the argillic Bt claypan. Plan view of the tread of a soil creep-related terrace with the most rapid movement through the center, leading to lateral termination of the landform. by earthquakes. Models of landform development 80°SW and N88°W; 80°SW for the San Emigdio involving soil creep postulate that these features form and Wheeler Ridge key areas, respectively (Figs. 5 in response to a downslope decrease in the velocity of and 6). Thus, in order for the fold growth model to be moving soil (Clayton, 1966; Benedict, 1970, 1976). accurate, the strike of the axial planes would have to This usually occurs when there is a downslope deviate from the strike of front limb attitudes by 15° decrease in gradient such as on concave lower slopes, for the eastern and 14° for the western key areas. We and leads to the development of buckle folds (Fig. 11). argue that the lack of alignment between terracette A maximum amplitude that can be obtained by any treads and hypothetical axial plane surface traces given fold exists, necessitating the development of serves to eliminate the coseismic folding hypothesis several subparallel structures. The rate of soil creep is of Mueller and Suppe (1997) as a viable mechanism highest at the surface and abruptly decreases at depth, of terracette development on the San Emigdio and with movement confined to a surface zone of a meter Wheeler Ridge anticlines. or less in depth (Fig. 11) (Carson and Kirkby, 1972). Consequently, soil creep is confined to the extremely 7.2. Flexural slip faulting hypothesis thick A and AB horizons that delaminate from the underlying rigid Bt claypan. According to the flexural slip faulting model, the coseismic slip that produces any given terracette is roughly equivalent to tread width (Fig. 7). Total 7. Discussion station profiles and field observations show that the average tread width is 4.4 m, average limb dip is 7.1. Coseismic folding hypothesis 19.9°, and total thickness of bedding in between hypothetical flexural slip faults (as dictated by ter- The discrepancy between moment magnitudes for racette spacing and limb dip) is approximately 1.22 historic earthquakes in southern California and fault m (Fig. 3). The model of chevron fold development slip (Dolan et al., 1995) in comparison to the defined by Ramsay (1974) shows that the amount of extremely large values of Mw predicted by the fold flexural slip is directly proportional to bedding thick- growth terracette model serves to discredit this ness and the amount of shortening which has hypothesis for terracette development. Furthermore, occurred. This relationship was formulated in the Figs. 5 and 6 illustrate that the hypothetical axial equation: surface traces do not remotely correspond to the pattern of terracette development observed in either s ¼ t Á tanðaÞð1Þ of the key areas. In order to successfully match a surface trace of an axial plane to terracettes in DEM, where s is the amount of flexural slip, t is the total the attitude of the axial surface must be N80°W; thickness of bedding in between slip surfaces, and a A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 147 is limb dip in degrees. Although the forelimb syn- terracettes are absent (Zepeda, 1993). Moreover, the cline of San Emigdio and Wheeler Ridge anticlines mound-building behavior of animals dictated by the does not satisfy the definition of a pure chevron fold, fossorial-rodent hypothesis is inconsistent with the Eq. (1) does provide an accurate approximation of long linear terracettes found along the northern-facing the amount of flexural slip that will occur within the slopes of San Emigdio and Wheeler Ridge anticlines. front limbs given the observed geometry (Bielecki, In addition, the California (Beechey) ground squirrel 1998). Using Eq. (1), the amount of flexural slip that (S. beecheyi) is a colonial animal, which is incon- occurs on 1.22-m thick bedding rotated to an angle sistent with the solitary behavior required by the of 19.9° is only 0.44 m which is an order of fossorial-rodent hypothesis. Although the effect bio- magnitude less than the 4.4 m of flexural slip needed turbation has on the soil mantle that makes up these to create the terracettes according to this model. landforms is significant, we argue that fossorial activ- Moreover, Figs. 8 and 9 clearly illustrate that these ity of rodents is not the primary cause of their bedding contacts do not match the pattern of terrac- development. ette distribution observed in either of the key areas to within a reasonable deviation. For the flexural slip 7.4. Mima-like mound: gilgai hypothesis terracette model, no attempts were made to derive a best-fit by manipulating the bedding attitudes The gilgai hypothesis of landform development is because it is impossible to create north-dipping based on the action of shrink–swell clays which are planes that ‘‘v’’ to the south in gullies. In addition, common in the argillic Bt horizon found in the the terracettes do not follow the ‘‘rule of v’s’’ northern San Emigdio Mountains. This hypothesis, predicted by the model and instead ‘‘v’’ to the south as applied to the terracettes found in SSJV, was first (upstream) (Figs. 1 and 2). The large deficit in the proposed by Seaver (1986), who noticed a correlation slip budget and lack of geometric alignment of between the landforms and Q4 geomorphic surface on terracettes with their predicted orientation serves to the front limb of San Emigdio anticline. However, a eliminate the flexural slip-faulting hypothesis for detailed model of landform development was not terracette development on the front limb of the two developed for these features and no data was pre- folds. sented to demonstrate the possible link between shrink–swell clays and landform development (Sea- 7.3. Mima-like mound: fossorial-rodent hypothesis ver, 1986). In addition, the relationship of Q4 geomorphic surface does not translate to eastern Wheeler The fossorial-rodent hypothesis is a possible Ridge where terracettes can be found below Q4 sur- explanation for the landforms observed because the face (Keller et al., 1998). Furthermore, the complete rigid Bt horizon could act as the relatively shallow absence of the landforms in adjacent regions with downward barrier to burrowing activity. Furthermore, similar soil characteristics and clay content, including the semiarid steppe ecosystem in SSJV forces animals the south limb of Wheeler Ridge (Zepeda, 1993), to burrow in an effort to conserve water, store food, serves to argue against their possible relationship to foster offspring, hibernate/estivate, and find protection shrink–swell clays. The presence of terracettes on from predators and intense summer heat. The obser- predominantly northern-facing slopes thus cannot be vation of numerous burrow openings and rodent explained by the gilgai hypothesis. heaps, along with the extensive bioturbation observed in the trenches, suggests that animals are actively 7.5. Mima-like mound: anthropogenic hypothesis burrowing within the soil that makes up the terracettes. However, these burrows are not limited to the Studies of Mima-like mounds throughout a variety northern-facing front limbs where the landforms are of regions demonstrate a complete lack of archeolo- found. In fact, both open burrows and krotovina with gical evidence that suggests that the landforms were diameters of 10–30 cm were observed predominantly built by humans (Aten and Bollich, 1981). Further- within the A and AB horizons of soil profiles on the more, trench excavations in SSJV showed no evi- crest and backlimb of Wheeler Ridge where the dence of anthropogenic activity, including the absence 148 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 of midden or agricultural tools left behind by native produce. The model of landform development based Americans. on slab slides (Fig. 10) relies on the opening of crevasses that infill with sediment and degrade into 7.6. Livestock grazing hypothesis the terracette tread observed today. This would result in co-planar terracette risers which is not the morphol- The treads of grazing-step terracettes are typically ogy observed (Fig. 12). Consequently, the slab slide 0.2–0.5 m in width, which is an order of magnitude model provides an insufficient explanation for land- smaller than the 3- to 5-m tread width of the form development. terracettes observed along the front limb of active The process of solifluction depends on the pres- folds in SSJV. Livestock tend to follow one another ence of waterlogged soil above a barrier to downward along grazing-step trails, eliminating the possibility soil water percolation. In general, solifluction is that the terracettes are a result of animals walking considered a periglacial process and is most prom- side-by-side (Howard and Higgins, 1987). The com- inent in regions where permafrost is present, although plete lack of terracettes on slopes with similar gra- permafrost is not essential for solifluction to occur as dient where grazing takes place, including the back long as some other layer exists that acts as a down- limb of Wheeler Ridge, contradicts the livestock ward barrier to water percolation (Carson and Kirkby, grazing hypothesis. Howard and Higgins (1987) also 1972; Embleton, 1975; Gerrard, 1992). However, propose the possibility that larger features known as there is a lack of impermeable beds that could act ‘‘catsteps’’ could start out as grazing-step terracettes to perch the ground water table along the San Emig- that are later amplified by other processes. This is not dio and Wheeler Ridge anticlines (Keller et al., 1998). a plausible explanation for the terracettes in this Mean annual temperature for Kern County Airport in region because grazing has only occurred since the Bakersfield, located 40 km north of Wheeler Ridge, mid-19th century. Moreover, the front limbs of San for the years 1964–1993 is 18.3 °C; and minimum Emigdio and Wheeler Ridge anticlines are still temperatures during the winter were rarely below actively grazed today, but lack fresh grazing-step freezing (National Climatic Data Center, 1993). This terracettes on the same surfaces where the landforms suggests that any processes associated with the freez- of interest are found. In general, field observations ing of ground water, including permafrost and along the front limbs show that slopes in the range of freeze–thaw activity, do not currently occur to a 10–25° are dominated by the large terracettes, while significant extent in SSJV. There is, however, the grazing-step terracettes typically occupy the slopes possibility that the landforms along the front limb of steeper than 25°. We interpret this evidence to dis- these folds are fossil structures that were formed in prove the livestock trail hypothesis for terracette development.

7.7. Mass wasting hypotheses

The slip-dominant mass movements of the slumping hypothesis are not as likely to occur as earthflows on the relatively shallow dipping slopes along the front limb of these folds. In general, slumping usually occurs on concave-up spoon-shaped slip surfaces that have limited lateral extent (Easterbrook, 1993). While this may provide a reasonable explanation for the shorter terracettes, it is not consistent with the long linear terracettes that can be traced more than 300 m along eastern Wheeler Ridge. Furthermore, these slip Fig. 12. Diagram illustrating the difference between co-planar surfaces would have to be quite pervasive and as terrace risers (A) and the landform morphology observed (B) along regularly spaced as the landforms that they might the front limb of San Emigdio and Wheeler Ridge anticlines. A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 149 the past and are no longer active. Since contemporary observed on slopes with aspect ranging from N70°W climate is classified as an interglacial cycle, temper- to N40°E at Comanche Point, another active structure atures and precipitation during the past glacial/pluvial found 12 km northeast of Wheeler Ridge (Bielecki, periods may have been more conducive to periglacial 1998). The presence of terracettes on northern-facing processes. The last glacial maximum occurred slopes elsewhere in the San Joaquin Valley suggests between ca. 20 and 18 ka. For this region of southern that this aspect is a prerequisite for their development California at a latitude of 35°N, this time period was and provides corroborating evidence for the soil characterized by higher precipitation and a mean creep hypothesis. annual temperature that was 8 °C cooler than the The over-thickened A and AB horizons found in present day (18.38.0=10.3 °C) (Pewe, 1983; Frenzel soils at the base of the front limb of these folds are et al., 1992). This mean annual temperature is still likely to promote soil creep because the increased ~10 °C warmer than the 0 °C mean annual temper- thickness of the active layer increases the downward ature required for periglacial processes to occur distance to frictional resistance at the top of the rigid (Carson and Kirkby, 1972; Embleton, 1975; Pewe, Bt claypan. In fact, these cumulic horizons can be 1983). In addition, none of the typical features asso- explained by the soil creep process itself since it ciated with solifluction lobes is observed in the land- provides a mechanism for the downslope accumula- forms along the front limbs of San Emigdio and tion of soil from higher slopes. This is supported by Wheeler Ridge anticlines. This includes the absence the fact that the terracettes developed completely of oversteepened toes, shingled stacking of land- within the A horizons (Fig. 4). Furthermore, the forms, and the lack of an accumulation of coarse well-developed A and AB horizons have a loamy material at the toe. Furthermore, the soils that make texture and moderate silt content which is optimal up the terracettes in SSJV are thicker in the tread for flow-dominant mass wasting due to the lower regions than the risers which is the opposite of the permeability and consequent increase in soil mois- solifluction model. ture. The grassland vegetation found throughout the Soil moisture conditions have the biggest effect on region also increases soil moisture content and the rate and distribution of soil creep (Harris, 1972; decreases potential evaporation by capturing surface Young and Saunders, 1986; Easterbrook, 1993). In runoff and increasing infiltration. In addition, rela- general, soil texture, vegetation, and slope angle play tively shallow root systems cannot provide signifi- secondary roles in determining the rate of flow- cant slope stabilization effects through the entire A dominant mass movements. Consequently, the rate and AB horizons. of movement would be fastest during the rainy The dominant wind direction in SSJV is out of season (October–April) or during past pluvial peri- NW (National Climatic Data Center, 1993). These ods. Since the average climate in SSJV is warm and winds are relatively unhindered as they travel across semiarid to arid, both evaporation and transpiration SSJV until they reach the front range of the northern play important roles in determining the amount of San Emigdio Mountains, where a decrease in veloc- soil moisture that is retained by any given surface. In ity causes eolian dust to fall out of suspension. the northern hemisphere, northern-facing slopes will Therefore, we argue that the eolian influx of loess lose the least amount of soil moisture to evapotrans- may be an important factor contributing to the over- piration due to their orientation in relation to the sun thickened soil deposits found along the front limbs of (Russell, 1909). Consequently, soil creep is more San Emigdio and Wheeler Ridge anticlines. Studies likely and will occur at faster rates on northern-facing show that a thick accumulation of loess and the slopes (Pierce and Colman, 1986). Landforms with presence of an argillic B horizon are a pre-requisite similar morphology and dimensions as the terracettes for flow-dominant mass wasting in the western Great on the front limbs of San Emigdio and Wheeler Basin where solifluction lobes developed entirely in Ridge anticlines have been mapped on N20°W- to A horizons over a planar Bt contact have been N45°E-facing hillslopes on the Kettleman Hills anti- discovered (Sawyer, 1988, 1990). In addition, Wal- cline, located 150 km NW in the west-central San lace (1991) ascribes elongation of similar landforms Joaquin Valley. In addition, terracettes can also be into long linear terracettes found along the San 150 A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152

Andreas Fault in central California to wind modifi- Homogenization of the upper A and AB soil hori- cation. We propose that winds out of NW that flow zons allows creep to occur over long distances (100– over terraced hillslopes appropriately oriented in 300 m) along the hillslopes while a downward SSJV may cause translation of loess from the orig- decrease in dip at the base of concave upward slopes inally lobate mounds in a direction across slope, leads to sharper and more pronounced terracettes. leading to the development of the long linear terrac- Formation of terracettes on active folds in the south- ettes in the areas we studied. ern San Joaquin Valley, thus depends on a number of The extensive bioturbation that occurs in the A factors including slope orientation, soil stratigraphy, and AB horizons which make up the terracettes can age and possibly the presence of large blind thrust also serve to explain their increased thickness. Fos- faults. sorial activity facilitates soil development through mixing and aeration (Butler, 1995). Moreover, the burrowing activity of animals can also cause a Acknowledgements specific type of soil creep known as ‘‘biogenic creep.’’ Biogenic creep occurs when burrows collapse We thank David Rabine (Laser Altimeter Process- or are refilled with material moved in from upslope, ing Facility in the Geodynamics Branch of the causing a net downslope transport of colluvium. In Laboratory for Terrestrial Physics at NASA’s Goddard regions where bioturbation is extensive, biogenic Space Flight Center) for his efforts in the post-flight creep can be the most significant contributor to processing of the RASCAL laser altimetry data, overall soil creep within the depth of soil occupied Jocasta Champion for her mapping efforts on Kettle- by the animals (Lehre, 1987). man Hills, and Maria Hertzberg for her assistance with soil profile descriptions. This research was supported by NASA Grant NAG5-3320 as part of 8. Conclusions the Topography and Surface Change Program, Na- tional Science Foundation Grant EAR-9614675 to K. Soil creep occurring within the front limbs of Mueller, and Geological Society of America Grant Wheeler Ridge and San Emigdio anticlines is inter- 6193–98 to A. Bielecki. preted to be the primary cause of terracette development. McGill (1951) suggests that these landforms References are the result of flow-dominant mass wasting that occurred at a time in the past when the climate was Aten, L.E., Bollich, C.N., 1981. Archeological evidence for pimple more humid. Although soil creep is likely in the (prairie) mound genesis. Science 213, 1375–1376. present day during the rainy season when there is a Barry, R.G., 1983. Late-Pleistocene Climatology. Univ. of Minne- rapid increase in soil moisture, we argue that the sota Press, Minneapolis, pp. 390–407. terracettes are primarily developed during past glacial Becker, A., 1994. Bedding-plane slip over a pre-existing fault, an example: the Ramon Fault, Israel. Tectonophysics 230, 91– periods when the climate of the American southwest 104. was much more humid and rates of mass movement Benedict, J.B., 1970. Downslope soil movement in a Colorado would have been faster (e.g., Barry, 1983). Terrac- alpine region: rates, processes, and climatic significance. Arct. ettes develop within deep A and AB horizons above Alp. Res. 2, 165–226. the rigid Bt claypan on northern-facing slopes due to Benedict, J.B., 1976. Frost creep and gelifluction features: a review. Quat. Res. 6, 55–76. the favorable aspect for soil moisture retention. Bielecki, A.E., 1998. Structural and geomorphic analysis of enig- Eolian modification could serve to elongate the land- matic terraced hillslopes formed on active folds in the southern forms and cause accumulation of loess contributing San Joaquin Valley using high resolution laser altimetry. Un- to the extreme thickness of A and AB horizons published MS Thesis, Department of Geological Sciences, Univ. which underlie up the terracettes. Bioturbation serves of Colorado, Boulder, 175 pp. Butler, D.R., 1995. Zoogeomorphology: Animals as Geomorphic to facilitate the development of the over-thickened A Agents. Cambridge Univ. Press, New York, NY, pp. 108–147. and AB horizons and causes biogenic creep that Carson, M.A., Kirkby, M.J., 1972. Hillslope Form and Process. augments simple gravity-driven mass movement. Cambridge Univ. Press, Cambridge, UK, pp. 172–300. A.E. Bielecki, K.J. Mueller / Geomorphology 42 (2002) 131–152 151

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