Effects of Pocket Gopher Burrowing on Archaeological Deposits: A Simulation Approach

 

Christopher Pierce

 

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The construction of burrows and movement of sediment by pocket gophers alter archaeological deposits by causing vertical size-sorting of artifacts, destruction of fragile artifacts, disruption of sedimentary structures, and organic enrichment of the subsurface. To evaluate the long-term effects of exposure to burrowing, a simulation was developed based on quantitative information on pocket gopher burrows and rates of sediment movement. Simulation results indicate the development of a distinct stone zone composed predominantly of particles greater than 6 cm after 4000-5000 years, and a logarithmic pattern to the rate of strata disruption. The patterns produced by the simulation compare well with patterns exhibited by actual archaeological deposits belonging to California's Milling Stone Horizon. These results suggest that current notions concerning the Milling Stone Horizon and other aspects of California prehistory may require revision, and that more emphasis must be placed on formation process research in such settings.

 

Published as:

Effects of Pocket Gopher Burrowing on Archaeological Deposits: A Simulation Approach. Geoarchaeology Vol. 7, No. 3, pp. 185-208, 1992.

 

Download a PDF reprint of the published version of this paper.

 

Run the simulation program discussed in this paper. 

 

 

INTRODUCTION

 

Over the past two decades, studies of the processes that affect the form and position of artifacts have occupied an increasingly prominent role in archaeology (e.g., Binford 1981; Schiffer 1972, 1987). Recognition and control of post-depositional processes that alter or disturb the archaeological record have received the greatest attention (Nash and Petraglia 1987; Wood and Johnson 1978). However, it is frequently difficult to identify and unravel the processes responsible for patterns and variation observed in the archaeological record. As a result, we often lack the ability to reliably recognize the nature and extent of disturbance of archaeological deposits, and thus, the knowledge necessary to minimize the effects of these impacts on our understanding of prehistory. A study of the effects of pocket gopher burrowing on archaeological deposits in California provides an example of this problem and how it may be resolve.

 

Recent research indicates that extensive pocket gopher burrowing may have substantially altered numerous archaeological deposits in California. Several studies demonstrate the potential for vertical mixing and size-sorting of artifacts and soil particles by pocket gopehrs (Bocek 1986; Erlandson 1984; Erlandson and Rockwell 1987; Johnson 1989; Pierce 1982; Wood and Johnson 1978). Some (Johnson 1989; Pierce 1988) suggest that archaeological patterns atttributed to different cultural processes may have resulted instead from the cumulative action of pocket gophers over time. However, none of these studies provides a model of pocket gopher distrubance with sufficient detail to allow prediction of long-term effects. This paper pesents a general, partly quantitative model of disturbance by pocket gopher burrowing. The model is based on patterns of sediment movement and burrow structure documented through studies of living pocket gopher (Thomomys sp.) communities. Quantitative portions of the model are translated into a computer simulation to evaluate the long-term effects of burrowing on vertical distributions of artifacts and stratigraphic integrity. Simulation results are compared to data from heavily burrowed archaeological deposits to assess the validity of the simulation.

 

The model and simulation produce patterns of vertical size-sorting, straigraphic mixing, and destruction of fragile remains comparable to those found in actual archaeological deposits. In fact, many of the characteristics of archaeological deposits assigned to the Milling Stone Horizon (Moratto 1984; Wallace 1955, 1978) in southern and central California match those generated by pocket gopher burrowing. The model allows formulation of specific expectations regarding artifact distributions, assemblage content, and stratigraphic integrity in deposits from various contexts and ages. These expectations demonstrate the utility of computer simulation for understanding archaeological formation processes. Furthermore, the results have implications for how we record, interpret, and report the archaeological record. Currently, inconsistencies in describing the archaeological record and failure to treat the record as a product of geological processes severly limit our ability to gain accurate knowledge of formation processes and therefore, past cultural systems.

 

A MODEL OF POCKET GOPHER DISTURBANCE

 

In California, burrowing rodents constitute over half of the population of all native mammals (Grinnell 1923:138). Of these rodents, the most abundant as well as the most prodigious burrower is the pocket gopher (Thomomys sp.), four species of which live within California (Hall 1981:454-480). The model presented here derives primarily from informaiton on the burrowing behavior of Thomomys umbrinus, the most common pocket gopher in California and the southwestern United States (Hall 1981:469-475). Debate exists over the accurate taxanomic status of pocket gophers in the southwestern U.S. and northern Mexico. Some argue for two species, Thomomys bottae to the north and T. umbrinus in the south, while others lump all pocket gophers of this area into one species, T. umbrinus, recognizing only subspecific distinctions due to the presence of genetic mixing. In this article, Thomomys umbribus following Hall (1981) is used. In presenting this model of pocket gopher disturbance, I use the term sediment to refer to the particles, including artifacts, moved by burrowing and burrow collapse.

 

Figure 1.
Figure 1. Plan view (A) and vertical section (B) of a typical pocket gopher burrow system (from Miller 1957 and Hickman 1977).
 

 

Burrows and Burrowing Behavior of Thomomys umbrinus

 

Pocket gophers feed primarily on subsurface plant parts. As a result, pocket gophers spend virtually all of their lives underground. Individual burrows consist of two main parts -- tunnels and chambers (Figure 1). Each pocket gopher maintains one or more main tunnels excavated within the root zone parallel to the ground surface. These main tunnels can be quite long (up to 90 m), and are segmented by frequent lateral tunnels. The lateral tunnels are used for grazing on roots, and as passages through which loose sediment is taken to the surface. Tunnels average approximately 6.5 cm in diameter, slightly larger than the pocket gopher. Tunnels form the majority of the burrow, occurring primarily between 10 and 30 cm below the surface. Chambers used for nests, food storage and waste disposal and deep, vertical sump tunnels occur between 40 and 70 cm below the surface in deeper extensions of the main tunnel. These deeper chambers are regularly 15-25 cm in diameter, and located beneath rocks, tree roots, or other objects for protection from predators (Buechner 1942; Grinnell 1923; Ingles 1947; Miller 1957; Storer 1933; Storer and Usinger 1963:339). Although open tunnels occasionally extend as deep as 2 m below the surface (Miller 1957; Reichman et al. 1982), well over 90% of the average total burrow volume occurs within the upper 70 cm, with most of the volume concentrated in the upper 30 cm (Table 1).

 

Table 1. Pocket gopher burrow volume in 10 cm depth intervals.

Depth Interval
(cm)
Tunnel Volumea
(cubic cm)
Chamber Volumeb
(cubic cm)
Total Volume
(cubic cm)
Percent
Volume
0-10 56,982 0 56,982 5.3
10-20 523,195 156 523,351 48.8
20-30 219,636 966 220,602 20.6
30-40 50,248 6,580 56,828 5.3
40-50 71,486 11,730 83,216 7.8
50-60 68,378 8,690 77,068 7.2
60-70 30,045 7,560 37,605 3.5
70-80 6,732 870 7,602 0.7
80-90 4,146 0 4,146 0.4
90-100 4,146 0 4,146 0.4

a Data from Miller 1957: Figure 4, Table 1. b Data from Miller 1957: Table 1.

 

Sediment loosened during burrowing is either periodically expelled from the burrow or transported elsewhere within the burrow to plug old tunnels or chambers. Pocket gophers move most of the loosened sediment to the surface through short, vertical passages opened every few inches along the main tunnel (Grinnell 1923). The rate of sediment movement to the the surface by gophers varies greatly depending primarily on the population density at a given location. Table 2 presents estiamtes for the rates of sediment movement by different species of pocket gopher in various contexts and population densities.

 


Table 2. Estimates of the rate of sediment movement to the surface by pocket gophers.

Taxon LocationRate
(mtons/h/yr)
Population
(gophers/h)
Source
Thomoys umbrinusDavis, CA
  alfalfa field
95
126
89
124

Miller 1957
Miller 1957

T. monticolaHuntington L., CA
  mountain meadow
2025Ingles 1952
T. monticolaYosemite Natl. Pk.0.003?Grinnell 1923
T. talpoidesLogan, UT
  garden
10474Richens 1966
T. talpoidesWasatch Plateau, UT1410-40Ellison 1946
Geomys brevicepsCollege Station, TX
  ungrazed grassland
1?Buechner 1942
G. brevicepsCollege Station, TX
  ungrazed grassland
19?Buechner 1942

 

Pocket gopher burrows grow by the addition of basic building units consisting of a main burrow segment and its associated lateral branch (Reichman et al. 1982). The individual burrows and burrow communities are highly structured systems that vary in size and spacing according to the size of the individual rodents, population size, soil type, and resource productivity (Davis 1938; Davis et al. 1938; Hansen and Remmenga 1961; Reichman et al. 1982). In a study of two pocket gopher communities in northern Arizona, Reichman et al. (1982) found that interburrow spacing is quite regular and consistent between the two locations, and that burrow length is inversely related to plant productivity. However, other studies show that burrow size and the density of burrows vary with soil conditions and plant resource productivity (Hansen and Remmenga 1961; Howard and Childs 1959). Hansen and Remmenga (1961) also demonstrate that burrow size and spacing vary inversely with burrow density and that burrow density increases with a increase in habitat quality.

 

Effects of Pocket Gopher Burrowing on Archaeological Deposits

 

The often rich, friable earth in archaeological middens presents optimal habitat for burrowing rodents. In California, such deposits often support large populations of pocket gophers. Burowing by pocket gophers can affect archaeological deposits in four ways: (1) displacement or movement of sediment; (2) selective destruction of fragile artifacts such as bone, shell, and charred plant pieces; (3) disruption or obliteration of sedimentary structures; and (4) organic enrichment of the subsurface. The extent to which these processes affect a given archaeological deposit is determined by several interrelated factors including geomorphic context, rodent population size/burrow density, rate of burrowing by rodents, and particular cultural and noncultural depositional histories.

 

Movement of Sediment

 

In the course of excavating their burrows, pocket gophers push loosened sediment into small mounds on the surface or use it to fill other portions of the burrow system. This process results in both vertical and horizontal movement of sediment. Horizontal movement occurs as sediment excavated from one spot is pushed horizontally some distance before it is taken to the surface or left in an old chamber or tunnel. Although horizontal displacement of artifacts may be minimized by the great regularity with which pocket gophers expel sediment onto the surface, it certainly occurs to some unknown degree (Bocek 1986). Very little information is available concerning the patterns and distances of horizontal transport of sediment by pocket gophers, making this process difficult to model systematically.

 

In contrast, vertical movement resulting from pocket gopher burrowing is quite well documented, and has been the focus of all archaeological studies of pocket gopher disturbance. Three types of vertical movement are evident. First, gophers transport particles smaller than the diameter of their tunnels during the course of excavating and filling burrow systems (Hansen and Morris 1968). Most of this excavated material is brought directly to the surface. Second, particles move down through open tunnels or through the collapse of tunnels under the force of gravity and/or moving water. The particles involved in these first two types of vertical movement rarely exceed 6 cm in diameter, and are generally less than 4 cm in diameter (Johnson 1989). Third, particles larger than the diameter of the tunnels move down in the deposit. As gophers remove sediment from around larger objects, voids are ceated that eventually collapse under the weight of overlying sediment. The material that moves down during burrow collapse includes particles of all sizes. The larger particles, which the gophers cannot move upward, continue to sink until they reach the bottom of the burrow.

 

The effects of the vertical movement of sediment by pocket gophers at particular locations depend on several factors. Johnson (1989) idenitifies the depth of burrowing, rate of burrowing, rate of sediemnt aggradation, and rate of sediment erosion as important in recognizing the characteristics and amount of time involved in the production of stone zones by pocket gophers. To these can be added the particular history of artifact deposition (continuous vs. discontinuous, short duration vs. long duration), and the size and composition of the artifacts deposited.

 

Destruction of Fragile Remains

 

Pocket gophers use their powerful jaws and sharp claws to loosen sediment while burrowing. As a result, fragile artifacts such as bone, shell, and charred plant remains can easily be broken. When pocket gophers push the loosened material onto the surface, the fragile remains are subjected to increased weathering by sunlight, extreme temperature and moisture changes, and trampling by large mammals. The combination of these processes can lead to the rapid destruction of such objects, and can also obscure, and perhaps even create wear on the edges of stone artifacts (Behrensmeyer 1978; Driscoll 1967, 1970; Gifford 1980; Gifford-Gonzales et al. 1985; Hare 1980; Lyman and Fox 1989; Miller 1975; Shipman 1981).

 

In specific deposits, the tendency for burrowing to destroy artifacts varies with the kinds of artifacts deposited and the rate of deposition. In a deposit containing a variety of floral and faunal remains, weathering associated with burrowing can alter the relative frequencies and vertical distributions of taxa and specific elements due to their differential susceptibility to weathering (Driscoll 1970; Lyman 1984). For example, changes in proportions of remains of various shellfish species with depth noted by Bocek (1986) may result from the differential weathering of more fragile mussel versus the more robust horn snail and oyster shells rather than from differential movement as Bocek suggests. In contrast, fragile remains deposited in stable surfaces may be initially protected from intense weathering at the surface by burial with sediment brought to the surface by pocket gophers. Continued burrowing may eventually lead to the destruction of virtually all fragile remains within the zone of burrowing. The amount of time necessary for this destruction is currently unknown because it depends on several factors, such as weathering rates, that are poorly known.

 

Disruption of Sedimentary Structures

 

Through excavation of burrows and transportation of sediment, pocket gophers disrupt or obliterate the structure and boundaries of strata and features within archaeological deposits. Strata boundaries and structures contained within at least the upper 70 cm of a deposit are particularly susceptible to the burrowing activities of pocket gophers. The degree to which burrowing disrupts strata in particular deposits depends on several factors including the rate and depth of burrowing, the rate of deposition or erosion at the site, and the extent to which burrowing involves the reuse of existing tunnels rather than the construction of new ones.

 

Organic Enrichment of Subsurface

 

Pocket gophers carry organic matter from the surface into their burrows where it is deposited in storage chambers, nests, and abandoned tunnels. In addition, feces are often concentrated in defecation chambers  dispersed along the burrow system. Both of these processes lead to the organic enrichment of subsurface deposits. As this material decomposes, the nutrient level of these subsurface zones increases. In addition to the purposeful transport of organic matrerial below the surface, sediments from an organically enriched A horizon or cultural midden can filter downward into lower portions of the burrow through the collapse and filling of tunnels and chambers. Whether this enrichment process produces measureable changes in subsurface zones depends on the original organic content and chemical composition of the zone of accumulation.

 

SIMULATION OF POCKET GOPHER DISTURBANCE

 

The qualitative model presented above describes the processes involved when pocket gophers burrow in archaeological deposits. However, from this model alone, it is difficult to appreciate the nature of specific effects over various time intervals. By converting portions of the model into a dynamic, quantitative computer simulation, the effects of burrowing over time intervals can be evaluated. Computer simulations have recently been used to assess the time-dependent effects of other post-depositional processes with considerable success (Wandsnider 1989; Yorston et al. 1990).

 

Translation of the model into a simulation program invovles two steps: (1) the processes to be simulated must be made explicit and quantitative; and (2) specific values for the quantitative portions must be provided. The first step requires that all ambiguities in the model be resolved while the second step furnishes the values to actually run the simulation. At both steps, distortions can be introduced that affect the validity of the simulation or how well the simulation fits the real world (Stanislaw 1986). These two steps are discussed in detail below so that assumptions and potential distortions can be evaluated.

 

Step 1: The Simulation Algorithm

 

There is currently sufficient information to simulate only two aspects of the burrowing effects model -- the vertical movement of sediment and the disruption of sedimentary structures. These two processes are controlled by the rate and depth of burrowing and by the rate of deposition and erosion. In addition, the vertical movement of artifacts is controlled by the history of artifact deposition and the sizes of artifacts invovled. To translate the model into a simulation program, I make the following assumptions:

 

  1. The rodent population and amount of burrowing is constant through time.
  2. The volume of the deposit is constant through time.
  3. No artifacts or other sediment are added to the deposit after the initiation of the burrowing simulation (no deposition).
  4. No artifacts or other sediment are subtracted from the deposit after the initiation of the burrowing simulation (no erosion).
  5. All sediment less than 6 cm in diameter excavated during burrowing are moved to the surface of the deposit.
  6. Downward movement of sediment of all sizes occurs through the collapse of burrows resulting in their filling with sediment from directly above the collapsed tunnel.
  7. Any movement of sediment through burrowing or the collapse of tunnels results in the disruption of any stratification or structure that may have been present in that portion of the desposit.
  8. The amount of sediment moved to the surface from a given depth interval is proportionate to the volume of the burrow system in that depth interval and to the total volume of sediment moved to the surface.

The first assumption may not be correct in all situations, but is simple and therefore its impact on the simulation is easily analyzed. The third and fourth assumptions of no deposition or erosion were chosen arbitrarily to simplify the simulation, but limit the applicability of the simulation results to deposits on geologically stable surfaces and in which artifact deposition occurred over short duration relative to the duration of burrowing. The fifth and sixth assumptions specify how pocket gopher burrowing moves sediment. The 6 cm size cutoff for upward movement is based on studies by Johnson (1989) and information on the average size of pocket gopher burrows through which the sediment is carried. Virtually all of the excavated sediment less than 6 cm is either expelled onto the surface or used to fill the lateral grazing tunnels located within the upper portion of the deposit. However, pocket gophers also use a small proportion of the loosened sediment to fill or plug deeper portions of the burrow system. The effect of this violation of assumption 5 on the validity of the simulation cannot be specified exactly, but is probably minimal. Assumption 6 is also a distortion of the model because some particles fall or are carried down by water and fill deeper portions of the burrow. The degree to which downward movement occurs through collapse versus filling probably varies considerably among different kinds of deposits. I chose to limit downward movement to collapse as the most conservative approach. However, because filling by transport through open burrows includes only material smaller than the diameter of the burrows, we can expect that the downward movement of these particles will, in some cases, be more rapid and exhibit a different pattern than portrayed in this simulaiton. The eighth assumption drives the simulation by allowing the calculation of burrowing rates for different depth intervals using the equation:

 

     (1)     PVM = PM * PBV 

 

where

PVM = proportion of volume of sediment moved to surface per year per depth interval;

PM = a constant derived by dividing the volume of sediment moved to the surface per year by the total volume of deposit containing the burrow system; and

PBV = proportion of total burrow volume at given depth interval.

 

The simulation program monitors the movement of particles greater and less than 6 cm in diameter in 10 cm thick depth intervals from the surface to 1 m below the surface. The simulation runs in 1-year increments. To begin the simulation, the operator enters the initial vertical distributions of the large  (>6cm) and small (<6cm) particles, the rates of sediment movement per level per year, and the number of years the simulation is to run.

 

The first  procedure in the simulation involves moving small particles from each of the levels in proportion to the burrow volume in each level, and adding this material to the upper most level. This is done by multiplying the volume of particles less than 6 cm in diameter in each level by the rate of sediment movement by pocket gophers (PVM) calculated with eq. (1) given above. The product of this operation in each level is subtracted from that level, and the sum of the subtracted material from all levels is added to the volume of the small particles already present in the uppermost 10 cm level (Level 1).

 

The next step in the simulation moves both small and large particles down to fill the voids left by the movement of small particles to the surface. Beginning with level 9 (80-90 cm below the surface), the amount of small and large particles to be moved down to Level 10 (90-100 cm) is calcualted by multiplying the relative volumes of each size class in Level 9 by the amount of burrow void or tunnels in Level 10. The product of each size class is subtracted from Level 9 and added to Level 10 so that the void is filled in Level 10 with small and large particles in the proportions with which they occur in Level 9, then Levels 7 and 8, and so on until all of the voids created by moving material to Level 1 are filled.

 

The resutls of the first iteration of the simulation are used as the starting point for the next, and this form of iteration continues until the number of years selected at the outset have been simulated. Although both time and the processes of burrow excavation and collapse are continuous, the 1-year time increment used in this program is small enough to be functionally continuous relative to the time depth and intervals treated by most archaeologists. If a smaller time increment is necessary, data are available to increment the simulation at intervals of at least 1 day. However, daily and seasonal variations in the amount of burrowing by pocket gophers (Bandoli 1981; Miller 1948) make the annual increment optimal.

 

The program monitors the disruption of strata by keeping track of the percentage of the volume of material in each level that has moved either up or down. With each interation of the program, the material moved for the first time is transferred to a disturbed category. The simulation is designed so that the probability that burrows will be excavated in undisturbed deposit is proportionately equivalent to the volume of undisturbed deposit in a given level.

 

Step 2: Rates and Starting Values

 

The only values needed to run the program are the relative volume of burrows in each depth interval, the rate of sediment movement to the surface, and the initial values for the vertical distribution of large and small particles. Values for the rate of deposition and erosion are unnecessary as these are held constant (at zero) in the simulation. I derived the information needed to calculate the depth and rate of burrowing from Miller (1957), who investigted 17 Thomomys umbrinus burrows in an irrigated alfalfa field in Davis, California. While excavating nine burrow systems, Miller recorded: (1) the linear footage of tunnels found within 5.1 cm (2 in.) depth intervals extending down 101.6 cm (40 in.); (2) the diameter of burrow tunnels; (3) the length of open and plugged tunnels; and (4) the depth, size and contents of food caches and nests. I selected these data in part because they provide sufficient quantitative information to calculate the necessary values. Miller's data also have two additional advantages. First, the lack of barriers to burrowing in the deep alluvium of the site he studied limits the constraints on burrow construction to behavioral patterns of the pocket gophers alone. Although this may not reflect conditions in many archaeological deposits, it is the best starting point for the simulation. Second, the vegetation in the alfalfa field is roughly similar to lush vegetation growing on many archaeological midden deposits. This similarity in habitat may maximize the similarity of values for pocket gopher population size and burrow density.

 

To calculate the volume of burrows in different 10 cm depth intervals (See Table 1), I multiplied the length of tunnel measured for each interval by the average cross-sectional area of the tunnels (34sq. cm), and added the volume of chambers (nests and food caches). Because Miller (1957: Table 1) provided only a single depth measurement for the chambers, a set of calculations were required to determine the amount of volume per 10 cm depth interval. For each chamber, I divided the volume by the height to determine tha amount of volume per centimeter of depth and, using the single depth given as the center of the chamber, allocated the appropriate volume to each 10 cm depth interval.

 

To calcualte the rate of sediment movement using eq. (1), values are needed for the volume of sediment moved to the surface per unit of time and the total volume of the matrix containing the burrow system. Miller (1957: Table 2) presents the volume of sediment in fresh piles at the mouths of five burrows over a period of 27 days following irrigation of the field. An average of 1,543 cubic cm of sediment was transported to the surface per burrow per day or 0.563 cubic m per burrow per year. The average volume of deposit containing each burrow is approximately 67 cubic m (average burrow area multiplied by 1 m of depth). With these values, the constant PM in eq. (1) is calculated at 0.008. Thus, over the course of a year, slightly less than 1% of the total volume containing an average burrow is excavated and carried to the surface by pocket gophers. This constant (PM) is multiplied by the porportion of burrow volume in each depth interval to solve eq. (1) and calculate the rate of sediment movement to the surface per year for each depth interval (Table 3).

 

Table 3. Calculation of proportion of sediment moved to the surface per depth interval per year by pocket gophers (Thomomys umbrinus).

Depth Interval
(cm) 
Proportion of Burrow
Volume (PBV) 
Proportion
Moved (PM) 
Proportion Volume
Moved (PVM) 
 0-100.0532 0.008
0.00043 
 10-20 0.4884 0.008 0.00391
 20-30 0.2059 0.008 0.00165
 30-40 0.0530 0.008 0.00042
 40-50 0.0777 0.008 0.00062
 50-60 0.0719 0.008 0.00058
 60-70 0.0351 0.008 0.00028
 70-80 0.0071 0.008 0.00006
 80-90 0.0039 0.008 0.00003
 90-100 0.0039 0.008 0.00003

 

The final set of values needed to run the simulation is the inital distribution of large and small particles within the deposit. Because this version of the simulation assumes a geoligically stable surface, artifacts must have been left on the surface with the exception of those deposited in pits and other excavated features. Thus, for the simulation results reported in this paper, all large and small artifacts begin the simulation within the uppermost level (Level 1, 0-10 cm). In addition, no artifacts are added or lost from the deposit after the beginning of the simulation.

 

Figure 2
Figure 2. Simulated vetical distribution of small artifacts (6 cm) over various time intervals.

 

Simulation Results

 

Figures 2 and 3 show the vertical distributions of small (<6 cm) and large (>=6 cm) artifacts produced by the simulation program as it was run over increasing time intervals. Both small and large artifacts move downward initially as sediment and artifacts from the surface fill burrows excavated in deeper portions of the deposit previously devoid of artifacts. The large artifacts continue to sink at an ever slower rate as they accumulate in the lower reaches of the burrow system. After 4000 to 5000 years, the simulated pocket gopher burrowing has sifted all of the large artifacts from the upper 30 cm of the deposit, concentrating them a lower depths. Because the sediment moved to the surface by pocket gophers can contain small artifacts, the artifacts less than 6 cm enter into a cycle of upward and downward movement. Thus, small artifacts tend to become evenly dispersed through the upper 50 to 60 cm of the deposit rather than concentrating at lower depths as do the large artifacts. After 3000 years, the upward and downward movement of small artifacts equalizes and it becomes difficult to distinguish the vertical distributions of small artifacts over subsequent time intevals.

 

Figure 3
Figure 3. Simulated vertical distribution of large artifacts (>6 cm) over various time intervals.
 

 

Figure 4 depicts the rate curve for the simulated disruption or mixing of sediment of all sizes in an average cubic meter over 10,000 years. Burrowing disrupts the deposit rapidly at first, affecting almost 60% fo the deposit within the first 1000 years. As the proportion of the deposit affected by burrowing increases, the rate at which new burrows encounter and mix undisturbed material decreases in an almost lagarithmic fashion. Although slightly less than 20% of the deposit remains intact after 10,000 years of simulated burrowing, all of this undisturbed material occurs below a depth of 60 cm. Figure 5 shows the differential effect of disturbance with depth over increasing time intervals. The upper 30 cm of the deposit, the location of most burrowing activity, is almost completely mixed within the first 1000 years of the simulation. Below this zone, the rate of mixing decreases roughly as the burrow volume decreases, leaving the lowest level relatively intact.

 

Figure 4
Figure 4. Simulated rate of disturbance per cubic meter of deposit.

 

 

Figure 5
Figure 5. Simulated amount of disturbance with depth over various time intervals.
 

 

Testing the Simulation Validity

 

To what extent do these simulation results accurately reflect the actual processes of mixing and artifact movement in archaeological deposits exposed to pocket gopher burrowing? Answering this question requires testing the validity of the simulation. This commonly entails comparing the output of the simulation to values derived by exposure to the same conditions in the real world (Stanislaw 1986). To accomplish this, archaeological deposits possessing the following characteristics must be located: (1) large (>6 cm) and small (<6 cm) artifacts deposited on a surface at a known point in time; (2) this surface must have been geologically stable since artifact deposition; and (3) pocket gophers must have been present in the deposit since artifact deposition. Unfortunately, archaeological site reports rarely contain sufficient information on stratification, artifact size, or geomorphic setting to determine if the deposits meet the conditions listed above. However, two reports form southern California provide adequate information to serve as initial tests of the simulation short of reanalyzing collections or conducting new excavations.

 

Sweetwater Mesa

 

The Sweetwater Mesa site (Ca-LAn-267) lies on the top of a grass covered Upper Pleistocene marine terrace 76 m above sea level and 330 m north of the shore at Malibu, California (King 1967). The terrace is deeply dissected to the east and west of the site, and slopes gently up to the base of the Santa Monica Mountains nearly 1 km to the north. Approximately 16 cubic m of deposit were excavated from 11 pits measuring 1.5 X 1.5 m (5 X 5 ft) in 15.2 cm (6 in.) levels down to 61 cm (24 in.) in three field sessions between 1961 and 1963. The deposit contains relatively well-developed soil horizons, but lacks eveidence of depositional layers or any sign that significant nonartifactual deposition has occurred since the exposure of the marine terace. Between 30 and 45 cm below the surface, the excavation expsoed a stone line or "floor" composed primarily of large artifacts. The most prominent soil feature is a Btk horizon beginning about 33 cm below the surface, which shows Stage II to Stage III carbonate accumulation. Two radiocarbon dates, 6310 +/- 100 (UCLA-918F) and 6870 +/- 100 (UCLA-918G), were obtained on marine shells recovered from the deposit along with other artifacts. After calibration and correction (cf. Taylor 1987:126-131), these dates suggests an age of roughly 7000 B.P. for the occupation resulting in the deposition of artifacts. Given the Pleistocene age of the surface, it is likely that the artifacts were all initially deposited on the surface. King observed evidence of pocket gophers living in the site both in the form of krotovina and hills of loose sediment at the mouths of burrows.

 

King (1967:33-52) provides the range of length, width and thickness measurements on artifacts assigned to each type. Using these data, I divided the artifact types into large , small and mixed categories. The large category includes all types whose minimum length is greater the 6 cm. The small category includes all types with maximum lengths of 6 cm or less. The mixed category includes all the remaining types. The small artifact category at the Sweetwater Mesa site includes the following artfact types: knives (Type I), gravers and drills (all types), small cores, domed scrapers (all types), thin flake scrapers, other illustrated scrapers, concave endset scrapers (Types I, III, and IV), biface scrapers (Types I, IIa, and IIb), quartz crystals, bone artifacts, and shell ornaments (all types). The large artifact category includes: all groundstone types (manos, small handstones, metates, pestles, mortars, mortar depressions, pitted stoens, grooved stones, and cogged stones; excluding fragments), periphery cores, battered flakes, cobble biface, cobble multiface, large scrapers (Type Ia), nosed scrapers, concave endset scrapers (Type II), and biface scrapers (Type IIc). The frequencies of artifacts for the two size categories in each excavated level were obtained from King's (1967) Figure II.

 

Figure 6
Figure 6. Vertical distributions of large (>6 cm) and small (6 cm) and small at the Sweetwater Mesa site in comparison to simulated distributions.
 

 

Figure 6 shows the vertical distributions of artifacts in the large and small categories. For comparison, the graphs also include vertical distributions of large and small particles generated by 7000 years of simulated burrowing. The small artifacts at Sweetwater Mesa occur in simialr frequencies across the upper three levels and then their abundance decreases markedly in the lowest level. The relative frequency of large artifacts peak between 30 and 45 cm, coinciding with the stone line observed by the excavators. The form of these distributions compares well with the simulated distributions, although the positions of peaks in relative frequency values clarely differ for the large artifacts. For both the large and small artifacts, the distributions appear compressed vertically in comparison to the simulated distributions. This could result from excavation of shallower burrows by the pocket gophers at Sweetwater Mesa than the gophers that formed the basis of the simluation. The presence of the moderately cemented Btk horizon may have inhibited deeper burrowing at Sweetwater Mesa. The accumulation of large artifacts at the top of the cemented zone supports this conclusion. Alternative explanations for differences between the actual and simulated distributions, such as inaccurate burrowing rates or incorrect dating of the occupaton, do not account for the marked accumulation of large artifacts in a stone line after their initial deposition on the surface.

 

Sayles Site (Locas A)

 

The Sayles Site (Ca-Sbr-421) is located in Crowder Canyon near Cajon Pass, California, between the San Gabriel and San Bernardino Mountians at an elevation of about 980 m. The site is divided into four Loci (A, B, C, and D) spread out on a dissected alluvial terrace and hill slope on the west side of Crowder Creek (Basgall and True 1985). Locas A, the site of excavations in 1965 and 1966 (Kowta 1969), lies on a level terrace surface approximately 20 m above the normally dry bed of Crowder Creek. Four areas were excavated within Locas A. In one of these areas, also designated Locus A, 43 cubic m were excavated from a total of 11 pits (1.8 X 1.8 X 1.2 m). Kowta (1969:9, Figure 1 and Plate 1) describes three soil horizons encountered in the excavation: a friable to loose, light yellowish-brown A horizon with a clear lower boundary at approximately 30 cm; a hard, dark B horizon with an irregular lower boundary produced by krotovina at approximately 76 cm; and a light colored, loamy coarse sand with occasional quartzite cobbles (possible C horizon) extending into the floor of the excavation. Slope wash may contribute some sediment to the site, but the degree of soil formation and lack of visible sedimentary layers suggest that the surface has generally been stable. Artifacts occur throughout the deposit, but are most frequent in the B horizon. Hydration rind measurements on eight obsidian artifacts from Locus A produced seven readings between 4.1 and 4.6 microns and one reading of 8.5 microns (Jackson 1985). Based on a locale specific hydration rate, Basgall and True (1985:9.3) suggests an age range for the occupation at Locas A from 1800 to 2400 B.P. with a possible earlier occupation aroung 3000 B.P.

 

In his report on the Sayles Site, Kowta (1969:11-31) provides the minimum and maximum values for the length, width, and thickness of artifacts included in each artifact type. Using the same criteria as with the Sweetwater Mesa Site, I divided the artifacts recovered from Kowta's Locas A into large (>6 cm), small(<=6 cm), and mixed size categories. Small artifacts at the Sayles Site include: pendants, paint stones, projectile points, small steep-edged scrapers, prismatic flakes, quartz crystals, and small flaked objects. Large artifats include: milling stones, striaght-edge scrapers, cleaver scrapers, bifacial chappers, and flaked hammerstones. Manos are not included in this study beacuse of the presence of fragments. Frequencies of large and small artifacts in each excavated level were compiled from Kowta's (1969) Table 4.

 

Figure 7

Figure 6. Vertical distributions of large (>6 cm) and small (6 cm) and small at the Sayles Site, Locas A in comparison to simulated distributions.

 

Figure 7 show the vertical distributions of large and small artifacts along with the distributions created by 2000 years of simulated burrowing. Comparing the two sets of distributions indicates that the Sayles Site contains a greater number of large and small artifacts below 60 cm than the simulated distributions. This disparity may be accounted for by artifacts deposited during an earlier occupation (3000 B.P.), or possibly sufficient accumulation of colluvial sediment on the site causing the zone of burrowing to rise thorugh time. In the upper portion of the deposit, the correspondence between the simulated and actual distributions is striking.

 

DISCUSSION AND CONCLUSIONS

 

The similarities between the simulated distributions and the vertical distributions at the Sweetwate Mesa and Sayles sites validate the rates of sediment movement and the notion of size sorting employed in the simualtion of pocket gopher burrowing. The disparities between the model and actual distributions appear to relate to situation specific differences in burrow depth, and geomorphic setting. The validity of the patterns of disruption of strata and depositional structures produced by the simulation cannot be assessed with the two examples used here because the lack of such layers and structures at the sites may result from other pedogenic processes. However, the rapidity of disturbance by pocket gopher burrowing is attested to by the mixed and discontinuous condition of strata at an aboriginal village site abandoned in the early 19th century and subsequently buried by alluvial deposits (Pierce 1982).

 

Given the success of the simulation at tracking the long-term effects fo pocket gopher burrowing, we can now assess the impacts these patterns of size sorting, strata disruption and destruction of fragile remains have had on the archaeological record and our understanding of California prehistory. Despite their differences in age, both the Sweetwater Mesa and Sayles sites have been assigned to the same culture historical unit, the Milling Stone Horizon. The concept of the Milling Stone Horizon was developed by Wallace (1954, 1955) to draw attention to a group of sites located along the southern California coast that produced similar artifact assemblages and were thought to date to the early or middle Holocene. Milling Stone Horizon sites are typically large, deep deposits containing great quantities of milling stones (basin metates and manos) and large core tools (hammerstones and scrapers), fewer projectile points, flake tools, and polished stone discoidals ("cogstones" and "charmstones"), and little or no shell, bone, and charred plant remains. The sites are often found in elevated locations such as hill tops and terraces that receive little or no natural deposition. The larger artifacts such as milling stones and core tools commonly occur in stone lines or zones like those at Sweetwater Mesa and the Sayles Site (e.g., Greenwood 1969; King et al. 1968; Pritchet and McIntyre 1979; Walker 1952). These shared characteristics have led to the view that the Milling Stone Horizon sites were occupied by large, sedentary, autonomous populations subsisting primarily on vegetal resources, particualrly small hard seeds, with a minor contribution from hunting, fishing, and shellfish collecting.

 

As with Sweetwater Mesa and the Sayles Site, several of the characteristics that have led archaeologists to group sites into the Milling Stone Horizon can be produced by pocket gohper burrowing rather than any close historical or genetic relationship. If true, this has implications for the veracity of culture historical constructs and interpretations of prehistoric adaptations currently employed in southern California. The 8000 year range of dates on Milling Stone Horizon sites (Basgall and True 1985:10.20) has already undermined this construct for culture history. In addition, recent work on early Holocene deposits protected from pocket gopher burrowing by deep burial document subsistence patterns that include significant shellfish collection and hunting (Erlandson 1988; Glassow 1988). The pattern of occurrence of Milling Stone Horizon sites on elevated landforms may be due to the presence of stable surfaces on the landforms, providing exposure to burrowing and increased visibility of early material.

 

The effects of pocket gopher burrowing are not limited to the Milling Stone Horizon sites. Many arcaheological deposits in California lack stratification (commonly referred to as undifferentiated midded deposits), and, consequently, archaeologists excavate and analyze these deposits using arbitrary depth intervals (normally 10 cm). If the lack of stratification is due to extensive burrowing, the assumption of superposition that equates depth with time is incorrect for these deposits. The "stratigraphic reversal" of radiocarbon dates noted by Erlanson and Rockwell (1987) may be only one example of a more general pattern of mixing that weakens chronological interpretations based on the vertical position of artifacts within deposits affected by burrowing. In addition to chronological problems, patterns identified in regional studies involving the comparison of subsistence assemblages and vertical distributions of artifacts from upland (stable) and lowland (depositional) environments could derive from differential exposure to burrowing rather than behavioral differences between people who occupied the two areas (e.g., Dreyer and Deal 1982). Studies based on surface assemblages could also be affected by pocket gopher burrowing. Although gopher burrowing may facilitate the discovery of buried deposits by bringing artifacts to the surface, it can also bury artifacts depostied on the surface and create surface assemblages depleted of large artifacts.

 

Unfortunately, the extent to which the problems discussed above color our understanding of California prehistory cannot now be thoroughly evaluated. Archaeologists have not adequately described the deposits in which they dig nor have they regularly produced the information needed to conduct reliable taphonomic studies from published reports. However, existing evidence warrants a cautious reappraisal of current views on the Milling Stone Horizon and other topics. At the same time, if would be inappropriate to use the information presented in this paper to regard heavily burrowed deposts as insignificant or useless for archaeological research. Such deposits may be used to address questions of site structure because horizontal movement, although not treated specifically here, appears to be minimal. However, extraction of chronological and subsistence information requires application and possible development of new methods together with a greater concern for the context and history of deposits. Clearly, the processes that formed the archaeological record as we find it today greatly influence our understanding of the past. If we are to decipher these formation processes, we must treat the archaeological record as a product of geological processes of transport, deposition, and post-depositional alteration.

 

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