Proceedings of the Indiana Academy of Science

2006·14.3 MB·English

by  Indiana Academy of Science

Proceedings
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Indiana of Science
VOLUME NUMBER
114 2
PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE
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Coverphoto: The photo shows a wolfspider (Araneae: Lycosidae) from Grand Sable Dunes on the
shore of Lake Superior, Pictured Rocks National Lakeshore, Alger County, Michigan. Spiders in
this family are more commonly observed on the ground surface rather than in vegetation. Without
examining the spider under the microscope, it is not possible to do more than guess about the spe-
cific identityofthisanimal.This illustratesonereasonwhytheacquisitionofdataonthisubiquitous
group ofanimals has been a slow process. See related paper on page 111. Photo by Gary Fewless
(University ofWisconsin-Green Bay).
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Publication date: 31 January 2006
Thispapermeetsthe requirementofANSI/NISOZ39.48-1992(PermanenceofPaper).
ISSN 0073-6767
2005. Proceedings of the Indiana Academy of Science 1 14(2^:83-104
PALAEOECOLOGICAL INTERPRETATION OF POLLEN,
MACROFOSSILS, POLYGONAL FISSURES, AND TAPHONOMY
OF THE SHAFER MASTODONT LOCALITY,
WARREN COUNTY, INDIANA
Anthony L. Swinehart: Department of Biology, Hillsdale College, Hillsdale,
Michigan 49242 USA
Ronald L. Richards: Indiana State Museum, 650 West Washington Street,
Indianapolis, Indiana 46204 USA
William Petty Rivers1: University of Tennessee, Center for Quaternary Studies,
Knoxville, Tennessee 37996 USA
Robert D. Hall and Allyson K. Anderson: Department of Geology, Indiana
—
University Purdue University at Indianapolis, Indianapolis, Indiana 46202 USA
ABSTRACT. Discovery of thejaw of an American mastodont (Mammut americanum) and an unusual
sedimentary profile in a cornfield in Warren County, Indiana, prompted a multidisciplinary study of the
palaeoenvironment of the site. Wood taken from near the base of the deposit was dated at 15.540 ybp.
m
Stratigraphic and textural analyses of the 2.3 sedimentary profile reveal a series of inundation and
desiccation events marked by polygonal fissures. Analysis of pollen from the sediment profile indicates
that a boreal flora predominated during much of the time period represented by the profile. Pollen cor-
relation indicates that the sedimentary record was truncated by unconformities around 10,000 ybp. Mac-
rofossil analysis indicates a local environment that began as a forest dominated by white spruce and
tamarack. Later inundation of the forest was indicated by the appearance of fish (Perca flavescens),
meadow voles (Microtuspennsylvanicus), and submergent aquatic macrophytes (Myriopyllum exalbescens.
Potamogeton pusillus, Ceratophyllum demersum, and Najasflexilis). The aquatic environment was inter-
ruptedby periods ofexposure anddesiccation as indicatedbythedisappearanceofidentifiablemacrofossils
and by the stratigraphy. The Shaferfossil assemblage is compared with otherlocalities, andthe taphonomy
and palaeoenvironment of the mastodont are discussed.
Keywords: Mastodon, polygonal fissures, paleoecology, macrofossils, pollen, Late-Pleistocene. Holo-
cene, Quaternary biota
On 5 July 1992, while digging a trench for mains of the mastodont and other associated
drainage tile in his Warren County cornfield, fossil material. Little additional mastodont or
Larry Shafer encountered teeth and jawbone other vertebrate material was recovered. How-
fragments of the American mastodont {Mam- ever, the complex stratigraphy encountered at
mut americanum). Upon discovery of the re- the site, including well-preserved fissure fill-
mains, digging was suspended; and Shafer ings and a lower stratum rich in plant mac-
contacted Purdue University archaeologists, rofossils, provided an opportunity to charac-
who referred him to the Indiana State Muse- terize the paleoenvironment of the Shafer
urn. A thorough excavation of the site (re- Mastodont. The discovery contributes to in-
ferred to as the Shafer Mastodont Locality)
laeoecological understanding of Late-glacial
was then conducted by staff and volunteers of and post.glacia] environments in Endiana. The
ttheembIenrdiiannaanSatattteemMprtustoeuremcofvreormad1di7t-i2o5nalSerpve-- oub,mj•'eenctt..i.tv.,hees"*foorcssti.ulh,eubip-ort.easeont,t-._t,pheapSCe1hrata,-erre*M.tao.st11xo)dd,,oonct-
'Formerly published as William H. Petty; Current Locality. 2) describe and discuss the taphon-
address: Department ofBiology; St. LawrenceUni- omy of the Shafer Mastodont. 3) determine
versity; Canton, New York 13617 the origin of polygonal fissure fillings at the
83
PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE
84
site, and 4) reconstruct the palaeoenvironment
of the area using pollen, macrofossils, verte-
brate remains, and stratigraphy.
STUDY AREA
The study area is in Section 7, T23N, R6W,
Chatterton Quadrangle, Warren County, Indi-
ana (40°27'21"N; 87°07'52"W) at an elevation
of 210 m (—690 ft. asl). It occurs within shal-
low, marginal wetlands associated with Otter-
bein Bog, a glacial kettle depression with a
maximum depth of approximately 13 m
(Richards 1938). The site is situated on a por-
tion of the Crawfordsville End Moraine, part
of the Cartersburg Till Member of the Trafal-
gar Formation, created approximately 16,000
ybp (years before present) during Wisconsin-
age glaciation (Wayne & Zumberge 1965;
Fullerton 1986). The peat in nearby Otterbein
Bog originated from grass and sedge remains,
and Richards (1938) reports that much of the
recent vegetation ofthe wetland is Phragmites
and Calamagrostis. The wetland surrounding Figure 1.—Map of Otterbein Bog watershed
Otterbein Bog is referred to on the United (Sections 5, 6, 7, 8, 17 & 18, T23N, R6W, Warren
States Geological Survey (USGS) topographic County, Indiana), showing the location ofthe study
map as "Cranberry Marsh," suggesting that area. Contour interval is 10 ft.(3 m) a.s.l.
the wetland may have harbored cranberry
(Vaccinium cf. V. macrocarpon), a plant usu-
ally restricted to peatlands with a boreal cli- encountered any additional bone. Gasoline-
mate or similar microclimate. The peatland is powered pumps removed water that seeped
best described as a fen or sedge-meadow be- into the excavation site, and also pumped
cause it is not dominated by Sphagnum moss- clean water from adjacent Holder Ditch to
es and is extremely mineral-rich (as indicated wash excavated sediments for the recovery of
by the predominant flora). The soils immedi- microfauna and plant macrofossils. A site da-
ately around the site are mainly poorly- tum and baseline were established for leveling
drained silty loams and silty clay loams ofthe and mapping. Profiles and floor "bench" ar-
Brenton, Drummer, and Williamstown-Rains- eas were created to facilitate mapping and
ville series (Barnes 1990). The Shafer Mas- photography of the complex stratigraphy.
todont Locality is situated on the southwest Bulk material from the trenches was variably
mm mm
edge of Cranberry Marsh that formed within washed through 1.2 (0.05 in) or 6.3
a kettle depression (Fig. 1). (0.25 in) mesh screens. When bulk screening
METHODS from both trenches failed to yield any mas-
— todont or other discernible biotic remains, the
Field procedures. Only a few fragments excavation strategy changed toward develop-
of mastodont bone at the Shafer Locality were ing stratigraphic profiles, down to till if pos-
found in situ; the remainder was disturbed by sible, to understand the geologic context of
excavating. Disturbed soils were removed by the mastodont jaw. In doing this, an organic
shovel and trowel down to undisturbed soil. silt, rich in conifer cones, was encountered.
Thereafter, two intersecting trenches were ex- The soils from this "cone zone" were exten-
mm
cavated by shovel and backhoe near the lo- sively sampled and washed through 1.2
cation of the original bone fragments in an mesh screen. Bulk soil samples were taken
attempt to locate additional mastodont re- from each of the distinct strata in the profile
mains. Neither the 9.5 m long east-west trench for textural analysis and recovery of macro-
m
nor the 10.2 long north-south cross trench fossils. Additionally, small plastic canisters
SWINEHART ET AL.—SHAFER MASTODONT LOCALITY 85
—
were driven into a freshly exposed profile at Macrofossil analysis. In addition to anal-
10 cm intervals from the surface ofthe profile ysis of macrofossils obtained from bulk
mm
to the underlying glacial diamicton. These screening in the field using 1.2 (0.05 in)
mm
were capped and taken to the laboratory for or 6.3 (0.25 in) mesh sieves, more careful
pollen analysis. analysis of the sediment was conducted in the
During the excavation of a drainage pit at laboratory to locate smaller or more delicate
the south end of the north-south trench, the material. Subsamples of 300 cm3 were care-
backhoe encountered a deeply-buried conifer fully broken by hand and inspected for leaf
log. This prompted eight exploratory pits and impressions. To dissociate the soil, samples
trenches several meters beyond the site perim- were then soaked in a 50 g per 1 solution of
eter. None produced any additional vertebrate sodium phosphate for three days and rinsed
material. A final widening and deepening of through a 0.4 mm sieve. Macrofossils were
all the trenches and pits around the site still identified and counted with the aid of a dis-
failed to yield bone of any—kind. secting microscope. Excess bulk material that
Stratigraphic analysis. Analysis of soil was not used in the quantitative subsampling
texture was accomplished using the Bouyou- was placed in a white enamel pan forrecovery
cos Procedure (Bouyoucos 1936). Percent or- of large or infrequent macrofossils. Voucher
ganic carbon in the soils was determined using specimens of macrofossils were deposited at
the loss-on-ignition—method (Storer 1984). INSM. —
Pollen analysis. Extraction of pollen and Radiocarbon dating. A single radiocar-
spores from the sediment samples was accom- bon date was obtained from a piece of spruce
plished using standard methods modified from wood located at a depth of 2.1 m ("cone
Faegri & Iverson (1975). A 1 cm3 sample zone"), approximately 1.8 m below the level
from each level was used in processing. Pa- from which the mastodont molars and jaw
lynomorph identification was based primarily fragment were found and approximately 20
on the key by MacAndrews et al. (1973), cm above the glacial till. The wood sample,
along with the aid ofthe pollen reference col- sent to Beta Analytic, Inc., was processed for
lection at the Center for Quaternary Studies at a standard radiocarbon age determination
the University of Tennessee, Knoxville. Tax- (Beta-62640). An attempt was made to date
onomy follows Gleason & Cronquist (1963). the mastodont, but mandible fragments failed
Identification ofblack spruce (Picea mariana) to produce recognizable collagen for dating
and white spruce (Picea glauca) pollen was (Beta Analytic, pers. commun.).
based on the morphometries developed by
& RESULTS & DISCUSSION
Birks Peglar (1980). Twenty spruce grains —
were measured for each level and assigned to Stratigraphy. The Shafer Mastodont Lo-
either black spruce, white spruce, or undiffer- cality, like most of the other lakes and bogs
entiated spruce, following Hansen & Engstron that have yielded mastodonts in Indiana, is a
(1985). These values were then used to assign shallow deposit of aquatic sediments that was
the remaining spruce pollen to one of those truncated early in the Holocene. In this case.
categories. For each level a minimum of 300 the profile represents a time period ofapprox-
terrestrial pollen grains was counted. Due to imately 5000 years, beginning just after de-
the existence of only a single radiocarbon glaciation nearly 16,000 ybp. Four major di-
date, it was not possible to calculate pollen visions of sediments were identified in Profile
influx rates for the profile. 4 and designated as A through D. oldest to
Interpretation of the pollen diagram along youngest (Fig. 2). Division A has also been
with assignment of chronology was augment- designated the "cone zone" (as well as Unit
ed by comparison with several studies, cited XI in Fig. 2). This unit consists of wood and
herein, which place the Shafer Mastodont Lo- plant remains in a matrix of silt loam. A piece
cality in a regional context. The pollen tally of wood from this zone yielded a radiocarbon
data are on file at the Center for Quaternary age of 15,540 ± 70 !4C ybp (Beta-62640).
Studies, University of Tennessee, Knoxville, which provides a minimum age for deglacia-
Tennessee 37996. Duplicate slides for each tion of the site and a maximum age for all
level are held by the Indiana State Museum other sediments above. Division B consists of
(INSM). interbedded silt and clav that is grav at the
PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE
86
m
only about 2.3 deep (Fig. 2), yet changes
in the paleoenvironment are indicated by both
the sediments themselves and the plant mac-
rofossils and pollen they contain. The area ap-
pears to have been in a relatively shallow
fringe zone, or shelf, around the margin of a
much deeper kettle at the south end of the
Shafer site. Back-hoeing well below the cone
zone exposed clay and logs. Based on the to-
pography of the area, it is likely that the Sha-
fer Site was on a shelf, and erosion ofthe hill-
slopes (including sand and allochthonous
organic matter such as wood, cones, and
bones) may have transferred debris to the
shelf. Waves and currents probably moved
some of the debris to the deeper kettle.
As the landscape became less of a direct
source of sediment for the kettle, sand per-
—
Figure 2. Diagram of Profile 4, showing sedi- centages in the sediment declined from more
ment divisions (capital letters) and units (Roman than 10% in the cone zone (Unit XI) to only
numerals). Dots represent the location of pollen a few percent in the younger sediments (ex-
samples taken from the profile at 10 cm intervals cept for Unit VI). The size ofthe organic mat-
(see Table 1 for sedimentological details). ter also declined, and the finer sizes charac-
teristic of the younger sediments were more
disseminated throughout the matrix rather
base (Unit X) then changes upward to blue- than being concentrated as in the cone zone.
green (Unit IX), dark brown (Unit VIII), and The textures of Sediment Division B are silt
brown (Unit VII) overlain by pure sand (Unit loam, silty clay loam, and silty clay, textures
VI). This sand also in-fills fissures (Figs. 3, 5) common among glacio-lacustrine sediments.
that may extend downward into Unit X. Be- Mean organic carbon content was relatively
m
sides a thickness of up to 1 of this sand as low at 15%.
fissure in-fillings, there is another 5-25 cm as Delivery of organic sediments ceased tem-
a blanket over younger units (Fig. 3). The unit porarily during deposition of the sand of Unit
both blankets the older deposits and fills the VI. The extremely high percentage of sand in
fissures within them. Sediments in Division C this unit (96%) and the fine to very fine sand
are similar to those below it and consist of size suggest eolian deposition during an epi-
brown silt (Unit V) that was buried, and fis- sode of aridity. A backhoe pit dug north of
sures that were in-filled with brown silt loam the original excavation revealed a meter of
(Unit IV) (Fig. 4). Unit IV of Division C is massive sand and may be part of a sand dune.
the source of the mastodont mandible. Divi- However, it is not known whether this eolian
sion D consists of a cap of brown loam (Unit sand is the same age as Unit VI.
III), gray loam (Unit II), and black humus Organic deposition then resumed during the
(Unit I). The oldest sediment reached in the formation of the deposits of Sediment Divi-
excavation was late Wisconsinan till of the sion C. These younger sediments have about
Wedron (?) Formation. This deposit of un- the same grain size, but have considerably
known thickness presumably underlines the more organic matter than those of Sediment
entire basin of Otterbein Bog. It was not de- Division B (Fig. 2, Table 1). Resumed depo-
scribed at the Shafer Locality. Two samples sition in a quiet-water lacustrine or wetland
were taken from below the cone zone, appar- environment seems likely. The youngest sed-
ently from the matrix of till (Fig. 2), for lab- iments of Sediment Division D are probably
oratory analyses (Table 1, Fig. 2). The texture the result oferosion in the drainage basin after
of these samples is silt loam. Loss-on-ignition European settlement. Sand percentages in-
averages 3.5%. crease, but organic content is very high with
The sediments of the Shafer Locality are a loss-on-ignition value of 50% in Unit III.
SWINEHART ET AL.—SHAFER MASTODONT LOCALITY *7
wan
v*
.
—
Figures 3-5. Photographs of fissures encountered in sediment divisions B and C. 3. Profile; 4. Cros
section of sediment division C; 5. Cross section of sediment division B.
—
Origin of the fissure in-fillings. Three origin of polygonal fissure-fillings used in the
main hypotheses for the origin of the fissure present study include Lachenbruch (1962).
in-filled polygons at the Shafer Locality were Bertouille (1974), Nissen (1985). Mears
considered (periglacial, desiccation, and sand- (1987), Nissen & Mears (1990). and Johnson
blown mechanisms). Any reasonable hypoth- (1990). The periglacial hypothesis requires
eses must include an explanation of two temperatures cold enough to maintain per-
events: 1) what opened the vertically oriented mafrost. As the temperature drops, the per-
fissures, and 2) how and when they were mafrost-laden soil will crack. Meltwater will
filled. Moreover, there is the possibility that enter the crack and freeze, increasing the vol-
the upper and lower sets of fissure in-fillings ume by 9%. Cracking may occur again and
were formed by different mechanisms. again, gradually building up an ice wedge.
Criteria from several sources by which the When the climate warms, the ice wedge melts.
periglacial, desiccation, and sand-blow mech- and sediment begins to till the fissure. Most
anisms may be compared, both to each other of the sediment is derived from the walls of
and to the observations and data from the po- the crack. Some small amount ofmaterial may
lygonal in-fillings at the Shafer Locality, are be washed or blown in. The most critical con-
presented in Fig. 6. dition for this hypothesis to be valid is tem-
Periglacial hypothesis: The primary peratures cold enough to first form permafrost.
sources of information about the periglacial then contract and crack it. The time of the
PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE
88
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SWINEHART ET AL.—SHAFER MASTODONT LOCALITY 89
Observationor Peri-glacial SeismicSand- Desiccation Shafer ShaferMastodont
Criterion hypothesis blowHypothesis andIn-filling Mastodont Locality, UpperSet
Hypothesis Locality,Lower
Set
1)In-fillsoccuras Yes,activeice Usuallydike-like, Crackshould Almostallin-fills Someinfillstaperwith
wedges,tapering wedgestaperwith with propagate arewedges depth;manydonot
withdepth depth mushroomingif downward,thus taperingwith
surfaceisreached. bewideratthe depth
Usuallywiden surface
withdepth
2)Map-view Yes,asshownby Notlikely,unless Yes,widely Well-defined Well-definedpolygons
polygons numerousactive injectionoccurred known polygonspresent present
areas alongolder mudcrackingis
fissures polygonal
3)Dimensionsof Diametersof3to Nopolygons. Verylittledata Diametersof Diametersofabout75cm.
polygons: a.Diamter 20m,butmostly Depthsofseveral available. about30cm, depthofabout90cm.
b.Depth <1 m. Depthsof meters. Thickness depthofaboutI thicknessatlandsurface
c.Thicknessat 1 to3m. of< 1 cmto>60 m,thicknessat about30cm
junctionwithland Thicknessatthe cm. landsurface
surface surfacemay about15cm.
averageabout50
cm
4)Timingofin-filling Ifburied,ice Sedimentinjected Withfissuring Formationof Climatetoowarmtoform
vs.Assuring wsleodwgley,spcearnhmaeplst swiitmhulftiasnseuoruisngly odrcyciunrgr,incgradcukrsing faitss1u2r,e0s0e0sNtiCmayted fpeorrimgleadciianlllya.stfMewa'ydehcaavdees
over2000yrs. unlessfissures maydevelop BP,basedon afterditching.
Sedimentswould formedpreviously veryquicklyand pollen
replaceiceasit byperiglacial mayremain assemblage.
melts activityor openformany
desiccation. centuries.
5)Temperature -6to-8deg.C Anytemperature. Any Probablytoo Definitelytoowarmto
conditions temperatureas warmtoform formperiglacially.
longasdrying periglaciallyafter
canoccur. ~14,000l4Cy
BP
6)Compositionofin- Similarto Sand-injected Almostalways Nearlypuresand. Organicsedimentsvery
filledsedimentsvs. sediments frombelow;host eoliansiltor, similartothoseadjacent
adjacentsediments surroundingthe usuallyfine- morefrequently,
wedges;delivered grained sand(few
byslumpingfrom impurities)
wallsoffissure.
7)Presenceof Canbedeposited Onlylocallyif Canbe Sandfillingthe Sedimentfillingfissures
blanketinglayer oncefissuresare surfaceisreached; depositedonce fissuresalso doesnotblanket,but
filled. mushrooming. fissuresare blanketsolder youngersedimentsdo.
filled sediments.
8)Structureofin- Layeringparallel Usuallymassive, Probablyrather Nonenoted. Nonenoted.
fillings tothewalland butmaybe massive,may
horizontal vertical,stress- locallybe
layeringboth producedbanding. laminated.
possible,butmay
alsobemassive.
9)Effectson Oftendeformed None. Crackingcan Nonenoted. Nonenoted.
adjacentmaterial asicetakesup cause
space. deformationin
hostmaterial.
—
Figure 6. Observations and criteria ofthree hypotheses to explain the polygonal fissure fillings at the
Shafer Mastodont Locality, Warren County, Indiana.
cracking is estimated at about 13,000 l4C ybp upper in-fills are tapered and some are not
based upon pollen correlation. Ice-wedge casts are usually polygonal: both
When the characteristics of periglacially- upper and lower fissure fillings at the Shafer
produced fissure in-fillings (i.e., ice-wedge Locality are polygonal. The dimensions ofthe
casts) are compared with the observations and wedges and polygons are within the range ex-
data at the Shafer Locality, there are both sim- pected for ice wedges.
ilarities and differences (Fig. 6). Periglacial Other differences between ice-wedges and
in-fills are wedge-shaped; at the Shafer Lo- the fissure in-fillings at the Shafer Locality in-
cality, the lower fissure fills are, too, but some clude a verv different sediment for lower in-
90 PROCEEDINGS OF THE INDIANA ACADEMY OF SCIENCE
fillings (pure sand) compared to the much fin- blows do match the conditions at the Shafer
er surrounding materials, the lack of layering Locality. These include dimensions and lack
in the in-fillings at the Shafer Locality, and of internal structure. Considering all the cri-
the lack of deformation in the adjacent mate- teria for seismic sand-blows in Fig. 6, this
rials. The periglacial origin for the upper set mechanism is not likely the source for either
of fissure fillings at the Shafer Locality can set of in-fillings at the Shafer Locality.
likely be dismissed because the late Pleisto- Desiccation and in-filling hypothesis: The
cene and Holocene climate was much too third hypothesis relies on drying and cracking,
warm to support permafrost. The possibility followed by in-filling with eolian sand at a
of a periglacial origin for the lower sequence later date. Primary sources of information
remains. Such a case is supported by the about desiccation phenomena used in the
wedge-shaped in-fills, their polygonal pat- study were Conybeare & Crook (1968), Cal-
terns, and range of dimensions for the in-fills, abresi & Burghignoli (1977), Haigh (1978),
but these characteristics are not unique to peri- and Selley (1988). This hypothesis is not con-
glacial mechanism. If the periglacial origin tradicted by any of the criteria in Fig. 6. Sim-
hypothesis is considered for the lower fissure ilarities between desiccation-caused in-fillings
fillings, then the permafrost would have to be and both sets of fissure fillings at the Shafer
present at a time after all the silt, sand, wood Locality include the wedge shape, although
fragments, bone fragments, cones and other this is more convincing in the case ofthe low-
organic debris were washed into the lake; then er set. Similarly, both sets form polygons. We
organic sediment accumulated under boreal have found very little data on the size of des-
forest conditions for about three millennia iccation polygons, but ordinary mud cracks
(i.e., by about 13,000 l4C ybp). By that time, are examples, and they seem to have dimen-
it seems quite unlikely that permafrost was sions that sometimes match those at the Shafer
around in Indiana. Mean annual temperatures Locality.
were probably warmer than —6° to —8° C. The older set of fissures probably formed
Seismic sand-blow hypothesis: Primary by 13,500 14C ybp (based on estimations de-
sources of information about sand-blow phe- rived from the pollen profile) as a result of
nomena used in the present study are Morris desiccation. In-filling with eolian sand (Unit
1983, Gohn et al. (1984), Obermeiere (1987), VI) must have followed quickly after Assuring
Selley (1988), Munson et al. (1993), and Tut- because no other sediments are present in the
tle & Barstow (1996). Sandy sediments can fissures. The upper set of fissures may have
be mobilized at the time of an earthquake formed by desiccation following ditching and
through a liquification process. The sand is lowering of the water tables in the early
squeezed upward as a dike. The pressure will 1900's. Such fissures were filled with re-
cause Assuring of older silty and clayey ma- worked organic sediment (Unit VI) before the
terial as the sand is forcefully injected toward site was blanketed with sediment resulting
the land surface. If it reaches the surface, the from cultivation ofthe surrounding landscape.
injected materials will spread laterally for a Fissures of the upper set are still forming as
short distance, forming a mushroom. witnessed by some fissures that were not filled
Although the seismic sand-blow hypothesis with sediment. The fact that the upper fissures
was considered as an origin for the fissure fill- do not extend deeper is probably controlled
ings at the Shafer Locality, it was noted im- by the level of the lowered water table.
mediately that seismic fissure in-fillings are All the observations and data at the Shafer
not wedge-shaped and do not occur as poly- Locality seem to be consistent with the con-
gons (Fig. 6). The seismic sand-blow origin clusion that both sets offissure fillings are the
can be considered only as part of a complex product of desiccation. Neither the periglacial
mechanism in which injection followed As- hypothesis nor the seismic sand-blow hypoth-
suring by one of the other mechanisms. Other esis is a viable alternative to the desiccation
aspects of seismic sand-blows that do not fit hypothesis.
the Shafer Locality are the tendency of dikes Both desiccation events (represented by
to widen with depth, the apparent lack of a Units V & VII) resulted in deep cracks (—80
sand source with depth, and lack of structure cm) in the sediment. This suggests a nearly
in the fissure infillings. Some criteria for sand complete loss of the wetlands water source.