Using Insights from the New England and
Scandinavian
landscapes to solve the “mystery” of the Mima Mounds (Thurston County,
Washington).
DRAFT, NOT FOR CITATION
Keywords: Mima Mounds, natural disturbance, microtopography,
landscape history.
Ever since Charles Wilkes led the United States Exploring
Expedition through “the level prairie” near what is today Olympia,
Washington, the Mima Mounds have provoked curiosity, speculation,
debate, and disagreement, all without giving rise to a broadly accepted
solution to the question of their origin (Wilkes, 1845).
Hypotheses have ranged from Wilkes’s improbable suggestion of
construction by natives of the region, to the more widely accepted
fossorial rodents (Dalquest and Scheffer, 1942), to the more recent
sorting by seismic vibration (Berg, 1989). Dozens of other
theories have been advanced over the years; a summary and discussion of
which can be found in Washburn (1988). The excessive passions the
mounds have aroused over the years, in the form of a torrent of letters
in response to any mention of the subject, drove one editor, F. L.
Campbell of the Scientific Monthly, to declare in 1948 the discussion
“concluded” and to request his readers stop
writing the journal and to instead correspond directly with the authors
involved
(F. L. Campbell, 1948).
This paper takes a different, less speculative
approach. It proposes that a solution may be found by examining a
well known but overlooked cause of mound formation and then extending
the potential geographic, temporal, and biotic contexts of mound
formation to include forested landscapes. It engages in what D.
R. Foster calls a process of “thoughtful consideration of nature’s
history” (2000). By doing so, a hitherto rejected causal process
emerges as a simple and elegant solution (Farnsworth, 1906, M. R.
Campbell, 1906). The Mima Mounds are now regarded, perhaps
improperly, as the “type mounds” for many different mounded
landscapes. This paper confines its discuss to the type mounds
themselves, and not others that may or may not have resulted from
similar processes.
About the only thing most researchers have been able
to agree on is the location and rough physical description of the
appearance of the Mima Mounds. Their physical characteristics,
dimensions, range, and density remain well described by Bretz
(1913). They are located in the southwest corner of Thurston
County, Washington, on the gravely outwash plain of the Vashon
Glacier. See detail of map, illustration 1. They originally
covered about 30,000 acres before being regraded and leveled for
farming, roads, and housing development. Their density ranges
from 17-30 per acre, for a what was once a total of perhaps 900,000
mounds. They are round or elliptical in plan, with a profile of a
roughly spherical section. Their height ranges from a barely
noticeable rise to 7 feet. See illustration 2, Bretz diagram. The
mounds are relatively uniform in shape and apparently random in
distribution. See illustrations 3 and 4, aerial
photographs. They resemble other mounds elsewhere in North
America and
around the world. The mounds are now mostly covered by an
assemblage
of [fire-tolerant] prairie grasses and forbs ?? and the prairie is
bordered
by Western hemlock - Douglas fir second-growth forest (source for
plants??).
The mounds are composed of unstratified soil; within many mounds there
is
a layer of darker soil with organic matter, including charcoal, on top
of
gravel bed, with dark intrusions extending downward; there are some
small
boulders mixed in. See illustrations 5 and 6, cross sections of
mounds.
The mounds are unlikely to be older than the retreat of the Vashon
glacier,
14,000 years before present, or younger than the earliest carbon 14
date
from soil on the mounds, 4500 years before present (Washburn, 1988;
other
source for Carbon 14 date??).
This paper proposes to get past the deadlock in
comprehending the mounds’ origin by proposing a radical cognitive
shift: abandoning the too-narrow definition of the surrounding
Biome as a “prairie,” an assumption no one has questioned since Wilkes
first employed the term in his Report (1845).
Although always described as prairie, this paper proposes that the
known
time scale, less than 170 years, is too short to generalize the full
range
of the biome’s potential. There no reason to believe that this or
any
other prairie plant community is static, namely that it has always been
a
prairie in the floristic sense since the Vashon glacier receded.
Research
into prairie landscapes elsewhere lead us to question the assumption of
prairie
biota as a constant since the beginning of the Holocene. However,
at
least since Omer Stewart’s work half a century ago, we have suspected
that
at least the extent of the Great Plains in North America is a result of
human-caused
fire regimes (Stewart, 1956; Pyne, 1982; Flannery, 2001).
Charcoal
in soil samples and recent invasions by trees give reason to believe
that
the Mima prairie are also human-caused. In fact, since fire
suppression began over 100 years ago, the invasion (colonization?) by
Douglas fir challenges the assumption of having always been a
prairie. Arboreal pollen further tests assumptions about the
unchanging nature of the prairie landscape. Yet, all but one of
the nearly 100 articles consulted for this paper take the unchanging
prairie biota as a given.
Local fossil arboreal pollen and layers of charcoal
from soil cores suggest that the Mima Prairie was likely to have been a
different community and that one reason for its current prairie is
periodic burning. See pollen profiles in illustrations 7 and
18. The sequence of pollen shows succession of boreal conifers
(Picea and Pinus), followed by Douglas fir (Pseudotsuga menziesii),
Western Hemlock (Tsuga plicata), western red cedars (Cupressaceae), and
opportunistic hardwoods such as Alders (Alnus), [common name??
(Betula), and poplars (Populus). At all times is a significant
quantity of non-arboreal pollen such as and prairie grasses
(Gramineae), bracken
(Pteridium), and camas (Camas) (Boyd and Leopold, 1999, 142), which
increase
after the Mount Mazama eruption of 5800 years BP (cite). From the
pollen
sequence it is easy to imagine huge, widely spaced trees with grasses,
bracken,
and camas growing between them.
Furthermore, recent conservation management
discussions center on what to do about the “encroachment” of trees onto
the prairie, now
that fire is suppressed, suggesting if nothing else, that the “prairie”
landscape
is still quite hospitable to “invasion” by forest species (Washburn,
1956;
Kruckeberg, 1991). Notice encroaching trees in illustration 9.
By removing the conceptual barrier of requiring the
mound-building processes to take place in a “prairie” biome, and
limiting
potential processes to those geologic events typical of prairies, or
species
endemic to prairie landscapes, we can widen the scope of research to
include
other processes, especially those well known to cause mound building,
even
those occur in a forested landscape. As it turns out, there is a
well
known and well documented processes by which mounds are built in a
forested
landscape: the upheaval of root balls when wind knocks down and
uproots
trees (Foster and Boose, 1992; Wessels, 1997; Boose, Chamberlain, &
Foster,
2001). The phenomenon of wind-thrown trees as a geologic agent is
especially
well studied in New England and Scandinavia (Troedsson, and Lyford,
1973). The sizes of mounds created in these regions are in rough
proportion to the size of the trees uprooted since the root ball is in
proportion to the rest of the tree and most recorded instances are in
secondary forests, none of the root balls or resulting mounds is
anywhere near the scale of the Mima mounds. As the roots decay
the size of the mound decreases and becomes more regular. Erosion
further reduces the size of mounds by washing away fine
particles. See illustration 10 and 11 of windthrown stumps and
resulting mounds in a New England forest.
Such observations of analogous land forms in
forested regions raise two questions: could there have been trees
large enough to cause Mima-scale mounds, up to 7 feet high and 40 feet
?? in diameter? Could there have been winds in the region strong
enough to topple and uproot such trees? The answers to both
questions lie within easy reach. As noted above, pollen samples
from soil cores reveal the Western Hemlock- Douglas Fir forest
community typical of western Washington. Specimens of trees with
diameters easily exceeding 3 m [set sizes to metric or English but not
both] exist not only in the temperate rainforest of the Olympic
Peninsula, but also in protected areas in Thurston County [check
location of State forest;
cite Jerry Franklin’s booklet].
Historical records of storms, especially the 29
January 1921 “Olympic Blowdown” x? miles west of the Mima Mounds,
[and the Lincoln
Day ?? blowdown event in the 1960s??] demonstrate winds of adequate
force
(and over comparable or greater area) have visited the region twice in
the
twentieth century and that even the largest old-growth specimens of
these
trees are vulnerable to being blown down.
Photographs of damage from the 1921 storm show
rootballs several times taller, but of lesser diameter than the Mima
mounds. This
is as one would expect if the woody components of the rootballs were
subject
decay and the soil to erosion. See illustration 12 of uprooted
stump
on Olympic peninsula. If the Mima mounds were the result of a
blowdown
of an old growth Hemlock-Fir forest at some time between the last
glaciation
and the present, one would expect to find the soil within the mounds to
be
disturbed to a greater extent than the surrounding soil and even after
several
thousand years, one would expect some at least some traces of the
biomass
that a forested landscape would have once contained. Geologic
analysis
of the mounds and surrounding soil have found both unstratified
structure
and organic material (Washburn, 1988). Furthermore, decayed roots
would
help explain the dark “rootlike intrusions” that have not only puzzled
researchers
since Bretz described them (Bretz, 1913); but which have also leant
credibility
to the fossorial rodent hypothesis. (Even Scheffer 1947 called
them
“mound roots” according to Pewe, 1948). See illustration 13 of
dark,
root-like intrusion.
Based on existing pollen data, this paper argues
that some time between 5000 and 8000 years before present, a severe
wind
event blew down part of an old growth forest, uprooting thousands of
trees
(Washburn, 1988; Kruckeberg, 1991; Boyd and Leopold, 1999). Since
then,
a combination of erosion and decay reduced the root balls to their
current
shape and size and that periodic fires, perhaps human-induced, have
kept
the forest species from re-colonizing the landscape. This
hypothesis
is not new. In fact, it was first suggested in a letter to
Science
in 1906 by Iowa physician P. J. Farnsworth. He had been following
an
ongoing discussion of similar mounds in Texas and wrote to offer a
hypothesis
based on his own observations of windblown trees in his family’s farm
in
Vermont (Farnsworth, 1906). Since then, only one other writer has
cited
him, only to dismiss his argument along with 11 others (Campbell,
1906).
Dr. Farnsworth, used the same tools of observation that D. R. Foster,
advocated
in 2000. Perhaps it is no accident that they both knew forests in
New
England, where landscapes have their longest recorded history in the
United
States; likewise, some of the difficulty in finding an acceptable cause
for
the Mima Mounds could stem from their being in the part of the country
with
the shortest recorded history.
How have so many capable scientists working for over
a century and a half failed to consider the uprooting of giant trees as
a
possible cause? Three factors emerge above all others: They
shared
the assumption that the landscape’s prairie flora remained static and
constant for the past 10,000 years, an assumption that limited
researchers to considering processes that could occur in a prairie, or
to peri-glacial events concurrent with the prairie’s formation.
Second, because the assumed context was a prairie, most investigators
came from fields within geology that dealt processes
that could form or have an impact upon prairie landscapes. The
assumption
of a prairie landscape furthermore limited the consideration of
biological
agents to those which associated with prairies, namely the pocket
gophers
that so dominated the discussion since Dalquest and Scheffer’s posited
hypothesis
(Dalquest and Scheffer, 1942). Having ruled out other biotic
communities,
no one but Farnsworth thought to consult specialists in forested
landscapes.
The third and final reason is contingency: no one familiar with the pit
and
mound microtopography of New England, Scandinavia, or other forested
regions
happened to make the connection to the much larger mounds of Thurston
County.
The third factor bears out Foster’s assertion that “although we are
geographically
and culturally separated and frequently do not draw adequately on each
other’s
literature, scientists in every can benefit greatly by sharing
and
learning from each other’s studies in historical-geography” (Foster,
2002).
Further Research:
Questions for future research include: could a high water table
or hard pan below the soil surface limited depth of roots, thereby
increasing vulnerability to blowdown? Analysis of plant material
using Henry and Swan’s methods (1974). Careful re-examination of
pollen record and new
samples. Statistical comparison of tree and mound distribution
patterns.
Refining environmental chronology.
ACKNOWLEDGMENTS
James R. Karr for advising me to begin with the known. Christina
Anderson, for sharing the initial “aha” moment with me after reading
Tom
Wessels’s book together. Britton Pettit, now State forester with
the
Washington State revenue department, for confirming that the density
patterns
fell within the range of what would be expected in an old-growth forest
in western Washington;
State Geologist Pat Pringle for listening attentively and encouraging
me
to put my thoughts to paper; Jerry Franklin for the stem maps of
old-growth
forest; Dave Knowblach for photographs, citations, and encouragement;
Dana
O’Day Senior for lending her geologic insights to my ongoing discussion
of
the subject; Kim Limond, the reference librarian at Clinton, Iowa, for
biographical
information about P. J. Farnsworth. Tom Wessels, Arthur
Kruckeberg, and the members of the Columbia History of Science Group
for patiently listening while reserving judgement. All errors, of
course, remain my own.
Copyright 2002-2003
Michael KucherUniversity of Washington
1900 Commerce St
Tacoma WA 98402
REFERENCES
Berg, Andrew (1989) Formation of mima mounds–a
seismic
hypothesis. Geology 18, 281-284.
Boose, E.R., Chamberlain, K.E. & Foster, D.R.
(2001) Landscape and regional impacts of hurricanes in New
England.
Ecological Monographs, 71, 27-48.
Boyd and Leopold (1999)
Bretz, J. Harlan (1913) Glaciation of the Puget
Sound
Region in Washington Geological Society Bulletin 8: 9.
[Campbell, F. L.] (1948) Remarks on the Mima Mounds
[ appended to Troy L. Pewe,’s article in] Scientific Monthly 66, 296.
Campbell, Marius R. (1906–check date??) Natural
mounds
Journal of Geology 14, 715.
Dalquest, W. W., and Victor B. Scheffer (1942) The
origin of the Mima mounds of western Washington, Journal of Geology 50,
81
Farnsworth, P. J. (1906) On the Origin of the Small
Mounds of the Lower Mississippi Valley and Texas Science 23:589
13
April 1906, 583-584.
Flannery, Tim (2001) The Eternal
Frontier: an Ecological History of North America and its Peoples.
Atlantic Monthly Press,
New York.
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bears: interjecting geographic history into ecological studies,
environmental
interpretation, and conservation planning. Journal of
Biogeography
27, 27-30.
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England
landscape. Journal of Biogeography 29, 1269-1275.
Foster D. R. and Boose, 1992.
Franklin, Jerry, et al. (1981) Ecological
characteristics of old-growth Douglas-fir forests USDA General
technical report PNW ; 118
Henry, J. D. and J. M. A. Swan (1974) Reconstructing
forest history from live and dead plant material– an approach to the
study
of forest succession in southwest New Hampshire. Ecology 55,
772-783.
Kruckeberg, Arthur (1991) A natural history
of Puget Sound Country. University of Washington Press, Seattle.
Limond, Kim. (25 Nov 2002) Reference librarian
at Clinton, IA personal communication
Pettit, Britton (2001) (personal communication)
Pringle, Patrick (2001), geologist with Washington
Department of Natural Resources, personal communication.
Pyne, Stephen (1982) Fire in America: a
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Appendix 1 Tentative Environmental
Chronology
14,000 BP Retreat of Vashon Glacier
13,000 BP Bretz-like flood (P. Pringle) on outwash plain of Mima Prairie
10,500 invasion of boreal conifers
9,900 first old-growth Douglas fir forest at maximum pollen
production
9,800 single greatest charcoal deposit suggests massive
fire. Douglas fir stand perhaps eliminated by a regime of regular
burning which shows up as charcoal in pollen cores.
9500-5000 bigger fires once or twice per century with burning
in between maintain Oak-savannah prairie.
6800 Mount Mazama ash later
5700 Osceola Mud Flow? (Pat Pringle)
5000 Douglas fir manage to re-invade prairie and
establish themselves. Result of cooling climate or change in
fire-regime?
4500 Trees are quite large, widely spaced with some
grassland in between, much as they appear today on edges of Mima
prairie.
4400 Blowdown: Typhoon/hurricane-force wind
fells thousands of trees.
4400 erosion of soil from root balls forms base of
mounds.
5,100 second largest charcoal deposit as stumps and tinder-dry
trees burn, scorching grass, causing more rapid erosion, reducing mound
heights. establishment of grass on disturbed landscape slows erosion
4400-1850 regular burning maintains grasses until
Europeans halt traditional fire regime
1850-present recolonization of “Prairie” by
trees Appendix 3