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ated by contraction caused by either freezing or desicca-
tion. these cracks filled with windborne soil particles,
and today their approximate form and depth are appar-
ent when the soil profile is exposed. Depending on the
cause of the cracks, the distinct patterns in the soil pro-
file are referred to as fossil ice wedges (see fig. 2.8) or sand
wedge relics, the latter having been formed in desicca-
tion cracks.
University of Wyoming soil scientists Lowell Spack-
man and Larry Munn dug trenches through mima
mounds in the Laramie Basin and determined that fos-
they concluded that the mounds could be attributed
to cryostatic pressure created from water entrapped
between a layer of permafrost or bedrock and a down-
ward-freezing frost layer from the surface. Pressure was
ultimately released through planes of weakness created
by large fossil permafrost sand wedges. they found no
evidence for the burrowing rodent hypothesis. Previous
research had suggested that the mima mounds in Wash-
ington were also formed in association with permafrost
and ice wedges.
26
Working in the same area, soil scientist Richard
Reider and his associates proposed a mound-forming
mechanism that involved low-pressure groundwater
finding that all of them had (1) shallow impermeable
bedrock, (2) a thin layer of alluvial gravel on top of the
bedrock, and (3) well-developed soil at the ground sur-
face. they also observed that the mound soil was lens-
shaped in cross section and that it had concentrations
of sodium in patterns suggesting a swirling ground-
water flow. Reider and his associates concluded that the
explanation probably lies in blending their explanation
with Spackman and Munn's cryostatic hypothesis, but
that no other explanation for mima-mound formation
seemed applicable to their study area. they also advised
that this hypothesis should not be applied to all mima-
like mounds.
For biologists, the most intriguing hypothesis in-
volves burrowing animals, usually pocket gophers. in
one of the earliest papers on the origin of mima mounds,
published in 1954, R. J. Arkley and H. c. Brown proposed
the following explanation:
the method by which the [pocket] gopher accumu-
lates soil into a mound is explained by his tendency
to place his nest in a well-drained spot where the
soil is deepest; thus generation after generation of
gophers may keep building nests near the crest of
any high spot in the land surface, or over a window
in a hardpan. When a gopher is tunneling, he moves
the soil beneath his body, and forces it backward to
a surface opening already established. . . . thus, over
a long period of time, the gopher, by digging out-
ward from his nest, tends to move the soil toward it.
. . . the mound rises very gradually over a consider-
able period of time and perhaps many generations of
gopher occupation.
28
others, primarily ecologist George cox and his asso-
ciates, concluded that burrowing animals are the uni-
“Fossorial rodents are the builders of Mima mounds,
the largest and most widespread landscape features pro-
duced by any mammal other than man.” cox reviewed
studies indicating that, in addition to pocket gophers,
mound building is caused by ground squirrels; badgers;
toads; fire ants; and, in Kenya, mole rats.
the rationale of cox, suggesting that burrowing created
all mounds worldwide, still needs to be reconciled with
the evidence obtained supporting other hypotheses in
the Laramie Basin. A new dimension to the debate was
added by geologist Andrew Berg, who concluded that
most mima mounds are the result of seismic vibrations
in earth's crust in areas with unconsolidated fine sedi-
ments on a relatively rigid, flat substratum. this led to
that the seismic hypothesis requires further work to
substantiate, but that it “points the way to the solution
of a geologic enigma that has been with us for more
than 100 years.” the most recent analysis, by Jennifer
Burnham and Donald Johnson in 2012, suggests there
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