Geomorphology 75 (2006) 408 – 419
www.elsevier.com/locate/geomorph
Degradation of unconsolidated Quaternary landforms in the western
North America
Jaakko Putkonen *, Michael O’Neal
Department of Earth and Space Sciences and Quaternary Research Center, MS 351310, University of Washington, Seattle, WA, USA
Received 15 October 2004; accepted 27 July 2005
Available online 3 November 2005
Abstract
One of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets
on the longevity of land surfaces. The latter has also turned out to be vital in modern applications of cosmogenic exposure dating
and interpretation of lichenometric data from unconsolidated landforms. Because the effects of landform degradation have not been
well documented, disagreements exist among researchers regarding the importance of degradation processes in the dating
techniques applied to exposures. Here, we show that all existing qualitative data and quantitative markers of landform degradation
collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of Quaternary
moraines or fault scarps. Degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply
sloping landforms. These conservative analyses are based entirely on field observations of decreasing slope angles of landforms
over the typical range of ages in western North America and widely accepted modeling of landscape degradation. We found that the
maximum depth of erosion on fault scarps and moraines is on average 34% of the initial height of the scarp and 25% of the final
height of the moraine. Although our observations are limited to fault scarps and moraines, the results apply to any sloping
unconsolidated landform in the western North America. These results invalidate the prevailing assumption of no or little surface
lowering on sloping unconsolidated landforms over the Quaternary Period and affirm that accurate interpretations of lichen ages
and cosmogenically dated boulder ages require keen understanding of the ever-present erosion. In our view, the most important
results are twofold: 1) to show with a large data set that degradation affects universally all sloping unconsolidated landforms, and 2)
to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that
significantly interfere with cosmogenic exposure and lichen dating.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Western North America; Erosion; Degradation; Soil; Moraine; Cosmogenic exposure dating; Lichenometry
1. Introduction
The degradation of unconsolidated landforms during
the Quaternary has been indirectly studied for decades.
Early studies in the eastern Sierra Nevada used moraine
slope angles as a relative age for correlating the extent of
* Corresponding author. Tel.: +1 206 543 0689; fax: +1 206 543
0489.
E-mail address: putkonen@u.washington.edu (J. Putkonen).
0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2005.07.024
past glacier advances in adjacent canyons (Blackwelder,
1931). The underlying assumption, based on field observations, was that as glacial moraines and fault scarps
mature, matrix materials from the crest or fresh slip face
are transported downhill and deposited on the flanks of
the landform. As this process continues over time, it
essentially smears the topography, produces smooth,
rounded landscapes (Fig. 1), and slope angles that decrease with increasing time. Subsequent to the arrival of
modern tools for dating, including radiocarbon and
J. Putkonen, M. O’Neal / Geomorphology 75 (2006) 408–419
409
Fig. 1. Moraine profiles (gray lines) of young (top panel, ~100 yrs, Mt. Adams, WA) and old moraines (bottom panel, ~ 120 kyrs, Bloody Canyon,
CA). These photos illustrate evolution of moraines from sharp crested, bouldery and narrow to broad and wide, devoid of boulders.
cosmogenic exposure techniques, interest in landform
shapes and other relative dating methods waned (Briner
et al., 2001; Gosse et al., 1995a; Licciardi et al., 2001;
Marsella et al., 2000; Phillips et al., 1996, 1997, 1990;
Porter and Swanson, 1998; Swanson and Porter, 2000).
Recently, the interest in understanding the degradation
of unconsolidated landforms has been renewed, however, in the context of explaining conflicting and counterintuitive results from cosmogenic exposure dating on
moraines and other Pleistocene age (10–2000 ka) landforms (Hallet and Putkonen, 1994; Putkonen and Swanson, 2003; Zreda et al., 1994) and interpreting the
accuracy of lichenometric dating of late Holocene (b4
ka) landforms (O’Neal, 2002).
When moraines are dated by assigning cosmogenic
exposure ages to exposed surface boulders at the moraine
crest, it is often assumed that no or negligible (~1 m)
matrix erosion has occurred (Gosse et al., 1995b; Owen
et al., 2002, 2001; Phillips et al., 1997, 1990; Shanahan
and Zreda, 2000; Zreda and Phillips, 1995; Zreda et al.,
1994). This assertion simplified the interpretation of boulder ages considerably and led to various simple
approaches for determining the ages of moraines from
the cosmogenic exposure dated boulders that span a wide
range of ages; some chose the oldest boulder age to
represent the age of the moraine, some calculate an average of the boulder ages. We found on our theoretical
analyses of boulder ages on an eroding moraine surface
that it is possible to completely miss a boulder that represents the formation age of the moraine and unintentionally
sample only younger boulders that have been exhumed
later by degradation (Putkonen and Swanson, 2003).
Although the Pleistocene moraines that are found in
western North America degrade continuously and sig-
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nificantly over a typical range of age of tens of
thousands of years, the degradation is most active in
the early stages right after the glacier recedes from the
fresh steep sided moraine. Understanding the resulting
pattern of degradation on late Holocene landforms is
crucial for applying many dating techniques. In a manner similar to the cosmogenic exposure dating of moraine boulders, lichenometrists use the diameter of
lichens on boulders as a proxy for exposure age. Because of the variety of methods in which lichenometric
data are collected and interpreted, arguments have ensued regarding the sample area, the number of boulders
or lichens to sample, and the portions of moraines to be
studied to obtain accurate lichenometric ages (e.g.,
Carrara and Andrews, 1973; Haines-Young, 1983;
Innes, 1984, 1986; Locke, 1983; Mahaney and Spence,
1985). Although researchers applying techniques of
lichenometric dating on late Little Ice Age (ca. A.D.
1650–1920) moraines have noted the effects of eroding
matrix materials on the distribution of boulders (e.g.,
Calkin and Ellis, 1980; Fuller, 1980), this insight has
not translated into any practical framework for assessing how boulder exhumation and redistribution affects
the ages of lichenometrically dated landforms.
In this paper, we show that landform degradation is
vigorous and considerable and it operates at time and
depth scales that significantly interfere with techniques
of exposure dating that are routinely used to determine
the ages of boulders on landforms comprised of unconsolidated materials (i.e., cosmogenic and lichenometric
dating). Because the surface exposure age of any portion of such landforms changes over time, the concept
of a stable form is arbitrary and our results suggest that
consideration of landform degradation can be omitted
only in rare cases. Because all landforms do not start
with, or proceed to evolve under, the same physical and
environmental conditions, degradation processes will
proceed at different rates and result in different age–
frequency distributions of boulders at any point in time.
No matter how accurate the technique of exposure
dating, each boulder that is dated must be considered
in the overall view of landform degradation.
Here, we analyze all existing and new data on
landform degradation/stability that consist of slope
angle observations, cosmogenic exposure ages of moraine boulders, lichen measurements, and degradation
modeling. We determine how widely the landforms in
western North America are affected by degradation
and define the absolute amount of degradation that
can be expected on an unconsolidated landform over
typical Quaternary time periods of tens of thousands
of years.
2. Methods and data
Data on slope angles are drawn from published
studies on wave cut scarps (Nash, 1980a), fault scarps
(Bucknam and Anderson, 1979; Hanks et al., 1984;
Nash, 1984; Wallace, 1977), cinder cones (Hooper
and Sheridan, 1998), and moraines (Birkeland and
Burke, 1988; Briner and Kaufman, 2000; Burke and
Birkeland, 1979; Colman and Pierce, 1986; Espizua,
1993; Kaufman and Calkin, 1988; Manley et al., 2001;
Miller, 1971; Shakesby, 1989). We briefly review and
expand our analyses of boulders dated by cosmogenic
exposure as it pertains to eroding moraines and present
new data on lichenometrically dated boulders on late
Holocene moraines. The majority of data are from
western North America; however, some supporting
data are drawn from observations outside this area.
2.1. Landform degradation modeling
A formal understanding and computer model of
landform degradation follows a widely accepted mathematical formulation (Carson and Kirkby, 1972; Hallet
and Putkonen, 1994; Nash, 1980b; Putkonen and Swanson, 2003). The formulation states that the mass transfer
along the ground surface is equal to the local slope
angle and a topographic diffusion coefficient. For detailed explanation of the degradation model, see Putkonen and Swanson (2003). The topographic diffusivity is
a key parameter in the model that integrates the substrate and climate and is generally poorly known. The
exact value is irrelevant for this study because we only
use the model to generate the complete cross-sections
of landforms whose slope angles are already known.
The form by itself is independent of the combination of
elapsed time and topographic diffusivity. The model
allows us to assign a reasonable crest height to a
given slope angle and then track the crest lowering as
the slope angle decreases over time.
2.2. Landform slope angles
The data set of slope angles is derived from an
exhaustive literature search for all measured slope
angles on fault scarps, wave cut scarps, moraines, and
cinder cones. We included published moraine slope
angles in this compilation only where more than one
slope measurement came from the same drainage basin
(Birkeland and Burke, 1988; Briner and Kaufman,
2000; Burke and Birkeland, 1979; Colman and Pierce,
1986; Espizua, 1993; Kaufman and Calkin, 1988;
Kiver, 1972; Manley et al., 2001; Miller, 1971; Sha-
J. Putkonen, M. O’Neal / Geomorphology 75 (2006) 408–419
kesby, 1989). Moraines in a given drainage basin are
consecutively older the farther away they are from the
cirque or the head of the valley. This way we were able
to generate plots of time-dependent moraine slope
angles without absolute dating control.
The fault scarp angles were collected from the literature and included only if data from more than one time
period existed (Bucknam and Anderson, 1979; Hanks
et al., 1984; Nash, 1984; Wallace, 1977). The fault
scarps were relatively dated by independent time markers (i.e., the relation of a given fault to a paleo-lake
strand line of known age). This established the relative
age scale.
Although we cannot calculate an exact lowering rate
for these landforms because of the lack of independent
age control, we maintain that they generally span an age
range that is typical of moraines and fault scarps in the
western North America and, therefore, give us valuable
information on absolute degradation that should be
expected when such landforms are studied. Based on
independent cosmogenic exposure dating in the same
general area, we know that the oldest moraines are
typically in the range of 100–200 ka, which gives us
an estimate for the survival of the oldest moraines that
still retain a recognizable form (among others, Phillips
et al., 1990).
The data on cinder cones (Hooper and Sheridan,
1998) and wave cut scarps (Nash, 1980a), although
rare, are included here only to illustrate the pervasiveness of the degradation processes on unconsolidated
landforms. In these cases, an absolute time scale exists.
The data on wave cut scarps comes from Michigan,
which is outside the general study area of western
North America.
2.3. Late Holocene landforms (b 4 ka)
Few quantitative studies exist regarding the degradation of late Holocene, unconsolidated landforms.
Bullard (2003a,b) and O’Neal et al. (2005) found that
models of landscape diffusion (e.g. Hallet and Putkonen, 1994; Putkonen and Swanson, 2003) were sufficient for quantifying degradation on the transportlimited slopes of human earthworks ranging in age
from 140 years to a few millennia. In an attempt to
expand our current understanding of the degradation of
late Holocene landforms, we analyzed the spatial distribution of lichens growing on two late Little Ice Age
moraines (O’Neal, 2005) in the Adams Glacier foreland, Mount Adams, Washington.
Because lichens grow in a radial pattern, the diameters can be equated with the exposure age of the
411
rocks on which they are growing. Thus, identifying the
largest lichen on each boulder and its location on the
moraine allows us to identify patterns of boulder ages
that can be related to slope degradation processes. To
our knowledge, this is the only existing data set that
represents the exact mapped locations of the largest
lichens measured on two moraines.
The late Little Ice Age moraines that we studied
have active, unconsolidated surfaces without any substantial vegetation competing with the lichens (Fig. 2).
The lichens were measured from two south-facing segments of the lateral moraines formed at ca. 150 and 50
years ago (O’Neal, 2005). Numeric ages for lichen
diameters were determined using the growth curve
presented in O’Neal and Schoenenberger (2003). The
largest lichen on each boulder N 0.3 m in diameter was
measured using digital calipers and the location of the
boulders within the 18-m-wide swath from the crest to
the toe of the slope was surveyed using a total station
with an accuracy of F 1 cm.
When interpreting lichenometric data, we assume
that as young and steep-sided moraines degrade,
boulders at the crest are gradually exposed and possibly become unseated, so that a net down-slope
transport of constantly emerging boulders occurs,
which are potential sites for lichen colonization. If
this rate of exhumation proceeds rapidly, an accumulation of boulders occurs at the base of the slope that
should bear early colonizing lichens. As the process
continues with time, the net deposition of matrix
materials will bury many of the boulders deposited
at the base. As a result, the average lichen population
at the crest will be younger than the initial colonization age or moraine age. As the degradation continues,
all of the boulders that have been deposited at the
base potentially will be buried by the matrix material
that is deposited on the flanks, eliminating the oldest
lichens.
2.4. Cosmogenic exposure ages of Pleistocene moraine
boulders
Because glacial moraines are used to establish accurate chronologies of terrestrial climate shifts, they are
the subject of numerous studies of cosmogenic exposure. Cosmogenic exposure dating requires sampling a
number of exposed surface boulders to determine the
age of the moraine. Typically, it is assumed that little or
no surface erosion occurred on the moraine surface. If
this assumption were correct, all moraine boulders from
one moraine crest would give an indistinguishable cosmogenic exposure age, the moraine formation age. It
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Fig. 2. Map showing location of the two late Little Ice Age moraines on Mt. Adams, WA.
has convincingly been shown that boulders that are
considerably younger than the moraine itself are frequently found on a given moraine crest (Putkonen and
Swanson, 2003). We make use of the same data set here
to see if the scatter of the ages of boulders increases
with the age of the moraine. If this holds true, it would
suggest continuous degradation of the surface rather
than an early period of rapid episodic lowering of the
surface.
2.5. Amount of Quaternary crest lowering
We calculate the crest lowering with the aid of
degradation model and field observations. When moraines that have been formed within the past few hundred years are observed in the field, they invariably
have relatively sharp crests and steep slopes close to the
angle of repose (Fig. 1, top panel). Therefore, the
commonly used initial condition in the degradation
modeling that moraines start as sharp crested seems
reasonable (Hallet and Putkonen, 1994; Putkonen and
Swanson, 2003). This particular assumption, however,
is often challenged by assertion that ice-cored or multiple crested fresh moraines have been observed in the
field and that they will rapidly degrade to smeared and
rounded form. It is then suggested that the long-term
erosion on these landforms is negligible after the initially rapid deformation.
In this paper, we are eager to alleviate the above
mentioned concern and, instead of assuming sharp
crested and steeply sloped initial moraine form that
degrades slowly, we calculate the mean slope angle
and corresponding moraine profile based on published
field observations. This means that (1) the initial forms
of moraine that we use in our degradation model have
already aged for thousands of years, and (2) the resulting quantitative estimates of the total lowering of the
crests are conservative minimums. This convention will
completely circumvent any disagreement of the assumed initial form of the moraine and guarantee that
the rapid early degradation or transformation does not
affect our results.
Because the general pattern of landform evolution
is fairly well understood and we have a number of
field observations on initial and final landforms, we
can model the degradation process in between those
end stages to track the transformation of the moraine
from a relatively steep and sharp to a more gentle and
smeared form. Although no absolute chronology
exists, we maintain that the sample size is large and
consists of moraines of all ages that are typically
found in the western North America and, therefore,
J. Putkonen, M. O’Neal / Geomorphology 75 (2006) 408–419
the amount of degradation is typical for the Pleistocene moraines.
As stated above, our intention is to use a non-controversial initial condition of the model to calculate
conservative minimum estimates of the lowering of
the moraine crest. To follow in this spirit, the modeled
initial form of the moraine has a rounded crest, not
sharp which would increase the crest lowering. To
consistently generate the smeared and rounded initial
moraine forms we start the model with sharp crested
steeply sloped moraine (34.58). Then we allow this
form to degrade until the slopes have reached the
mean angle found in the published literature. This is
now the initial form for modeling moraine degradation.
Then we allow the model to run until we attain the
mean final angle of the slope found in the published
literature. The advantage of this two-step process is that
the first smoothing of the moraine cross-section
removes all artificially sharp angles and straight slope
segments, and guarantees a minimum estimate of lowering the crest. The smoothed form of the moraine also
closely resembles the moraines commonly found in the
field that are older than few thousand of years.
To run realistic estimates of lowering the surface on
the fault scarps, the initial and final slope angles of the
fault scarps are calculated in the similar manner to
moraine data. The average of the young scarp slope
angles is taken to represent the initial scarp angles and
the average of the old scarps is used for final scarp
angle.
The modeling was not performed for the data from
cinder cones or wave cut scarps because they are presented only to illustrate the generality of the degradation processes of unconsolidated landforms.
3. Results and analyses
3.1. Landform slope angles
Although significant variability is noted in slope
angles between separate drainages, the moraine slope
angles generally decrease with increasing time. The
average slope angle of the youngest Pleistocene moraines (located closest to the valley head or existing
glacier) in a given drainage is 238 and the oldest
Pleistocene moraines (located furthest away from the
valley head or existing glacier) is 158. We do not have
any absolute dating control and the young moraines
may vary in age considerably; the same applies to old
moraines. If our conceptual understanding is correct,
however, the younger moraines are consistently steeper
than older moraines in a given drainage and the overall
413
trend should reveal decreasing slope angles between
these two end-members. The data for fault scarps
were organized with same principle.
The mean of the published young fault scarp angles
is 308 while the mean of the old scarps is 118. These
were used as the initial and final slopes in the degradation model for fault scarps. This convention again
assures a minimum estimate of the total lowering of the
surface.
When all the existing slope data from unconsolidated landforms (wave cut scarps, fault scarps, moraines,
and cinder cones) are compiled, a uniform picture
emerges of pervasive decrease of slope angles through
time (Fig. 3). This large data set reveals the robustness
of the landscape evolution. We find this qualitative
result to be the proof of the ubiquitous erosion that
uniformly affects all landforms with sloping surfaces.
The data on wave cut scarps and cinder cones affirm the
notion of decreasing slope angles with increasing time;
these two data sets also include an independent dating
control.
3.2. Lichenometry on late Holocene landforms
In the previous paragraph we listed available data
from landforms whose ages generally range from
thousands of years to tens of thousands of years old.
Now we turn attention to considerably younger landforms. In the lichenometric data that was collected for
this study, the spatial distribution of the lichens on the
surface of the moraine shows that the lichen diameters
at the crest are smaller and generally increase in size
down-slope (Figs. 4 and 5). As the surface exposure age
of a given boulder is related to the lichen diameter, the
boulder ages follow a generally similar pattern of degradation as the older landforms and show oldest exposed boulders on the flanks and youngest on the
moraine crest. Although a variety of environmental
controls can affect the distribution of lichens, the pattern that emerges from the lichen data is consistent with
the general understanding of the slope evolution (i.e.,
Carson and Kirkby, 1972). More importantly, it is in
excellent agreement with our expectations of the moraine degradation model (Hallet and Putkonen, 1994;
Putkonen and Swanson, 2003).
3.3. Cosmogenic exposure ages of moraine boulders
Additional and independent evidence of continuous
and ubiquitous lowering of moraine crests comes from
our own previous and here expanded analyses of cosmogenic exposure-dated moraine boulders (Putkonen
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Fig. 3. Four panels show the general degradation based on measurements of various landforms in the field: (A) moraines (the line is the linear trend
of all studies, rectangles 1 std, vertical bars show the maximum and minimum) (Birkeland and Burke, 1988; Briner and Kaufman, 2000; Burke and
Birkeland, 1979; Colman and Pierce, 1986; Espizua, 1993; Kaufman and Calkin, 1988; Kiver, 1972; Manley et al., 2001; Miller, 1971; Shakesby,
1989); (B) cinder cones (Hooper and Sheridan, 1998); (C) wave cut scarps (Nash, 1980a); (D) fault scarps (Bucknam and Anderson, 1979; Hanks et
al., 1984; Nash, 1984; Wallace, 1977). Symbols denote separate field areas.
and Swanson, 2003). A large number of studies have
utilized cosmogenic exposure dating of moraine
boulders in order to determine numerical ages of moraines. Typically the boulder ages from a single moraine
surface spread over a wide age range which is consistent with a large degradation of the moraine (Fig. 2 in
Putkonen and Swanson, 2003). If little or no degradation occurred, all the boulders would theoretically show
the same age, the moraine age.
In our analysis of all published exposure ages for
moraine boulders (Barrows et al., 2002; Briner et al.,
2001; Brook et al., 1993, 1995; Davis et al., 1999; DukRodkin et al., 1995; Gosse et al., 1995a,b; Gualtieri et
al., 2000; Ivy-Ochs et al., 1999, 1996; Licciardi et al.,
2001; Marsella et al., 2000; Owen et al., 2001; Phillips
et al., 1996, 1997, 1990; Shanahan and Zreda, 2000;
Steig et al., 1998; Swanson and Porter, 2000; Zreda and
Phillips, 1995; Zreda et al., 1994), we found that the
absolute scatter in boulder ages (range of boulder age)
from single surfaces increases as the moraines age and,
thus, the moraines have to degrade continuously. The
mean scatter increases with the moraine age
(t s = 0.45t + 2 ka; where t s = range of boulder age (oldest
boulder age youngest boulder age) [ka], t = moraine
age [ka]) (Fig. 6). If the degradation was limited to a
relatively short period after the ice recedes from the
moraine, then the age scatter in boulder ages from all
moraine surfaces would be about the same, which is not
the case. The scatters of individual ages are commonly
considerable and many greatly exceed the suggested
random error (8%) and systematic error (10%) (Gosse
and Phillips, 2001) or overall uncertainty of 3–5%
(Stone, personal communication, 2002). In our compilation, we followed the original author’s suggested
interpretation of the data to omit the older outliers,
which decreased the individual age variations. Younger
J. Putkonen, M. O’Neal / Geomorphology 75 (2006) 408–419
415
Fig. 4. Data from ca. 50-yr-old moraines on Mount Adams: (A) the dimensions (m) of the moraine profile (half of the profile from crest to flank);
(B) an 18-m-wide swath measured down the same moraine flank (black dots represent the diameter of the lichens); (C) the mean lichenometric age
for each 10 m bin along the horizontal moraine profile.
outliers were included in our data compilation because
these ages are predicted by the model of moraine
degradation. Additional details of the analyzed data
sets can be found in Putkonen and Swanson (2003).
3.4. Amount of Pleistocene surface lowering
The absolute amount of surface lowering is dependent on the size of the landform. The relative amount is
also weakly dependent on the size of the landform
because the slope length increases with increasing
size of the landform and strengthens the contribution
of overland flow to general erosion.
We modeled moraines that range between 13 and
100 m in initial height and between 10 and 80 m in
final height (the final height of the moraine crest is the
height observed today in the field). The conservative
estimate of crest lowering ranges from 22% to 27% of
the final height of the moraine crest and the average is
25%. The corresponding absolute lowering of the crest
is 2.4–21.8 m and the average is 10.3 m. These
calculations completely exclude the initial phase of
potentially rapid lowering of the crest right after the
ice recedes from the moraine and leaves the landform
steep sided and sharp crested. This also excludes the
possible rapid degradation caused by melting ice buried within moraine matrix. Our field observations
continue to indicate that the majority of the moraines
are, initially, sharp crested. Therefore, we maintain
that the ice retreat is followed by vigorous lowering
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Fig. 5. Data from ca. 150-yr-old moraines on Mount Adams: (A) the dimensions (m) of the moraine profile (half of the profile from crest to flank);
(B) an 18-m-wide swath measured down the same moraine flank (black dots represent the diameter of the lichens); (C) the mean lichenometric age
for each 10 m bin along the horizontal moraine profile.
of the crest (as shown by our lichen data) and would
in reality increase the above numbers of surface lowering by a large fraction. On the other hand, it has
been argued that some moraines may initially have
rounded crests and gentle slopes at the time when ice
recedes, although this is seldom seen in the field
today. In that case the above calculation would be a
true estimate of lowering the surface rather than a
conservative minimum.
For the fault scarps, the total amount of degradation
is calculated at the top of the footwall (top of the linear
fault face, where it intersects the level farfield ground
surface). This choice guarantees a maximum estimate
of the total erosion. The relative amount of lowering of
the scarp surface depends weakly on the scarp height.
The modeled scarps range in height 1–10 m. The
surface lowering ranges from 31% to 35% of the initial
scarp height and the average is 34%.
4. Discussion and conclusions
All existing data on slope angles show decreasing
slopes with increasing time from landforms such as
wave and fault scarps cutting through unconsolidated
sediments, cinder cones, and moraines. Collectively,
they attest to the pervasive and universal erosion of
unconsolidated landforms. Additional and independent
lichenometric and cosmogenic exposure evidence of
boulder ages on moraines show a pattern that is consistent with continuous erosion and emergence of fresh
boulders to the surface and strongly contradicts an
assertion of no or little degradation through the Qua-
J. Putkonen, M. O’Neal / Geomorphology 75 (2006) 408–419
Fig. 6. Boulder age range [oldest age (ka) youngest age (ka)] for
each individual moraine surface. An age range that is a large fraction
of the corresponding moraine age suggests that boulders have recently
exhumed to the surface and inadvertently sampled for moraine dating.
The linear regression (solid line) shows that the age range generally
increases with the moraine age. For printing clarity two data points are
excluded from the plot (moraine age = 582, 1389 ka, boulder age
range = 503, 489 ka, respectively). They are, however, used in the
regression calculation.
ternary period. When we analyzed the field observations in a systematic and consistent manner in a degradation model, the conservative results suggest
considerable erosion that generally is 25% of the final
height of the moraine and 34% of the height of the
scarp. This is equivalent to meters to tens of meters of
degradation on unconsolidated landforms, such as fault
scarps and moraines over typical moraine and fault
scarp ages of thousands to tens of thousands of years.
Our analyses were limited to fault scarps and moraines because of the importance in paleoclimatology
and seismic analyses and the resulting abundance of
published data. Our result of the vigorous and considerable erosion is obviously not limited to landforms for
which we were able to find data for. These results apply
to any unconsolidated landform with sloping surface,
which is highlighted by supporting data on wave cut
scarps and cinder cones.
From the lichenometric data, we conclude that degradation is a primary control on the ages of boulders
and subsequent distribution of lichens and provides
proof of preferential and continuous exhumation of
boulders at the crest. This is also seen in the cosmogenic exposure ages of moraine boulders, where the age
scatter from a single moraine surface generally
increases with the age of the moraine. The lichen data
suggest that no original boulders survived on the crests
of the late Holocene moraines that we studied. We
speculate that the steep and narrow crests are subject
to continual shallow sliding and creep that removes all
417
boulders that were originally located there. As the
moraine slopes become gentler and the landsliding
ceases, the slow surface lowering continues to exhume
new boulders to the surface. The age of these boulders
will always be less than the moraine age. These observations show that young and steep landforms experience considerable erosion in the early phase of
evolution, again emphasizing the notion of the dynamicity instead of stability of the landscape. The relative
degradation (amount of degradation/landform height)
increases with the size and percentage of matrix materials. If the moraine is small in size, the better is the
chance that the catastrophic land sliding and burial of
boulders is not going to affect the lichen ages. And of
course, bouldery moraines with minimal matrix content
are more resistant to erosion.
These results are crucial in interpreting lichen data to
derive moraine ages. Obviously, statistical methods that
average lichen diameters from the whole moraine surface can result in significantly younger estimated minimum age than from using the largest lichen, or a subset
of the largest lichens on the landform. We also suggest
that vigorous degradation can lead to instances where
none of the original lichens that first colonized the
surface survive and that the constant supply of fresh
boulders dramatically reduces the estimated limiting
age for the landform.
The most important results are twofold: 1) to unambiguously show that considerable degradation that
scales with slope angles affects uniformly all unconsolidated landforms, and 2) to show that conservative
estimates of the total degradation on two common landforms is so large that it has to be accounted for in any
research that is based on the preservation of the surfaces.
Acknowledgements
We are grateful to Bernard Hallet who started us on
this path, Alan Gillespie, Terry Swanson, and John
Stone for valuable discussions over the years, Ryan
Murphy and Pirjo Berg for field assistance, and the
Mazamas for funding the lichenometric analysis. We
also thank Ben Laabs and two anonymous reviewers for
constructive comments.
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