Comment
Editor Comments
Response
Manuscript Change
Could you pay particular
attention to the revision of figures
We have reviewed all figures and
made some modifications.
Several figures were removed so
as to shorten and simplify the
manuscript. We did not feel
these were vital to the overall
objective of this paper, but just
added more detail. We hope this
will help with the editorial
concerns of shortening the paper.
Figure 5 was referred to earlier
and referenced as Figure 4 now to
address the reviewers comments.
Also, the figure caption to (now)
Figure 4 was edited because of
formatting mistakes.
Figures have been reordered as a
result of this change.
We also reviewed and modified
the figure caption to Figure 8
(was figure 9) to address the
perspective issues of the
reviewer:
Figure 8. The Bursera Valley
rests entirely within the South
Mountain metamorphic core
complex (SMCC) of central
Arizona, USA. Its drainage basin
is outlined on the image and
flows to the southwest where it
grades into the Gila River and
basin fill deposits. A solid line
through the middle of South
Mountain delineates the boundary
between lithologies. West of the
line, drainages are underlain by
metamorphic rock and contain
alluvial fans as the dominant
alluvial landform. East of the line
the range is underlain by plutonic
rock, which forms semicontinuous fluvial terrace
surfaces. The perspective of
lighting in this image is from the
south so shadows are cast to the
north.
Figures 6, 11, 12, 14 were
removed.
to some simplification of the case
study so as to improve its
accessibility to a wider readership
We hope we have simplified the
case study enough for a broader
readership. Many extra details
were removed from both the text
and figures. We also highlighted
the “tourist” aspect of this area
that may attract more readers to
it.
to some reconsideration of
process- or time-based
The first paragraph was modified
to include this distinction.
The Figure caption for Figure 10
was modified to hopefully
simplify the description.
The Bursera Valley (BV) rests
entirely within the South
Mountains (SM) of central
Arizona (Figure 8). SM stretches
approximately 29 km in length, is
a city park preserve located just
south of metropolitan Phoenix,
Arizona, USA, and hosts 2
million visitors each year.
Toe-cut terraces (Figure 1) form
as a result of the interaction
between a basin’s trunk stream
and alluvial fans that originate
from tributary drainages. They
often form when a drainage
basin’s trunk stream laterally
migrates eroding the distal
portion of an alluvial fan. This
process is referred to as ‘toecutting’ (Figure 2; Leeder and
Mack, 2001). The resulting
landscape reflection of this
process is the abandonment of the
alluvial fan surface above the
modern floodplain (Figure 3).
Toe-cut terraces can also develop
when vertical incision of the
trunk stream lowers the local
base-level of the tributary fan
flowing into it, thus abandoning
the former alluvial fan surface at
a higher level (e.g. Figure 4;
Colombo, 2005; Colombo et al.,
2000). These two mechanisms
are the focus of toe-cut terrace
formation as they are modern
processes acting at the surface. It
is possible that exhumed toe-cut
terraces may be present when
deep vertical incision exposes
formerly buried fan deposits and
truncates them, however, the
focus of this manuscript is on
processes and deposits interacting
at the Earth’s surface. As
highlighted in the previous
literature, toe-cut terraces are
important features because of
their broader implications for
stratigraphic basin analysis,
groundwater studies (Leeder and
Mack, 2001) and landscape
evolution, especially in arid
environments (Larson, 2013).
definitional criteria as noted
The simplification of the case
study - with some shortening - is
an editorial concern which I have
identified, not a comment on
scientific quality
We have attempted to simplify
and shorten this section
considerably. Shortening was
primarily achieved through
removal of wording and figures
that added detail but was not
essential to the overall point of
the manuscript.
Simplification proved more
difficult. We felt that simplifying
this case study too much would
Very thorough editing and
“cutting” took place within the
Case Study section. We
attempted to remove confusing or
unnecessary discussion as it
relates to the larger point of this
manuscript. We believe these
substantial edits will be clearly
noticeable. We also removed
several figures as they were not
necessary in this context as well.
result in an inappropriate
representation of the difficulty in
interpreting, and thus the need for
the definition, of these landforms.
This is the purpose of this paper.
We did, however, try to make the
text more clear and concise.
Please see this section in the
manuscript to note changes.
They are far too long to put in
this table.
Reviewer #1 Comments
This paper attempts to interpret
terrace origins in a relatively
small Arizona drainage basin.
The discussion of the terrace
types will likely be the most
useful part of this paper. The
application of the discriminating
criteria to the Arizona basin,
while probably necessary for the
presentation, seems very
complex, and may not be read in
detail by most of the readership.
My first problem with the paper
began with the inclusion of a
second cause for toe-cut terrace
formation (lines 53-56). I didn’t
understand from the text what
was being described until I
looked at Figure 5a sometime
later. This second cause for toecut terraces needs to be added to
a redrawn Figure 1, or the reader
needs to be referred to Figure 5a
when that cause is first described.
Another problem (particular to
my own research) is featured in
lines 127 ff. At the location
described there are two sloping
terraces separated by a distinct 23 m scarp where a late
Pleistocene alluvial fan built into
the basin atop thick lake clays,
forming an erosion-resistant layer
atop the clays. The lower terrace
is a sloping structural terrace
caused by very heavy
cementation of the underlying
fan. The scarp described that
separates the two sloping terraces
formed when a major base-level
drop led to deep erosion of the
We agree that the most important
part of this paper is the
discussion/literature review on
terrace genesis and form.
However, we also believe the
definition of this proposed form
is equally useful.
The complexity of the Arizona
basin is complex, but necessary to
show the difficulty in
distinguishing these forms.
Complexity in geomorphology is
the norm in most studies.
We agree that for some not
familiar with this landform, this
could be confusing. We have
added some further clarification
and referenced figure 5a. Figure
1 is not meant to show the
mechanisms of formation, but a
simple depiction of the
complicated relationships of
terrace landforms one may
observe in the field.
The first paragraph describing
formation process and definition
was modified to explain exhumed
Toe-cut terraces and draw a
distinction between past and
present processes.
Toe-cut terraces can also develop
when vertical incision of a stream
lowers the local base-level of the
tributary fan flowing into it, thus
abandoning the former alluvial
fan surface at a higher level (e.g.
Figure 4; Colombo, 2005;
Colombo et al., 2000).
Toe-cut terraces (Figure 1) form
as a result of the interaction
between a basin’s trunk stream
and alluvial fans that originate
from tributary drainages. They
often form when a drainage
basin’s trunk stream laterally
migrates eroding the distal
portion of an alluvial fan. This
process is referred to as ‘toecutting’ (Figure 2; Leeder and
Mack, 2001). The resulting
landscape reflection of this
process is the abandonment of the
alluvial fan surface above the
modern floodplain (Figure 3).
Toe-cut terraces can also develop
basin, exhuming the heavily
cemented fan, and producing the
scarp solely by erosion of the
unprotected clays off of the
lower, cemented surface. By the
criteria of this paper, both
terraces could be toe-cut terraces
because both were caused by the
trunk stream experiencing a
major base level drop, combined
with the fact that both coarse
deposits originated as alluvial
fans. My concern is that the lower
structural terrace hasn’t been at
the surface for more than 190
thousand years, and did not
originate by any post-190 ka fanrelated process, and so I must
question whether the authors
really intend to describe the lower
terrace as a toe-cut terrace. If it is,
then this may mean that every
former alluvial fan surface (with
or without cemented horizons)
that is exhumed in a dissected
basin as a result of a base level
drop is a toe-cut terrace. Because
this paper’s present definition is:
“Toe-cut terraces form as a result
of the interaction between a
basin’s trunk stream and alluvial
fans that originate from tributary
drainages” (lines 48-49), the
surfaces the authors have describe
in line 127ff would be toe-cut
terraces.
However, I would like them to
reconsider carefully whether
there should be any other
process-based or time-based
limitations on the definition that
might exclude exhumed
landforms and exhumed
cemented fanglomerate horizons.
On line 246 the authors describe a
drainage basin incising headward
and laterally across a landscape. I
inferred that they might be
excluding knickpoint incision
here because it is described
separately in the following
paragraph. If line 246 is indeed
describing knickpoint incision,
then that needs to be explicitly
said. If, on the other hand, they
when vertical incision of the
trunk stream lowers the local
base-level of the tributary fan
flowing into it, thus abandoning
the former alluvial fan surface at
a higher level (e.g. Figure 4;
Colombo, 2005; Colombo et al.,
2000). These two mechanisms
are the focus of toe-cut terrace
formation as they are modern
processes acting at the surface. It
is possible that exhumed toe-cut
terraces may be present when
deep vertical incision exposes
formerly buried fan deposits and
truncates them, however, the
focus of this manuscript is on
processes and deposits interacting
at the Earth’s surface. As
highlighted in the previous
literature, toe-cut terraces are
important features because of
their broader implications for
stratigraphic basin analysis,
groundwater studies (Leeder and
Mack, 2001) and landscape
evolution, especially in arid
environments (Larson, 2013).
This paragraph was modified to
address this confusion.
Lithologic variations in
drainage basins can also affect
incision (Colombo et al., 2000;
Ritter et al., 2002). As a drainage
network evolves, it often incises
headward and laterally across the
landscape (i.e.knickpoint
migration or interfluve
denundation). This may result in
changes through time in the
materials entering from a stream
are intending to describe the
expansion of a drainage basin at
the expense of another (e.g., a
shifting interfluve), then that
needs to be stated clearly (since
that is a very slow process in the
softest of materials).
and its tributaries. For example,
changes that result in the
production of larger or smaller
clasts can lead to aggradation or
incision, assuming discharge rates
are static. Changes in particle size
can also impact the autogenic
processes within a drainage
network, where streams
originating in mountains have a
steeper gradient and higher
elevation than neighboring
tributaries originating over less
resistant lithologies in the
piedmont. This can lead to
drainage capture by the lower
elevation/gradient piedmont
stream, which, in turn, can cause
aggradation followed by incision
as the capturing stream adjusts its
longitudinal profile to variations
in sediment input from upstream
(Ritter, 1972).
Line 332: no criterion should be
specific to a given region. This
appears to be an inadvertent
mistake in how the paragraph is
worded.
The regional description was
removed.
Line 391: What does
“downstream convergence” refer
to specifically? Does it refer to a
convergence of terrace treads?
Clarify.
Yes, it does refer to terrace
treads. We have modified this
sentence to be more clear.
Line 592: “erodes the fan toe.”
This was changed. Thank you!
Lines 603-04: “but in reality
originated as toe-cut terraces.”
This was also changed. Thank
you, again!
At first Figure 9 was completely
confusing to me. It appeared that
the Bursera Valley boundaries are
We have modified the figure
caption to Figure 9.
Where debris-flow processes
dominate alluvial fan
aggradation, the morphology of a
fan surface can differ
substantially from that of fluvial
terraces derived from the
drainage basin’s trunk stream.
The one caveat to be
aware of when using this criterion
is that tectonic uplift of the
tributary valley will also produce
downstream convergence of
terrace treads (e.g. Pewe, 1978);
however, if tectonic activity can
be ruled out, then convergence
would indicate that the
abandoned alluvial surface is
likely a toe-cut terrace.
Alluvial fans commonly feature
scarps at their distal end produced
when a stream erodes the fan toe.
Employment of these criteria in
the Bursera Valley, central
Arizona, USA, illustrates their
potential to correctly identify
landforms that look very much
like a fluvial terrace, but in reality
originated as toe-cut terraces.
Figure 8. The Bursera Valley
rests entirely within the South
Mountain metamorphic core
in stream channels. When I
turned the image upside down, I
realized it might because shadows
should always point to the viewer
to avoid perceived topographic
reversals. If the image isn’t
reversed, this effect should be
pointed out in the caption. It
might also help if the Gila River
is labeled in the lower left corner
if, in fact, it is there.
complex (SMCC) of central
Arizona, USA. Its drainage basin
is outlined on the image and
flows to the southwest where it
grades into the Gila River and
basin fill deposits. A solid line
through the middle of South
Mountain delineates the boundary
between lithologies. West of the
line, drainages are underlain by
metamorphic rock and contain
alluvial fans as the dominant
alluvial landform. East of the line
the range is underlain by plutonic
rock, which forms semicontinuous fluvial terrace
surfaces. The perspective of
lighting in this image is from the
south so shadows are cast to the
north.