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U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Pamphlet to accompany

Scientific Investigations Map 3245

Geologic Map of MTM –30247, –35247, and –40247 Quadrangles, 

Reull Vallis Region of Mars

By Scott C. Mest and David A. Crown

SIM 3245

i

Contents

Introduction.....................................................................................................................................................1

Physiographic Setting...................................................................................................................................1

Data .............................................................................................................................................................2

Contact Types .................................................................................................................................................2

Fluvial Features ..............................................................................................................................................2

Waikato Vallis ........................................................................................................................................3

Eridania Planitia ....................................................................................................................................4

Reull Vallis..............................................................................................................................................4

Channels.................................................................................................................................................5

Gullies ....................................................................................................................................................5

Structural Features........................................................................................................................................5

Crater Counting Methodology .....................................................................................................................6

Stratigraphy ....................................................................................................................................................6

Highland Materials ...............................................................................................................................6

Plains Materials ....................................................................................................................................6

Vallis Materials......................................................................................................................................9

Surficial Deposits.........................................................................................................................................11

Crater Materials..................................................................................................................................12

Geologic History...........................................................................................................................................14

Acknowledgments.......................................................................................................................................15

References Cited..........................................................................................................................................15

Tables

1. Crater size-frequency data and time-stratigraphic determinations for Reull Vallis

region of Mars.............................................................................................................................19

2. Contact relations of geologic units in Reull Vallis region of Mars .....................................20

1

Introduction

Mars Transverse Mercator (MTM) –30247, –35247, and 

–40247 quadrangles cover a portion of southern Hesperia 

Planum and the highlands of eastern Promethei Terra, east 

of the Hellas basin (fig. 1). The map area (lat 27.5–42.5° S., 

long 110–115° E.) consists of cratered ancient highland 

materials of moderate relief, isolated knobs and massifs of 

rugged mountainous materials, extensive tracts of plains, and 

surficial deposits. Waikato and Reull Valles extend through 

plains and highland terrains. Regional slopes are generally to 

the southwest toward the Hellas basin (fig. 1), but local slopes 

(for example, highlands to plains) dominate the landscape.

The Martian highlands cover more than 60 percent of 

the planet’s surface and are primarily found in the southern 

hemisphere (Tanaka and others, 1992). Most of the highlands 

consist of rugged, densely cratered terrains believed to 

represent the final phase of heavy bombardment in the inner 

solar system ~4.0 billion years ago (Murray and others, 1971; 

Schubert and others, 1992; Tanaka and others, 1992). Parts of 

the Martian highlands show evidence of extensive degradation 

and modification. The map area exhibits landforms created 

by numerous geologic processes, including tectonism, fluvial 

activity, and mass wasting. The occurrence of features that may 

have been formed or modified by water, such as the eastern 

Hellas valles and valley networks, has significant implications 

for past Martian conditions. Determining the geology of 

the highlands east of the Hellas basin provides a better 

understanding of the role and timing of volatile-driven activity 

in the evolution of the highlands.

Geologic mapping at 1:1,000,000 scale from analysis of 

images—including Mars Observer (MO) Thermal Emission 

Imaging System (THEMIS) daytime infrared (IR) and visible 

(VIS), Mars Reconnaissance Orbiter (MRO) High Resolution 

Imaging Science Experiment (HiRISE) and Context Camera 

(CTX), Mars Global Surveyor (MGS) Mars Orbiter Camera 

(MOC), and Viking Orbiter (VO) images—and MGS Mars 

Orbiter Laser Altimeter (MOLA) topographic data complements 

previous local and regional geologic mapping and geomorphic 

studies of Reull Vallis (Mest and Crown, 2001, 2002, 2003) 

and the other eastern Hellas valles (Crown and others, 1992; 

Price, 1998), drainage networks (Mest and Crown, 2001, 2004; 

Ivanov and others, 2005; Mest and others, 2010), and highland 

debris aprons (Pierce and Crown, 2003). Crater size-frequency 

distributions have been compiled to constrain the relative ages 

of geologic units and determine the timing and extents of the 

observed geologic processes.

Physiographic Setting

Noachian materials are the principal components of the 

rugged highlands surrounding the Hellas basin and represent 

the oldest rocks on Mars (Murray and others, 1971; Tanaka, 

1986; Tanaka and others, 1988, 1992; Schubert and others, 

1992). Within the circum-Hellas region, erosion of highland 

materials began during the Late Noachian Epoch and continued 

throughout the Hesperian Period, resulting in deposition of 

extensive plains materials in and around the Hellas basin. 

Regionally, the units of the Hellas basin and its surrounding 

rim are grouped into the Hellas assemblage and range from the 

ancient basin-rim unit, consisting of rugged highland plateaus 

and massifs dissected by channels and valley networks, to 

younger plains units, some of which contain channels, ridges, 

scarps, and mesas (for example, Greeley and Guest, 1987; 

Crown and others, 1992; Tanaka and Leonard, 1995; Mest and 

Crown, 2001; Tanaka and others, 2002).

Highland volcanism began north and northwest of the 

map area in the Late Noachian/Early Hesperian Epochs with 

the formation of Tyrrhenus and Hadriacus Paterae (Crown and 

others, 1992; Tanaka and Leonard, 1995; Crown and Greeley, 

1993, 2007; Gregg and others, 1998), initially by explosive 

eruptions (Greeley and Spudis, 1981; Greeley and Crown, 1990; 

Crown and Greeley, 1993), and ending with effusive activity 

during the middle to Late Hesperian/Early Amazonian (Crown 

and others, 1992; Mest and Crown, 2001). The ridged plains of 

Hesperia Planum (Tanaka, 1986; Greeley and Guest, 1987), the 

basal referent for the Hesperian System, is one of many areally 

extensive plains-forming units found on the surface of Mars 

(Scott and Tanaka, 1986; Greeley and Guest, 1987) believed to 

have been emplaced as flood lavas (Potter, 1976; King, 1978; 

Greeley and Spudis, 1981; Scott and Tanaka, 1986; Greeley 

and Guest, 1987). However, despite the abundance of highresolution images from recent missions (MOC, THEMIS, CTX, 

and HiRISE), definitive evidence for the presence of volcanic 

features (edifices, flow fronts, rilles) has not been observed, 

and thus a volcanic origin of these plains is uncertain (Mest and 

Crown, 2001; Crown and others, 2005, 2007; Gregg and Crown, 

2005, 2007).

The abundance of channels and valley networks in the 

eastern Hellas region, along with the presence of several canyon 

systems, suggest water played a major role in modifying this 

part of the Martian landscape. Well-developed valley networks 

appear to be the oldest fluvial features in the area; they are 

found in rugged ancient highland terrains and are most likely 

Noachian to Hesperian in age (Crown and Mest, 1997; Mest 

and Crown, 2001, 2002, 2003). The presence of widespread 

plains units northeast of the Hellas basin provides evidence that 

extensive degradation and resurfacing occurred in the highlands. 

Subsequent erosion of these plains units is believed to have 

occurred in the Late Hesperian/Early Amazonian Epochs 

(Greeley and Guest, 1987).

Activity along the highland valles (Waikato, Reull, Dao, 

Niger, and Harmakhis) occurred primarily during the Hesperian 

Period, with some activity extending into the Amazonian. 

Recent studies of these canyon systems suggest an origin by 

subsurface flow and collapse of volatile-rich plains materials 

combined with erosion by fluvial processes, followed by 

modification by wall retreat (Crown and others, 1992, 2005; 
2

Tanaka and Leonard, 1995; Crown and Mest, 1997; Mest and 

Crown, 2001, 2002, 2003). Most of the valles head within or are 

adjacent to a major local volcanic feature: Dao Vallis cuts into 

the flank materials of Hadriacus Mons; Niger and Harmakhis 

Valles are located adjacent to the Tyrrhenus Mons lava flow 

field; Waikato Vallis is located in the ridged plains of Hesperia 

Planum; and Reull Vallis begins at the western edge of the 

depression of Eridania Planitia (figs. 1, 2). The fact that these 

canyons are in close proximity to volcanic features suggests 

channel formation may be in part due to mobilization and 

release of subsurface volatiles by volcanic heat (Squyres and 

others, 1987; Crown and others, 1992).

Lastly, evidence of mass wasting in the highlands east of 

Hellas basin has been observed in the form of debris aprons and 

flows that extend from highland massifs, interior crater walls, 

and valles walls and appear to be the youngest features observed 

(Crown and others, 1992, 1997, 2005; Crown and Stewart, 

1995; Tanaka and Leonard, 1995; Crown and Mest, 1997; Mest 

and Crown, 2001; Pierce and Crown, 2003).

The history of volatiles within Hesperia Planum and 

the surrounding highlands has been analyzed in broad-scale 

regional studies (Maxwell and Craddock, 1995; Crown and 

others, 2005; Mest and others, 2010), and some general 

consensus has emerged on the timing and sources of volatiles 

within the eastern Hellas/Hesperia Planum regions. Studies 

have suggested that hydrologic activity in this area began in 

the Noachian with fluvial dissection of the adjacent highlands 

(Mest and others, 2010). Ivanov and others (2005) suggested 

that water accumulated within a pre-ridged plains depression 

in the location of the present-day Hesperia Planum. According 

to Ivanov and others (2005), this volatile reservoir was 

later depleted (1) in the Early Hesperian by erosion prior to 

emplacement of the ridged plains, (2) in the Late Hesperian with 

formation of the eastern Hellas valles (Crown and others, 1992, 

2005; Mest and Crown, 2001), and (3) in the Early Amazonian 

by formation of lobate debris aprons (Pierce and Crown, 2003) 

and viscous flow features (Berman and others, 2009) in the 

highlands of Promethei Terra. Mid-latitude mantling deposits, 

now observed in various stages of degradation (Mustard and 

others, 2001), cover most surfaces in the southern part of the 

map area and are a recent source of volatiles to the region 

that may be directly related to the youngest fluvial and glacial 

landforms east of Hellas basin.

Data

Several data sets were used to map MTM –30247, –35247, 

and –40247 quadrangles. A THEMIS daytime IR mosaic 

(~230 m/pixel) was used as the base for displaying the map. 

THEMIS VIS (17–19 m/pixel), CTX (~5.1 m/pixel), and MOC 

narrow angle (1.5–8 m/pixel) images were provided by the U.S. 

Geological Survey (USGS) and, in addition to the THEMIS 

daytime IR mosaic, were used to identify contacts and features 

(craters, valleys, structures), to assist in characterizing geologic 

materials, and to measure the diameters of craters. A MOLA 

digital elevation model (DEM), with a resolution of 128 pixels/

degree, was used to evaluate the overall topography of the map 

area and assess the distribution of units and features within the 

map area with respect to their relative elevations.

Contact Types

This mapping effort uses standard contact types defined 

by the USGS, including certain, approximate, and inferred 

contacts. A “certain” contact type indicates a clear delineation of 

material units based on appearance and (or) relative age. Most 

units in the map area are delineated by a certain contact. An 

“approximate” contact type indicates certainty of the existence 

of the contact but uncertainty in the location of the contact 

due to data quality and (or) modification of the surface to 

obscure its precise location. For example, we use approximate 

contacts to delineate older impact crater ejecta deposits, such as 

Dowa/Pál ejecta materials, that do not display sharp contrasts 

with underlying plains due to the thinning of the distal parts 

of the deposit, as well as subsequent modification (erosion, 

mobilization, and deposition) of ejecta and plains materials 

by fluvial and eolian processes. Lastly, we use an “inferred” 

contact type in a handful of instances to delineate units that 

are similar in appearance and may be of similar origin but 

have clearly been modified differently. For example, inferred 

contacts are used to delineate highly degraded crater materials 

from highlands materials (mountainous material and basin 

rim unit), where a crater rim clearly identifies the presence of 

an impact crater, but the topography is too subtle and surface 

characteristics are too similar to the adjacent highland materials 

to support usage of another contact type.

Fluvial Features

Fluvial processes appear to have played a major role 

in the geologic evolution of the map area. Numerous valley 

networks, single channels, and gullies are incised within many 

of the geologic units, including the ridged and modified ridged 

plains, smooth plains (upper and lower members), etched and 

mottled plains, upper Reull Vallis floor material, and materials 

associated with impact craters. Furthermore, the presence of 

Waikato and Reull Valles indicate that significant amounts of 

material were eroded and transported from Hesperia Planum 

and Promethei Terra.
3

Waikato Vallis

Waikato Vallis1 (fig. 2) extends ~400 km through the plains 

of Hesperia Planum and is believed to be the source area for 

at least some of the fluids that carved Reull Vallis (Mest and 

Crown, 2001; Ivanov and others, 2005; Kostama and others, 

2007). The first expression of Waikato Vallis occurs as a 

shallow (150–200 m deep) elongated depression in the northcentral part of the map area that extends for ~140 km before 

transitioning to the main canyon system. The north boundary 

of the depression is gradual (slope ~0.5°), whereas the east 

and west boundaries appear as distinct low-relief scarps and 

(or) coincide with wrinkle ridges. The northernmost part of the 

depression is widest at ~47 km across, then narrows to ~24 km, 

where the depression intersects a fairly pristine wrinkle ridge 

and remains relatively consistent in width until it opens into the 

main canyon.

Two narrow channels are incised within the northern and 

southern parts of this depression for nearly its entire length 

(fig. 3A). The northern channel originates within a broad, flatlying, scarp-bounded area of the ridged plains where several 

small channels, low-relief scarps, and a streamlined teardropshaped island converge. This northern channel is sinuous, 

~500–800 m wide and ~2–5 m deep, and at several locations 

along its length the channel is braided (fig. 3B). These braided 

areas all occur on the north side of low-relief wrinkle ridges that 

cross-cut the channel, suggesting water within the channel may 

have ponded behind the ridges until it eroded through the ridges 

and became confined within the channel again. This channel 

continues south for ~60 km before terminating in the ridged 

plains and transitioning into a (subtle) positive relief feature 

(fig. 3C). Secondary craters from craters Dowa and (or) Pál 

(located ~160 km to the west) are superposed on this northern 

channel. The secondary craters appear morphologically similar 

and relatively pristine in both the plains and the channel where 

they cross, indicating that the impact events of Dowa and (or) 

Pál likely postdate incision of this channel.

The southern channel originates at a theater-headed alcove 

and displays steep walls along most of its length (fig. 3D). This 

channel is as much as 55 m deep and 0.5–1.5 km wide and 

generally widens prior to crossing intersecting wrinkle ridges 

and narrows where it cuts through and south of the ridges. 

Similar to the northern channel, the ridges likely impeded 

flow until the ridges were breached, but, unlike the northern 

channel, ponding water resulted in widening of the channel 

and not braiding. Unlike the northern channel, the southern 

channel contains a narrow channel incised on its floor for short 

distances along its length. The southern channel terminates prior 

to intersecting the main canyon of Waikato Vallis, where the 

channel has been impacted by a 5.5-km-diameter crater (a in 

fig. 3A). The pre-crater course of the channel, however, can 

be traced in the ejecta and the north and southwest rim of the 

crater, suggesting minor amounts of fluvial activity may have 

occurred after this impact event.

The main canyon of Waikato Vallis originates at the 

southern end of the elongated depression and continues south 

for ~260 km toward Eridania Planitia (figs. 2, 4). Waikato 

Vallis consists of a series of 40- to 70-km-wide, near-circular, 

~250-m-deep scarp-bounded depressions connected by a 

narrower (~4–15 km) canyon (Mest and others, 1998; Mest and 

Crown, 2001). Based on the high density of wrinkle ridge-rings 

throughout this part of Hesperia Planum, which are believed 

to reflect the locations of buried impact craters (Bryan, 1973; 

Chicarro and others, 1985), we presume that the near-circular 

depressions reflect impact craters that were buried by the ridged 

plains and exhumed during vallis formation. Waikato Vallis 

appears to have debauched into Eridania Planitia, where it 

terminates. Any visible connection between Waikato Vallis and 

Eridania Planitia is obscured by impact materials from craters 

Fiztroy, Avarua, Fancy, and Cayon, and a 34-km-diameter 

impact crater along the east edge of the map area (fig. 4A). 

However, MOLA data show that, despite the presence of this 

ejecta, it is likely water flowed to the east of Fancy and Cayon 

toward Eridania Planitia (fig. 2B). These two craters, as well as 

Fitzroy to the west and the 34-km-diameter crater to the east, 

provide important constraints on the timing of vallis formation. 

Fancy and Cayon contain knobs of mottled ridged plains 

material on their floors indicating both craters were inundated 

with ridged plains material; the remnant rims of two smaller 

craters to the east of Fancy and Cayon also appear to have been 

buried by plains materials, likely ridged plains material, but 

have subsequently been buried by smooth plains material and 

(or) ejecta. Following emplacement of ridged plains material, 

craters Fitzroy and Avarua formed and emplaced ejecta across 

this part of the plains. Waikato Vallis then formed, via a 

combination of subsidence and fluvial erosion, causing collapse 

of plains materials north of and within Fancy and Cayon and 

likely to the east of these craters. Water ponded within Fancy 

and Cayon and resulted in deposition of Waikato Vallis floor 

materials within these craters. The southwest rim of crater 

Fancy is breached; a small channel extends from this breach 

toward the smooth plains to the south (fig. 5), suggesting 

enough water was able to downcut the rim of Fancy and drain 

from the craters, thus eroding ejecta from Fitzroy and smooth 

plains material.

Grooves incised in Waikato Vallis floor materials in the 

northernmost part of the canyon (fig. 4B) and along the east 

edge of the depression just northeast of craters Fancy and 

Cayon (fig. 4A) suggest flow of water through the canyon 

eroded collapsed ridged and mottled ridged plains and Waikato 

Vallis floor materials. Inliers of ridged plains and modified 

ridged plains materials are found throughout Waikato Vallis and 

occur as streamlined islands and chaotic clusters of polygonal 

blocks. Ridges visible on the floor of Waikato Vallis have 

subdued morphologies and lack the characteristic broad rise 

and crenulated ridge crest observed in the ridges extending into 

the plains. This lack of primary wrinkle-ridge morphology, 

1

Mest and Crown (2001) defined three morphologically distinct segments for 

Reull Vallis, including Segment 1 (now Waikato Vallis) and Segments 2 and 3 

(now upper and lower Reull Vallis, respectively).
4

combined with notches observed in the ridges, indicates fluvial 

erosion of the ridges resulting from the Waikato outflow event. 

This further suggests that we are observing ridge structures at 

depth and providing constraints on the relative timing of plains 

emplacement, ridge formation, and vallis formation.

The overall morphology of Waikato Vallis and features 

identified within and around the vallis suggest this system 

formed by a combination of surface flow of water and collapse 

of ridged plains material by removal of subsurface water (Mest 

and Crown, 2001). The elongated northern depression and 

chaotic clusters of blocks within the canyon suggest removal 

of subsurface water and collapse of plains material along zones 

of structural weakness coinciding with buried impact craters. 

Subsidence may have been largely responsible for the overall 

shape of Waikato Vallis. Sinuous channels in the northern 

depression and plains adjacent to the main canyon of Waikato 

Vallis, grooves on the vallis floor, and streamlined inliers of 

ridged plains material suggest surface flow was also important 

in shaping the system and removing material from the canyon 

via erosion.

Eridania Planitia

An obvious surface connection between Waikato Vallis and 

Reull Vallis is not apparent. Previous mapping, prior to accurate 

topographic data, suggested the two valles were once connected, 

but the connection was subsequently buried by younger plains 

or eroded (Mest and Crown, 2001). However, MOLA data 

show there is no obvious channel-form connection between the 

two systems; rather, water evacuated from Waikato Vallis may 

have been stored temporarily in the depression recently named 

Eridania Planitia2 (Ivanov and others, 2005; Kostama and 

others, 2007).

Eridania Planitia (~3,100 km2) is located south of Hesperia 

Planum (fig. 1; lat 33–44° S., long 111–133° E.). Relative to 

the surrounding terrain, Eridania Planitia lies ~1 km below 

the ridged plains to the north and ~2 km below the adjacent 

highlands. The plains throughout Eridania Planitia are crosscut by numerous wrinkle ridges, but these ridges are more 

degraded than in the higher-standing ridged plains to the north, 

and they appear mantled in some areas, especially within the 

map area. In general, these low-lying plains would have acted 

as a depositional sink for fluvial sediments being eroded from 

the adjacent highlands, as shown by highland valley networks 

intersecting the plains along the western extent of Eridania 

Planitia (Ivanov and others, 2005). In addition, Eridania 

Planitia may have temporarily contained water evacuated from 

the ridged plains to the north via Waikato Vallis; crater Lipik 

(diameter, 56 km; lat 38.42° S., long 111.5° E.) and massifs 

within Promethei Terra may have acted as barriers that trapped 

water within Eridania Planitia (fig. 6). The materials covering 

Eridania Planitia, at least in the vicinity of the map area and 

possibly to the east, likely consist of sediments eroded from 

both the adjacent highlands and the ridged plains to the north 

(through Waikato Vallis) that have subsequently undergone 

eolian erosion, mapped in this study as etched plains materials 

(described below). This stored water was able to breach Lipik 

and highland massifs contributing to the formation of Reull 

Vallis (Ivanov and others, 2005; Kostama and others, 2007; 

Mest and Crown, 2010).

Reull Vallis

Reull Vallis (fig. 1), ~1,100 km long, displays complex 

morphology indicative of multiple styles and episodes of 

formation and modification. The Reull Vallis system was 

previously separated into three morphologically distinct 

segments (Crown and Mest, 1997; Mest and Crown, 2001); 

Waikato Vallis includes the former Segment 1 of Reull Vallis 

and was discussed in a previous section. The upper Reull Valles 

(formerly Segment 2) is oriented northeast-southwest and 

winds through Promethei Terra, exhibiting sinuous morphology 

where it originates within western Eridania Planitia and less 

sinuous, steep-walled morphology (Mest and Crown, 2001) 

down-canyon. The lower Reull Vallis (formerly Segment 3) 

starts at the junction of the upper Reull Vallis with Teviot 

Vallis (~350 km southwest of the map area), where it alters its 

course northwestward and terminates near the source basin of 

Harmakhis Vallis (Mest and Crown, 2001). Along its length, 

the upper Reull Vallis dissects highland and plains materials 

of Promethei Terra to its junction with Teviot Vallis (Mest 

and Crown, 2001, 2003) and channeled plains material where 

the lower Reull Vallis terminates near the head-canyon of 

Harmakhis Vallis (Crown and others, 1992; Price, 1998; Mest 

and Crown, 2001).

The upper Reull Vallis (6–13 km wide, ~350 m deep; 

fig. 6) displays sinuous morphology and extends for ~220 km 

through degraded highlands and smooth plains (Mest and 

others, 1998). Terraces are exposed along both walls of the 

canyon (figs. 6, 7). Some terraces are believed to be exposed 

strata and are mapped as smooth plains material (lower 

member), whereas some terraces appear to be erosional or 

depositional in nature, possibly representing various stages 

of flow within Reull Vallis, and these terraces are mapped as 

scarps. Floor materials in this part of the canyon are incised 

with a series of braided channels oriented parallel to the canyon 

walls but also cross from one side of the canyon to the other 

(fig. 7). These features suggest that at least this part of Reull 

Vallis is fluvial in nature (Mest and Crown, 2001).

Downstream of the sinuous part of Reull Vallis, in 

the southwestern part of the map area (fig. 6), a narrow 

(1.5–2 km wide), shallow (75–100 m deep) canyon downcuts 

into the main canyon floor (fig. 8) (Mest and others, 1998; Mest 

and Crown, 2001, 2002, 2003). This section of Reull Vallis 

is morphologically distinct from the preceding section, 

displaying steep walls and a relatively flat floor (fig. 9), and is 

2

 Previous work by Ivanov and others (2005) and Kostama and others (2007) 

informally termed this depression the “Morpheos Basin”.
5

narrower (~6 km) and shallower (~200 m) than the remainder 

of Reull Vallis to the west (Mest and others, 1998; Mest and 

Crown, 2001, 2002, 2003). The lack of abundant erosional 

features (terraces, floor grooves, fluvial channels incised in floor 

material, streamlined islands) here and in the lower part of Reull 

Vallis (to the west) suggests subsurface drainage and collapse 

of plains materials was likely a more dominant process in the 

formation of these parts of Reull Vallis than fluvial erosion or, 

at least, dominated the most recent stages of activity, removing 

indicators of an earlier fluvial phase (Crown and others, 2005).

Channels

Highlands and plains in the region are incised with 

channels that either occur as single units or join together to 

form small, poorly developed valley networks. Channels 

tend to occur in low-lying, low-slope (<2°) areas of crater 

ejecta and plains materials. Most features are single channels 

that are less than 1 km wide and several tens to hundreds of 

kilometers in length. Some channels merge to form networks 

consisting generally of one to three tributaries, but some have 

up to a dozen. Channels throughout this region have two basic 

morphologic types. The first type, which includes most of the 

channels observed in the map area and the longest features, is 

shallow, flat-floored, and sinuous in planform (fig. 10). Some 

of these channels display braided morphology at one or more 

locations along their length, suggesting unconfined and (or) 

sediment-loaded flow (Leopold and Wolman, 1957; Schumm 

and Kahn, 1972; Wohl, 2000). The second type (c in fig. 7) 

is V-shaped in cross section and deeply incised relative to 

the narrow valley widths, tends to form short theater-headed 

valleys, and is found primarily along the sinuous uppermost 

part of Reull Vallis that is incised within ejecta material from 

crater Greg. Although some sinuous channels intersect Reull 

Vallis, it is not clear whether they predate or postdate Reull 

Vallis, whereas the V-shaped channels display a deeply incised 

notch in the walls of Reull Vallis, providing clear cross-cutting 

relations.

Gullies

Gullies are found throughout the southern part of the map 

area and are incised in the walls of many impact craters and 

along the slopes of massifs of mountainous material (Berman 

and others, 2009). The gullies display typical morphologies as 

described by Malin and Edgett (2000). These gullies generally 

head within alcoves located just below the crater rim crest or 

peak of the massif and extend to the base of the slope. They are 

not deeply incised and some display a fan of material at their 

termination. Most gullies appear to be incised in the regional 

mid-latitude mantle (Mustard and others, 2001; Milliken and 

others, 2003) that superposes most surfaces in the southern part 

of the map area.

Structural Features

The most abundant and most obvious features in the 

map area that provide evidence for deformation of surface 

materials are wrinkle ridges and ridge rings. Formation of 

wrinkle ridges is generally attributed to folding and (or) thrust 

faulting resulting from compressional stresses either within the 

lithosphere or confined within the deforming unit (Lucchitta 

1976, 1977; Chicarro and others, 1985; Plescia and Golombek, 

1986; Sharpton and Head, 1988; Watters, 1988, 1991, 1993; 

Golombek and others, 1991; Zuber, 1995; Goudy and Gregg, 

2001, 2002; Goudy and Schultz, 2003; Neel and Mueller, 2007). 

Ridge rings are circular wrinkle ridge-like structures believed 

to delineate buried craters; it has been suggested that the 

overlying unit underwent subsidence resulting in compressional 

stresses over the buried crater rim (Bryan, 1973; Chicarro and 

others, 1985).

Wrinkle ridges form rings and linear features that consist 

of a broad arching rise (to 10 km wide and 100 m high) topped 

with a narrow (~1.5 km wide) crenulated ridge that zigzags 

from one side of the rise to the other (figs. 2, 4). Regionally, and 

within the map area, two dominant ridge trends are observed 

(figs. 1, 2)—northeast-southwest (Hellas radial) and northwestsoutheast (Hellas concentric)—indicating the stress regime 

shifted over time (King, 1978; Watters and Chadwick, 1989; 

Porter and others, 1991). Most ridges oriented northwestsoutheast display the broad rise and narrow crenulated ridge, 

whereas many ridges oriented northeast-southwest display only 

the crenulated ridge without the significant presence of the 

broad rise. Wrinkle ridges are found primarily within the ridged 

and etched plains materials in the map area, but the highest 

density of ridges is observed within ridged plains material. 

Ridges throughout the map area display pristine through 

degraded morphologies, and, overall, the morphologies do not 

follow a specific trend. Most of the degraded ridges are found 

adjacent to and on the floor of Waikato Vallis, within a zone 

of ridged plains extending from Waikato Vallis to the western 

edge of the map area and bounded by the ejecta deposits of 

craters Fitzroy, Avarua, Dowa, and Pál and within the etched 

plains found on the floor of Eridania Planitia (fig. 2). Based 

on their locations relative to features believed to have formed, 

or been modified by, fluvial processes, degradation of ridges 

may also be due in large part to fluvial processes, but some 

erosion may be eolian. In addition, some ridges in the smooth 

and etched plains may be buried, thereby subduing their 

characteristic morphology.

Cross-cutting and superposition relations suggest 

wrinkle-ridge formation in the map area occurred after plains 

emplacement and prior to formation of Waikato and Reull 

Valles. Several lines of evidence can be used to constrain the 

timing of wrinkle-ridge formation within the map area. First, the 

channel incised within the northern depression of Waikato Vallis 

cuts through several ridges, and ridges exposed on the floor of 
6

Waikato Vallis are notched and do not display the characteristic 

morphology observed in the ridges as they continue into the 

adjacent plains. Second, secondary craters and ejecta from the 

Hesperian-aged craters Dowa and Pál in the north and Greg 

in the south parts of the map area superpose wrinkle ridges 

formed within the ridged plains material and etched plains 

material, respectively. Third, ridges observed within the smooth 

and etched plains display knobby and pitted textures found 

throughout the plains, suggesting a regional unit may have been 

emplaced whose deposition is indifferent to topography, such 

as the mid-latitude mantle material that is believed to consist 

of ice-rich sediments deposited via airfall (Mustard and others, 

2001; Milliken and others, 2003). In the map area as a whole, 

ridge-forming deformation may have begun as early as the 

Early Hesperian Epoch and likely ceased by the mid-Hesperian.

Crater Counting Methodology

Relative ages for the geologic units in MTM –30247, 

–35247, and –40247 quadrangles were determined by the 

number and size distribution of superposed impact craters 

for each geologic unit (Tanaka, 1986; Werner and Tanaka, 

2011), as well as by the principles of relative dating. Unit 

contacts were digitized in ArcGIS on a THEMIS daytime IR 

mosaic (~230 m/pixel); unit areas were calculated in ArcGIS 

from the resulting unit polygons. Individual craters were 

identified on the THEMIS daytime IR base, as well as CTX, 

THEMIS VIS, and MOC NAC images, for each unit and their 

diameters were measured and recorded in ArcGIS. Due to near 

complete coverage of the map area by these high-resolution 

image datasets, craters ≥0.5 km in diameter were counted; 

however, given the complex history of erosion, deposition, 

remobilization, and mantling that could remove a large number 

of small-diameter craters within primary geologic units, age 

determinations are based on crater size-frequency distribution 

data for diameters greater than 1 km. Table 1 summarizes the 

crater size-frequency distribution data for N(1), N(2), N(5), 

and N(16) for each geologic unit mapped in MTM –30247, 

–35247, and –40247 quadrangles. N(1), N(2), N(5), and N(16) 

represent the cumulative number of craters with diameters >1, 

2, 5, and 16 km/106 km2, respectively (crater size-frequency 

errors = ± ((N1/2)/A) × (106 k2)). The N(1), N(2), N(5), 

and N(16) data were then plotted for each unit; these data, 

combined with observations of superposition, cross-cutting, and 

embayment relations, were used to determine the appropriate 

time-stratigraphic series for each unit.

Stratigraphy

Relative ages of the geologic units mapped in MTM 

–30247, –35247, and –40247 quadrangles were determined 

using crater size-frequency distributions (table 1) in 

combination with observed stratigraphic relations (table 2). 

With few exceptions, crater size-frequency distributions show 

relative ages that tend to be consistent with previous studies 

(Greeley and Guest, 1987; Crown and others, 1992; Tanaka and 

Leonard, 1995).

Highland Materials

Highland terrains contain the oldest materials exposed in 

the area, including mountainous material and materials of the 

basin-rim unit. Mountainous material (unit Nm) forms large, 

rugged, isolated, or clustered massifs throughout the map area, 

but most (and the largest) are concentrated in the southern part. 

Massifs of unit Nm are found within exposures of the equally 

ancient basin-rim unit or are surrounded by younger impact 

ejecta, plains materials, or lobate debris aprons. Mountainous 

material is believed to consist of ancient crustal material 

uplifted during impact basin formation (Scott and Tanaka, 

1986; Greeley and Guest, 1987) and (or) remnants of eroded 

crater rims.

Along with mountainous material, the basin-rim unit (unit 

Nh1) (Greeley and Guest, 1987) forms most of the highland 

terrain of Promethei Terra (Mest and Crown, 2001, 2002, 

2003) and is found within the southern part of the map area. 

This unit forms continuous exposures of rugged terrain with 

expanses of smooth and knobby textured surfaces. Exposures 

of the basin-rim unit are believed to consist of ancient crustal 

material uplifted during formation of impact basins and craters 

subsequently modified by fluvial processes and mass wasting.

In THEMIS day IR and Viking Orbiter images, the 

highland terrains appear mountainous and rugged (fig. 6). 

However, in high-resolution images (MOC, THEMIS VIS, and 

CTX), their surfaces appear rounded and their ruggedness is 

muted, which appears to be due to erosion of these highland 

materials followed by mantling by a thin but fairly continuous 

deposit. Images show that this mantle is being removed from 

the peaks of some of the steepest massifs and transported 

downslope via slumping and viscous flows (Milliken and others, 

2003) to form deposits of debris apron material (unit Ada) 

(fig. 9) or talus.

Mountainous material and the basin-rim unit are two of 

the oldest materials exposed in the map area; N(5) and N(16) 

crater size-frequency distributions indicate Noachian ages. 

The range in age for these units is due to (1) their small areas 

of exposure and (2) burial of small- and medium-diameter 

craters by mantling material. Based on crater size-frequency 

distribution statistics (N(5) and N(16)) and superposition 

relations, mountainous material and the basin-rim unit are most 

likely Early to Middle Noachian in age.

Plains Materials

Several plains units are mapped, including ridged (upper 

and lower members), smooth (upper and lower members), 

etched, and mottled plains materials. Combined, exposed plains 

materials occupy almost half (49%, ~111,000 km2) of the total 

map area; inclusion of the areas of the ejecta blankets that likely 
7

superpose large expanses of plains suggests plains materials 

could cover more than 75 percent (~177,000 km2) of the total 

map area.

Most of the map area is located within Hesperia Planum, 

which contains one of the largest exposures of ridged plains 

material on Mars (Greeley and Guest, 1987). In this study, 

ridged plains material, upper member (unit HNpru), covers 

most of MTMs –30247 and –35247 and likely extends under 

much of the impact crater materials that superpose the northern 

part of the map area. Here, unit HNpru is characterized by a 

high density of wrinkle ridges and ridge rings.

The ridged plains material (upper member) appears mottled 

at all resolutions displaying large areas of low and high albedo 

surfaces (figs. 3, 4). In THEMIS day IR images, most interridge areas display relatively smooth and featureless surfaces 

except for the presence of low-relief scarps and small sinuous 

channels. CTX and THEMIS VIS images show that inter-ridge 

areas contain dune features, accumulations of smooth materials 

in low areas, and small knobs adjacent to some ridges. Some 

areas within the deposit, especially near Waikato Vallis and 

within the zone of degraded wrinkle ridges, contain numerous 

sinuous, and in some cases braided, channels, scarp-bounded 

mesas, and long (hundreds of kilometers) arcuate scarps that are 

interpreted to be fluvial in origin (Mest and Crown, 2001).

Previous studies have suggested the ridged plains material 

of Hesperia Planum was emplaced as flood lavas that filled 

low-lying regions of the highlands (Potter, 1976; King, 1978; 

Greeley and Spudis, 1981; Scott and Tanaka, 1986; Greeley and 

Guest, 1987). However, in this study area, no obvious volcanic 

features (vents, rilles, flow fronts) are visible, despite the 

recent abundance of high-resolution images. Narrow sinuous 

valleys with adjacent dark deposits are found throughout the 

plains, including within the northern part of this map area, but 

subsequent modification obscures the precise origin for many 

of these features. The sinuous nature of many valleys suggests 

they could be rilles or lava channels, and the dark deposits 

could be volcanic material. However, numerous other valleys 

incised in ridged plains material suggest these features could 

also be fluvial channels with adjacent floodplain deposits. The 

presence of these fluvial zones, as well as the eolian-dominated 

inter-ridge areas, suggests unit HNpru has been heavily 

modified by processes subsequent to its emplacement. Here, 

we interpret the ridged plains to consist of layered sedimentary 

and (or) volcanic material, large areas of which have undergone 

subsequent fluvial and eolian modification that resulted in the 

mottled appearance of this unit. In addition, chains and clusters 

of secondary craters, likely originating from craters Dowa and 

Pál, are found in the central part of the map area within ridged 

plains material (fig. 3). The ejecta deposits from these two 

craters (described in more detail below) have also been highly 

modified, but a thin mantle of these ejecta deposits may be 

present, contributing to the mottled appearance of this part of 

the ridged plains material (upper member).

The ridged plains material of Hesperia Planum has been 

used as the basal referent of the Hesperian System (Tanaka, 

1986). However, since the original mapping of these plains 

(Potter, 1976; King, 1978; Scott and Carr, 1978; Greeley 

and Guest, 1987), significant improvements in data (such as 

acquisition of higher-resolution image, spectral, and topographic 

datasets) have resulted in reclassification of large areas of 

the Hesperia Planum ridged plains as materials unrelated to 

flood-basalt-type eruptions (Gregg and Crown, 2005, 2007; 

Crown and others, 2007). For example, the extent of Tyrrhenus 

Mons volcanic materials has been reevaluated resulting in a 

significantly larger area classified as Tyrrhenus Mons shield, 

flank, and flank flow materials rather than ridged plains material 

(Porter and others, 1991; Crown and others, 1992; Gregg and 

others, 1998; Gregg and Crown, 2009; Williams and others, 

2008). Also, some areas within Hesperia Planum contain the 

characteristic wrinkle ridges but are generally less dense than 

the characteristic ridged plains material and (or) the ridges have 

been greatly modified; areas of ridged plains material within 

the south-central part of Hesperia Planum have been remapped 

as “smooth” and “dissected” plains (Mest and Crown, 2001, 

2002, 2003). Lastly, portions of ridged plains material within 

its southern zone have been subjected to significant amounts of 

erosion by fluvial processes and (or) burial by eolian and midlatitude mantling materials.

These subdivisions of Hesperia Planum have ages ranging 

from Noachian for parts of Tyrrhenus Mons to Amazonian for 

the Tyrrhenus Mons flank lava flow field. Interestingly, the 

parts of Hesperia Planum still considered to be ridged plains 

material show Early Hesperian ages, as defined originally 

for Hesperia Planum as a whole (Potter, 1976; King, 1978; 

Scott and Carr, 1978; Greeley and Guest, 1987). Crater sizefrequency distribution statistics for the area of ridged plains 

material (upper member) mapped in this study, combined with 

superposition relations with ejecta from craters Dowa and 

Pál, suggest Late Noachian (N(16)) to Early Hesperian (N(2) 

and N(5)) ages.

Ridged plains material, lower member (unit HNprl), has 

limited exposure along much of the canyon of Waikato Vallis 

and forms chaotic clusters of irregularly shaped flat-topped 

polygons and knobs within the canyon (fig. 4A). Most surfaces 

of unit HNprl are smooth and appear to have similar to slightly 

higher albedo than the upper member of ridged plains material, 

and some areas are also mottled in appearance. Other areas 

display surface textures that are more irregular; these areas tend 

to be incised with narrow, sinuous valleys that are likely fluvial 

in nature. Exposures of unit HNprl are typically topographically 

lower than unit HNpru. The location and appearance of unit 

HNprl suggests some exposures consist of stratigraphically 

lower materials exposed during formation of Waikato Vallis. 

Other exposures may consist of unit HNpru that has undergone 

collapse and subsequent erosion by fluvial processes. Mottling 

within the plains is caused by a mantle of eolian and (or) fluvial 

sediments. Given the nature of Waikato Vallis, as with the 

source areas of other Martian outflow channels, chaotic areas 

are interpreted to have formed by collapse of plains materials. 

Irregular textures within some exposures of unit HNprl reveal 

these areas are incised with sinuous channels and contain 

streamlined bedforms, suggesting fluvial modification, likely 

during vallis formation.
8

Crater size-frequency distribution statistics for unit 

HNprl show ages of Early Noachian to Early Hesperian. This 

unit displays limited extent, but it is superposed by the ejecta of 

several fresh small-diameter craters. Based on our interpretation 

of unit HNprl—exposed and (or) collapsed and eroded plains 

material—we indicate a Late Noachian to Early Hesperian age 

for the lower member of the ridged plains material.

Smooth plains material consists of an upper member 

(unit HNpsu) and a lower member (unit HNpsl). Within the 

map area, the upper member is found primarily adjacent to 

upper Reull Vallis (figs. 6–8), where it heads within the lowlying plains of Eridania Planitia. Previous mapping of central 

Reull Vallis to the west of the current study area showed smooth 

plains material forms much of the plains extending from the 

canyon of Reull Vallis to the highlands (Mest and Crown, 2001, 

2002, 2003). The lower member of the smooth plains material 

is more limited in exposure, forms the lower wall/lower terraces 

along much of the length of the uppermost part of Reull Vallis, 

and is stratigraphically lower than the upper member (figs. 7–9). 

In THEMIS day IR images, HNpsu displays smooth, relatively 

featureless surfaces (fig. 6); however, in CTX and THEMIS VIS 

images, HNpsu displays low-relief scarps and ridges, small 

channels, and shallow pits or undulations. The upper member 

exhibits lobate terminations and appears to embay Noachian 

highland units (Nh1 and Nm) and highly degraded crater 

material (discussed below) where they are in contact. Outcrops 

of HNpsl display smooth and relatively featureless surfaces and 

appear lower in albedo than the overlying upper member.

Smooth plains materials, both upper and lower members, 

are interpreted to consist of sediments deposited (1) within a 

transient body of water, (2) by erosion of adjacent highlands 

via valley networks, and (or) (3) via mass wasting (Mest and 

Crown, 2001, 2002, 2003). Most of the materials that constitute 

the smooth plains materials likely predate formation of the 

existing Reull Vallis canyon system and were deposited within 

a transient body of water or by flooding of low-lying regions 

bounded by the adjacent highlands; a network of transient 

paleolakes may have extended eastward and been connected 

with Eridania Planitia (Ivanov and others, 2005; Kostama and 

others, 2007). The outflow event that formed the uppermost 

part of Reull Vallis eroded through the upper member of the 

smooth plains and exposed the lower member, now preserved as 

terraces along this part of the canyon (fig. 7). Some areas within 

the upper member could postdate formation of Reull Vallis, 

consisting of sediments eroded from the adjacent highlands 

and transported to the plains via valley networks and (or), more 

recently, as a result of mass wasting of volatile-rich debris 

from highland massifs. Channels and low-relief scarps found 

within HNpsu are believed to be fluvial in nature, resulting 

from drainage of the fluvial event that in part formed Reull 

Vallis. Shallow pits preserved within HNpsu could have formed 

by sublimation of volatiles within the plains and subsequent 

collapse of sediments and (or) could be craters modified by the 

wind and (or) water.

Crater size-frequency distributions for both members 

are similar, showing large uncertainties in formation age with 

younger ages for the lower member because of its limited 

exposure and exhumed nature and therefore lack of fresh 

superposed impact craters. Previous mapping by Mest and 

Crown (2001) showed the smooth plains material was Early 

Hesperian in age. However, the extent of this unit has since 

been reduced because of the ability to use higher resolution 

images to identify ejecta deposits from several of the local 

large (>50 km in diameter) impact craters, such as craters Greg, 

Fitzroy, and Avarua, that superpose these plains. We designate 

ages of Late Noachian to Early Hesperian for the upper and 

lower members of the smooth plains.

Etched plains material (unit Npe) forms the surface of 

the topographically low area of Eridania Planitia. The unit 

extends from the head of Reull Vallis to the east edge of the 

map area and to the south along the eastern margin of crater 

Greg. In THEMIS day IR images, unit Npe displays a fairly 

constant moderate albedo in the northern part of the deposit but 

is mottled with darker and lighter tones along the east edge of 

the map area (fig. 6). The northern part of the deposit displays 

fine-scale knobs and pits. This texture also superposes wrinkle 

ridges, which display degraded morphologies along their broad 

rises but fairly sharp unmodified crenulated ridges, and the 

peaks of small massifs of unit Nm. Two areas within the etched 

plains material display larger knobs that appear to superpose 

the plains and wrinkle ridges east of crater Greg and along the 

northern Npe /HNpsu contact. At these locations it appears 

that the ejecta from Greg and the smooth plains material grade 

into zones of knobs (fig. 11A). The mottled area of the etched 

plains material displays surface textures consisting of elongated 

knobs and pits with a general northeast-southwest orientation 

(fig. 11B). Here, the mottled appearance is due to eolian 

removal of dark material to expose underlying bright material; 

the lower albedo (relative to the overall moderate albedo of 

unit Npe to the north) could be due to concentration of eroded 

and redistributed material within this part of the plains. Eolian 

modification of the plains is supported by the presence of an 

abundance of dust-devil tracks throughout the southern part 

of unit Npe. Several sinuous, flat-floored, braided valleys are 

found in the southernmost part of unit Npe, suggesting fluvial 

processes also modified portions of the plains. Ejecta from 

several large craters superpose the etched plains material, 

including crater Greg and unnamed 34- and 20-km-diameter 

craters along the etched plains material’s northeastern and 

southern contacts, respectively.

The etched plains material is interpreted to be sedimentary 

in nature, emplaced (1) as lacustrine sediments, (2) from fluvial 

erosion of the surrounding plains and highlands, and (or) (3) as 

a wind-blown deposit. Lacustrine sediments suggest that water 

and sediment released from Waikato Vallis could have been 

stored within a temporary lake in Eridania Planitia until the lake 

breached its divide (in the area of crater Lipik) to contribute 

to Reull Vallis. Ivanov and others (2005) and Kostama and 

others (2007) suggest a lake level as high as the 650 m contour; 

however, based on the volume of Waikato Valles, Capitoli and 

Mest (2010a,b) estimated that a lake in Eridania Planitia likely 

did not extend above the 450 m contour. The presence of etched 

plains material on the broad rises of wrinkle ridges and peaks 

of massifs of mountainous material, as well as on the low-
9

lying plains, suggests a lack of direct topographic control that 

would be expected in a lake basin and is more consistent with 

a wind-blown deposit. If deposited in a lake, the depth of water 

required to submerge knobs and ridges would have extended 

to the 1,000 m contour, which is significantly greater than 

estimates by Capitoli and Mest (2010a,b), as well as Ivanov 

and others (2005) and Kostama and others (2007), and far 

exceeds the volume of water that appears to have been released 

from Waikato Vallis. Regardless of its origin, the materials 

composing the etched plains material appear to be easily eroded 

by the wind to form the characteristic knob-and-pit texture 

and yardang-like features. The large knobs just east of crater 

Greg and along the unit HNpsu/Npe contact are believed to 

consist of remnants of ejecta from crater Greg and unit HNpsu, 

respectively. Here, ejecta could have been deposited within a 

temporary lake located within Eridania Planitia, which drained, 

thereby exposing the ejecta to enhanced erosion and eolian 

modification. Numerous chains of degraded secondary craters, 

believed to have originated from the crater Greg impact event, 

are also found within unit Npe, supporting the ejecta origin 

for these large knobs. Sinuous valleys in the southern part 

of the deposit indicate erosion of plains materials by fluvial 

processes. Alternatively, the presence of degraded wrinkle 

ridges and ridge rings within the plains suggests that the etched 

plains material could consist of a thin mantle of eolian-derived 

sediments superposing ridged plains material (upper member), 

and the presence of knobs extending from the upper member 

of the smooth plains material suggests that portions of these 

plains may consist of a thin mantle of the lower member of 

the smooth plains material superposing ridged plains material 

(upper member).

From crater size-frequency distribution statistics (table 1), 

etched plains material shows Noachian ages at larger crater 

sizes (N(5) and N(16)) and a Late Hesperian or younger age at 

smaller crater sizes (N(2)). Superposition relations with ejecta 

deposits from several large impact craters along the edge of 

the deposit, including crater Greg, which shows an age of Late 

Noachian to Early Hesperian (discussed below), and the fact 

that the plains superpose wrinkle ridges and unit Nm massifs 

suggest that etched plains material is Late Noachian in age.

The mottled plains material (unit Npm) fills low-lying 

areas around highland massifs and degraded craters in the 

southern part of the map area (fig. 12A) and embays these 

highland units where they are in contact. The surface of mottled 

plains material displays broad areas of dark and light (mottled) 

tones throughout the deposit and contains smooth, knobby, 

and pitted textures. The western and southern parts of the 

deposit tend to show overall smooth textures in THEMIS day 

IR images. Some areas of mottled plains material, specifically 

those adjacent to the southernmost deposits of ejecta from 

crater Greg (fig. 12B), contain fluvial channels with degraded 

morphologies suggesting burial and (or) modification by 

collapse of adjacent plains materials. Mottled plains material 

along the southeastern edge of the map area displays a knobby 

texture (fig. 12C), with areas that exhibit an arcuate, almost 

lobate edge. Here, unit Npm consists of mostly small (tens of 

meters across) knobs surrounded by relatively smooth and pitted 

surfaces, and the knobs appear isolated from a more continuous 

deposit. Pitted textures appear to be the result of degradation of 

smooth areas within the plains by collapse of volatile-rich plains 

and (or) by eolian modification.

Mottled plains material is likely sedimentary in origin, 

derived from (1) highland materials transported via valleys 

and (or) emplaced via mass wasting (Mest and Crown, 2001, 

2001, 2003), (2) degraded ejecta from local impact craters, 

and (or) (3) eolian processes. The plains in the northern part of 

the deposit are found adjacent to highland terrains containing 

narrow, degraded valley networks that could have deposited 

sediments into low areas among highland massifs but whose 

termini are now obscured by subsequent deposits. Mottled 

plains material nestled within the highlands in the eastern and 

southern parts of the deposit are within a zone where debris 

aprons commonly surround highland massifs (see Pierce and 

Crown, 2003). Coalesced debris aprons could form these parts 

of mottled plains material (as suggested by Crown and others, 

1992, for a region near Harmakhis Vallis in western Promethei 

Terra). Several large, highly degraded impact craters, such as 

the 44-km-diameter crater located at lat 42° S., long 112.6° E., 

are located in the southern part of the map area, and their ejecta 

deposits would have covered a large portion of this terrain. 

The location of the knobby terrain relative to large craters, its 

similarity to the knobby terrains within etched plains material 

associated with the ejecta east of crater Greg and the smooth 

plains material, and the almost lobate-like appearance of several 

clusters of knobs suggest this part of the deposit could represent 

impact ejecta emplaced in a lacustrine environment that 

eroded into remnant knobs. CTX images show that the overall 

appearance of the surfaces of mottled plains material is similar 

to the mid-latitude mantle deposit where smooth surfaces grade 

into smooth knobs isolated by pitted surfaces due to desiccation 

of volatile-rich airfall deposits.

Crater size-frequency distributions suggest an Early 

Noachian to Early Hesperian age for mottled plains material. 

Based on superposition relations with ejecta from crater Greg 

and N(2) crater statistics, the age of mottled plains material 

is most likely Late Noachian. However, fluvial and eolian 

modification of these materials, as well as mantling by airfall 

deposits, suggests younger activity consistent with constraints 

from smaller diameter craters (for example, N(2)).

Vallis Materials

The floor of Waikato Vallis contains a deposit that is 

mapped as Waikato Vallis floor material (unit HNWVf). The 

deposit extends from the northernmost head of the canyon to the 

south, where the canyon terminates and has breached the rims 

of two nested craters. Waikato Vallis floor material displays 

smooth texture in THEMIS day IR images (fig. 4A). However, 

in high-resolution CTX and MOC images, parts of this deposit, 

especially within its central to southern extent, display fine-scale 

knobby and pitted textures (fig. 4C). Wrinkle ridges intersect the 

valley at several locations along the length of Waikato Vallis; 

these exposed ridges exhibit degraded morphology and are 
10

mantled with unit HNWVf. Many ridges exposed on the canyon 

floor are incised with notches at their topographic crests; the 

down-canyon side of the ridge contains smooth, high-albedo 

(relative to overall albedo of unit HNWVf) deposits on the 

canyon floor that are incised with shallow sinuous channels that 

initiate at these notches. Similarly, at several locations within 

unit HNWVf (unrelated to wrinkle ridges) there are smooth, 

bright deposits that are oriented parallel to the canyon walls and 

are incised with very shallow sinuous channels.

Waikato Vallis floor material likely consists of sediments 

eroded from the surrounding ridged plains material and 

emplaced as water moved through the canyon and eventually 

receded. The notched ridges, bright patches, and sinuous 

channels provide evidence for flow within the canyon. These 

features may be areas where transported sediment accumulated, 

such as within a bar. The overall fine-scale knobby and pitted 

texture is likely produced by eolian deflation of sediments. 

Crater size-frequency distribution statistics show that Waikato 

Vallis floor materials are Noachian in age; the presence of 

smaller diameter craters (N(2)) suggest Hesperian-aged 

resurfacing. Cross-cutting relations between Waikato Vallis 

and ridged plains materials indicates that unit HNWVf must 

be younger than both ridged plains and modified ridged 

plains materials, therefore we report a Late Noachian to Early 

Hesperian age.

Floor materials found in Reull Vallis display different 

surfaces within the morphologically distinct upper and lower 

reaches of the canyon. The upper reach, which is sinuous and 

displays terraces of the smooth plains (lower member) along 

its walls, contains materials mapped as older Reull Vallis 

floor material (unit HRVfo). These deposits transition from 

etched plains material where the head of Reull Vallis emerges 

from Eridania Planitia. Upper Reull Vallis floor materials 

display an overall smooth appearance at all image resolutions 

(figs. 6, 7). Older Reull Vallis floor material is generally 

smooth and is incised with narrow sinuous channels. Most 

channels are oriented parallel to the canyon walls, whereas a 

few meander from side to side along the canyon floor, and in 

places the channels are braided. A few channels converge at 

an ~2-km-wide canyon that downcuts into the main canyon 

of Reull Vallis (fig. 6). This convergence coincides with the 

location where the older Reull Vallis floor material ends and 

the younger Reull Vallis floor material (unit AHRVfy) begins, 

as well as approximately where the morphology of Reull 

Vallis changes from wide, sinuous, and terraced to narrow, 

straight, and steep-walled. Older Reull Vallis floor materials 

also display low-relief scarps that are oriented parallel to the 

canyon walls, suggesting layers exposed in the walls and (or) 

erosional terraces.

Older Reull Vallis floor material likely consists of 

sediments eroded primarily from upstream, with contributions 

from the adjacent plains and highlands. The sediments were 

transported by fluvial processes and deposited on the canyon 

floor. As the available water receded, these deposits were 

eroded and redistributed to form braided channels and sediment 

bars. Older Reull Vallis floor material contains few superposed 

impact craters ≥1 km in diameter (3 craters) and no craters 

≥5 km in diameter. N(2) data show an Early Amazonian age 

with significant uncertainty. From our assessment of the 

age of etched plains material (Late Noachian) and the upper 

member of the smooth plains material (Late Noachian/Early 

Hesperian), initial incision of Reull Vallis likely occurred 

in the mid-Hesperian, with most activity ceasing in the Late 

Hesperian. Overprinting the morphology of these materials is 

a texture characteristic of the mid-latitude mantling deposit 

(Mustard and others, 2001). In high-resolution images (HiRISE, 

CTX, and MOC), this deposit mutes the morphology of many 

craters and likely obscures many of the small-diameter craters 

superposed on the pre-mantle floor deposit. Older Reull Vallis 

floor material displays an ambiguous contact with etched plains 

material, where this unit begins within the headlands of Reull 

Vallis. Here, many of the surface textures and characteristic 

features (fine-scale knobs and pits, small sinuous and braided 

channels) are observed on both units in CTX images. Difficulty 

in identification of this contact is compounded by the presence 

of regional mantling materials on both units, and the exact 

location of the contact between units Npe and HRVfo may vary 

within a transitional zone extending several tens of kilometers 

to the east. From these constraints, we have designated the age 

of older Reull Vallis floor materials to be Late Hesperian.

The lower reach of the upper segment of Reull Vallis, 

which displays little sinuosity, a flat floor, and steep walls, 

contains younger Reull Vallis floor material. This deposit 

begins within the ~2-km-wide canyon incised within the 

main canyon of Reull Vallis (fig. 8) and continues west of 

the map area for several hundreds of kilometers (Mest and 

Crown, 2001, 2002, 2003). These materials display lineations 

and chains of elongated pits parallel to the canyon walls 

that are similar to those seen on most mid-latitude lineated 

valley-fill deposits (Squyres, 1978, 1979; Squyres and Carr, 

1986; Crown and others, 1992, 2006). Early work on lineated 

valley-fill deposits suggested they formed as interstitial ice/

water caused unconsolidated materials to undergo shear as 

the deposit flowed (Squyres, 1978, 1979; Squyres and Carr, 

1986), similar to flow in terrestrial rock glaciers (Wahrhaftig 

and Cox, 1959). However, recent work, with the help of 

improved topography and new orbital radar instruments (such 

as Mars Reconnaissance Orbiter Shallow Radar (SHARAD) 

and Mars Express Mars Advanced Radar for Subsurface and 

Ionosphere Sounding (MARSIS)), suggests lineated valley-fill 

deposits consist of a thin debris layer covering a massive ice 

core that results in glacial-type movement down-canyon (Head 

and Marchant, 2009; Head and others, 2010; van Gasselt and 

others, 2010).

Younger Reull Vallis floor material is believed to consist 

primarily of unconsolidated sediments, eroded primarily from 

upstream, but also from the adjacent plains and highlands that 

were fluvially transported and deposited on the floor of this part 

of the canyon. A portion of the deposit may consist of debris 

from the canyon walls emplaced via mass movements similar 

to debris aprons observed elsewhere in the highlands adjacent 

to Reull Vallis. Movement of the deposit down-canyon, and 

thus deformation of the surface of the deposit, was facilitated 

by interstitial ice/water or burial of a massive ice deposit. The 
11

observed lineations and elongated and deformed pits in unit 

AHRVfy are produced by differential shear of the debris/ice 

mass as it moved down-canyon (Squyres and Carr, 1986).

Previous analyses, based on crater size-frequency 

distributions and cross-cutting relations with adjacent plains 

units, have suggested the lineated floor material along the entire 

length of this part of Reull Vallis ranges from Late Hesperian 

to Early Amazonian in age (Greeley and Guest, 1987; Crown 

and others, 1992; Mest and Crown, 2001, 2002, 2003). In 

this study, younger Reull Vallis floor material displays only 

two superposed impact craters ≥500 m in diameter within the 

map area, and no craters ≥1 km in diameter, which we find is 

consistent with a Late Hesperian to Middle Amazonian age. 

Extending the age of AHRVfy into the Middle Amazonian 

compensates for the inclusion of mass-wasted debris in this 

deposit as debris aprons (see Surficial Deposits), which we 

believe were active during this time as well.

Surficial Deposits

Debris apron material (unit Ada) appears to have formed 

the youngest map unit. Exposures of this unit are found 

throughout the eastern Hellas region and are observed to border 

highland massifs, crater rims, and vallis walls (Crown and 

others, 1992; Crown and Stewart, 1995; Stewart and Crown, 

1997; Mest and Crown, 2001; Head and others, 2005). Massifassociated deposits (figs. 8, 12) have uniform or mottled albedo 

and lobate frontal morphologies and appear to be composed of 

multiple coalescing lobes. Crater-associated deposits (fig. 13) 

are relatively small and display mottled surfaces and arcuate 

to lobate fronts. In high-resolution images, massif- and craterassociated deposits display lineations and elongated pits. 

Lineations consist largely of ridge-and-furrow features, but 

some lineations are composed of closely spaced to merging 

elongated pits that are oriented roughly perpendicular to the 

proposed direction of flow. Deposits of debris apron material 

along the walls of Reull Vallis are morphologically different 

than massif- and crater-associated deposits and some vallisassociated deposits observed farther down-canyon (see 

Crown and others, 1992; Mest and Crown, 2002, 2003). Vallisassociated deposits (fig. 14) do not extend far from the wall and 

their fronts are steep and roughly parallel to the canyon wall; 

they are generally lower in albedo (in THEMIS VIS images) 

relative to adjacent plains and vallis floor materials, and in CTX 

and HiRISE images they display grooves perpendicular to the 

source wall and the areas of the deposits are superposed by or 

degrade into irregular pitted and knobby surfaces, usually on 

the deposit close to the source wall and along the base of the 

deposit front, respectively.

Deposits of debris apron material were interpreted to 

consist of unconsolidated sediments accumulated at the 

base of topographically higher terrains via mass wasting of 

highland massifs, crater rims, and vallis walls. Pits within 

massif- and crater-associated deposits may be from collapse 

of volatile-rich debris or degraded impact craters (Pierce 

and Crown, 2003). Lineations on Martian debris aprons are 

similar to features observed on terminal moraines on Earth 

and may indicate sorting of material within the flow (Squyres, 

1978, 1979). The overall morphology—lobate terminations, 

ridge-and-furrow texture, and elongated pits—of massif- and 

crater-associated debris aprons suggests downslope movement 

of the deposit. Similar to lineated valley fill, early work on 

debris aprons suggested mobility of these deposits was likely 

due to incorporation of water or ice (Squyres and Carr, 1986; 

Crown and others, 1992, 2006; Tanaka and Leonard, 1995; 

Mest and Crown, 2001). However, recent work utilizing image, 

topographic, and radar (SHARAD and MARSIS) data has 

suggested that these features may be more similar to debriscovered glaciers consisting of a thick (~10 m) ice core covered 

by a thin layer of rock and sediment (Head and others, 2005; 

Holt and others, 2008; Head and Marchant, 2009; Morgan and 

others, 2009; Head and others, 2010; van Gasselt and others, 

2010). Vallis-associated deposits in the map area appear more 

like talus deposits than typical mid-latitude lobate debris 

aprons. The irregular pitted and knobby texture displayed by 

parts of this deposit may be related to the mid-latitude mantle 

deposit that covers most terrains in this map area and is likely 

mass wasting from the vallis walls, as well as from the debris 

apron front.

Deposits of debris apron material contain few superposed 

impact craters on their surfaces (13 craters >500 m in diameter) 

and they are not areally extensive, resulting in limited age 

constraints. Except for the impact craters that superpose their 

surfaces, these deposits superpose all materials on which they 

occur, including highlands, plains, vallis floor materials, and 

ancient impact crater deposits. Examination of the crater sizefrequency distribution for the debris aprons in this map area 

shows that diameter bins in the 350–750 m range fit the slopes 

of the Martian isochrons (Hartmann, 2007) and correspond to 

Early to Middle Amazonian ages.

It should be noted that, although this material is not 

mapped, mid-latitude mantling deposits cover nearly all 

surfaces—highlands massifs, impact crater rims, floors 

and ejecta blankets, plains, vallis walls and floors, and 

debris aprons—south of ~30° in the map area and subdues 

characteristic morphologies. This deposit is not static and has 

undergone mass wasting from highland massifs, crater rims and 

walls, and vallis walls forming knobby deposits on the slopes or 

along the bases of these features and (or) viscous flow features 

(or lobate flows) along the interior walls of impact crater rims 

(Milliken and others, 2003), such as craters Greg and Fitzroy 

(Hartmann and others, 2003; Berman and others, 2009), and it 

fills alcove-headed channels incised along the interior walls of 

these craters (Berman and others, 2009). Mustard and others 

(2001) and Head and others (2003) estimated emplacement 

of the mid-latitude mantle to be Middle to Late Amazonian 

in age (~0.880 to 0.235 Ga) (Werner and Tanaka, 2011) with 

degradation of the mantle to be Late Amazonian (younger than 

~400 ka). We can use the age of the mantle to constrain the 

upper ages of most deposits in the map area. 
12

Crater Materials

Impact craters represent discrete events in the geologic 

record, and their ejecta deposits can serve as unique 

stratigraphic markers. Materials associated with impact 

craters—such as the materials that form their floors, rims, and 

ejecta blankets—are found throughout the map area and cover a 

significant portion of the surface (~49%, ~90,000 km2). Impact 

craters in this area range from the limits of image resolution to 

~70 km in diameter, and many have associated geologic units 

that have been mapped. Most craters have been designated a 

general unit name based on their morphology (fresh, moderately 

degraded, or highly degraded). However, some of the larger 

craters with extensive ejecta blankets, such as craters Greg, 

Fiztroy, Avarua, Dowa, and Pál (located just west of the map 

area), are assigned unique geologic labels and are arranged 

into assemblages.

Crater floor material (unit AHcf) is found on the floors of 

most craters in the map area, regardless of preservation state, 

and was previously mapped as smooth floor material (Greeley 

and Guest, 1987). In THEMIS day IR images, crater floor 

material appears smooth and featureless (figs. 6, 11A). However, 

in high-resolution images (MOC, THEMIS VIS, CTX, and 

HiRISE), these deposits show much detail on their surfaces. The 

margins of many of the crater floor deposits are lobate where 

they are in contact with the crater walls, and the edges of some 

deposits are superposed by debris apron material. Deposits of 

crater floor material display a variety of albedos and surface 

textures, including knobby, pitted, stucco-type, brain-like, 

ridge-crater fill (fig. 15B) (Carr and Schaber, 1977; Squyres, 

1978, 1979, 1989; Lucchitta, 1984; Squyres and Carr, 1986; 

Crown and others, 1992), are similar to textures observed in 

craters throughout the highlands, and suggest flow of volatilerich material across the crater floor, similar to debris aprons and 

lineated valley fill. Knobby and pitted textures may indicate 

eolian erosion of poorly indurated fine-grained materials 

causing deflation of these surfaces forming pits and (or) knobby 

remnants. Crater floor materials likely consist of (1) sediments 

derived from crater rim and wall materials, (2) wind-blown 

sediments derived from external sources (highlands, plains), 

and (or) (3) ice-rich airfall sediments forming the mid-latitude 

mantling deposits, especially in the southern part of the 

map area.

Few craters greater than 500 m in diameter (29 craters) are 

superposed on crater floor materials, but crater size-frequency 

distributions show that this unit ranges in age from Late 

Noachian to Late Amazonian. Some crater floor material, such 

as the lineated deposits, do not contain any superposed craters; 

these particular deposits may be composed of coalescing debris 

aprons, suggesting they formed during the same time as the 

debris aprons in this area, and these deposits of crater floor 

material are likely younger. Post-emplacement modification 

of many of these deposits could have also erased many small 

craters. Based on the range of surface morphologies and the 

potential range of sources for these materials, we designate an 

age of middle to Late Hesperian to Late Amazonian for crater 

floor materials.

Crater materials within the map area display various states 

of preservation, indicating degradation of these materials has 

been a continuing process throughout the evolution of this 

region. Well-preserved crater material (unit AHc3) shows the 

least amount of degradation and is characterized by pronounced, 

continuous crater rims that are elevated relative to the 

surrounding terrain and also by well-defined, continuous ejecta 

blankets. Many of these craters, especially within the plains 

(ridged and smooth plains materials) near Reull Vallis and in the 

southern part of the map area, have ejecta deposits that display 

single- or double-layer ramparts (Barlow, 2006), suggesting 

impact into volatile-rich materials (Squyres and others, 1992). 

Moderately degraded crater material (unit HNc2) shows 

moderate amounts of degradation, displays crater rims that 

exhibit minor relief above the surrounding terrain and may be 

discontinuous in places, and is characterized by discontinuous 

and (or) poorly exposed ejecta blankets. Highly degraded crater 

material (unit HNc1) shows the most degradation in the map 

area; discontinuous crater rims exhibit little to no relief above 

the surrounding terrain and ejecta blankets have either been 

completely eroded or mantled by younger materials, thereby 

obscuring all characteristic ejecta morphologies. Many craters 

of all types (well preserved to highly degraded) appear to have 

smooth, flat floors, containing crater floor material (unit AHcf).

Well-preserved, and moderately and highly degraded 

crater materials show generally consistent ages based on crater 

size-frequency distribution data and superposition relations. In 

the map area, these ages are designated as Late Hesperian and 

Amazonian for unit AHc3, Late Noachian to Late Hesperian 

for unit HNc2, and Early Noachian to Early Hesperian for 

unit HNc1. 

The impact events of craters Greg, Fitzroy, Avarua, Dowa, 

and Pál (west of map area) significantly influenced the geology 

of the region by distributing their ejecta materials over the 

surface, disrupting fluvial processes and peppering adjacent 

materials with their secondary craters, which makes these large 

craters stratigraphically significant in the geologic record of this 

area. The rim and ejecta materials were mapped as assemblages 

either individually (crater Greg), where their rims and ejecta 

could be separated from other materials, or combined (craters 

Fitzroy and Avarua, and Dowa and Pál), where their rim 

materials could be clearly identified but their ejecta blankets 

could not be separated.

The Greg assemblage consists of Greg crater material (unit 

HNGc) and Greg ejecta material (unit HNGe) that compose 

crater Greg (69.6 km in diameter; lat 38.6° S., long 113.0° E.) 

(figs. 6, 16A). Greg crater materials form the rim and central 

peak of Greg, which exhibit significant relief above their 

surrounding materials. All Greg-associated features (central 

peak, rim, floor) are or were superposed by the ice-rich midlatitude mantle. The north-facing wall of Greg is dissected 

by channels that generally head within alcoves located at or 

near the crater’s rim crest (fig. 16B); most of these channels 

tend to be filled with debris that exhibits knobby and chevron 

textures, suggesting the infilling material flowed down-channel 

(Hartmann and others, 2003; Berman and others, 2009). These 

channels appear to be incised in the crater wall material, 
13

predating the mantle deposit, which suggests the infilling 

material likely consists of the ice-rich mantle. The south-facing 

wall of Greg exhibits ice-rich mantle material that extends 

approximately halfway up the wall (fig. 16C); the deposit is 

dissected by narrow gullies that originate at the upper edge 

of the mantle. The south-facing wall, above the level of the 

dissected mantle, is also superposed by several viscous flow 

features (vff) (Milliken and others, 2003). The vff are generally 

composed of one or more moraine-like ridges that originate 

near the crater rim crest and merge to form lobes that narrow 

significantly downslope and terminate above the upper edge 

of the dissected mantle. The surface of the vff displays pits 

and elongated knobs and lineations oriented parallel to the 

inferred direction of flow. The mantle between the vff and 

dissected mantle is degrading into a knobby surface that extends 

downslope into the gullies, where the surface texture transitions 

to smooth fill that extends to the termini of the gullies.

Greg ejecta material forms several deposits located in the 

south-central part of the map area that surround Greg and are 

separated by ridges of mountainous material and Reull Vallis. 

Exposures of unit HNGe are generally characterized by smooth 

to hummocky surfaces, but some areas display etched texture 

or lineations. In CTX images (fig. 17), etched textures west 

of Reull Vallis are formed by yardang-like features oriented 

northwest-southeast (radial to Greg), and lineations found in 

the deposit north of Greg are oriented radial to Greg and may 

have resulted from emplacement of the ejecta deposit. Valleys 

are incised within all areas of the ejecta and display sinuous, 

shallow, and braided morphologies north and south of the crater 

and adjacent to Reull Vallis and short, straight, box-like canyons 

that intersect Reull Vallis west of Greg (fig. 7).

The Fitzroy/Avarua assemblage consists of Fitzroy crater 

material (unit HNFc), Avarua crater material (HNAc), and 

Fitzroy/Avarua ejecta material (HNFAe) that compose craters 

Fitzroy (38.3 km in diameter; lat 35.7° S., long 112.0° E.) and 

Avarua (47.4 km in diameter; lat 35.9° S., long 109.7° E.) 

(fig. 18). Fitzroy crater material and Avarua crater material 

form the rims of these craters, both of which exhibit significant 

relief above the surrounding terrain, but some areas of their rims 

are eroded and show minor relief. Fitzroy (fig. 19A) displays 

much of the same interior morphology that is observed in Greg, 

including a north-facing wall dissected with infilled channels 

that originate within alcoves at or near the crater rim (fig. 19B) 

and a south-facing wall displaying a dissected (probably icerich) mantle along the lower part of the wall, lobate vffs along 

the upper part of the wall, and degraded (knobby and pitted) 

mantle along the middle of the wall (fig. 19C). Only the eastern 

rim and part of the floor of Avarua are found in the map area. 

Although Avarua is similar in size, preservation state, and age to 

Greg and Fitzroy, the mantle deposit appears more continuous 

across Avarua and less modified than Greg and Fitzroy. For 

example, Avarua does not display the interior north-facing 

channels or south-facing gullies that characterize Greg and 

Fitzroy, and the vffs along Avarua’s south-facing wall show 

broad lobes along their length rather than tapering to a point 

(fig. 20). The west-facing wall displays smooth mantle on the 

rim and blocks along the wall and, similar to those observed in 

Greg and Fitzroy, the mantle appears to flow downslope and 

forms deposits exhibiting chevron-like texture between the 

blocks. One significant difference between Avarua and these 

other large craters is that an unnamed ~23-km-diameter crater 

impacted the southern rim of Avarua; ejecta from this crater was 

emplaced on the rim and wall of Avarua and possibly prevented 

mobilization of the mantle material via mass wasting.

Fitzroy/Avarua ejecta material forms a nearly continuous 

deposit that completely surrounds Fitzroy but only contacts 

Avarua along its east and north walls (in the map area), because 

of the superposed 23-km-diameter crater on its southern rim. 

The Fitzroy/Avarua ejecta deposit is characterized by a smooth 

to irregular surface and lobate edges; areas within the deposit are 

dissected by sinuous and braided channels that converge at or 

near Reull Vallis.

The Dowa/Pál assemblage consists of Dowa crater material 

(unit HDc) and Dowa/Pál ejecta material (HDPe) that compose 

craters Dowa (41.5 km in diameter; lat 31.7° S., long 110.3° E.) 

and Pál (78.7 km in diameter; lat 31.3° S., long 108.7° E.) 

(fig. 21). The rim of Pál is located outside of the map area so 

its rim materials were not mapped, but image analysis indicates 

ejecta from Pál is significantly blended with ejecta from Dowa, 

hence the combined ejecta deposit. Dowa crater material 

forms the rim of Dowa, which exhibits minor relief above the 

surrounding terrain, and parts of the rim are discontinuous 

(fig. 22A). Dowa exhibits terraces, resulting from slumping 

of large blocks of wall material along normal faults, along 

much of the interior wall; the terraces along the western wall 

are broad relative to the eastern wall, and all terraces display 

smooth surfaces, suggesting infilling by eolian or mass-wasted 

sediments. CTX and THEMIS VIS images show a smooth 

deposit that mantles most surfaces and has moved downslope 

in some areas to expose crater wall material (fig. 22B), 

especially (1) along the steeper parts of the wall where a lineated 

(lineations perpendicular to the wall) deposit forms at the base of 

the crater wall and (2) within gaps in terrace blocks that are filled 

with materials exhibiting flow textures.

Dowa/Pál ejecta material forms a continuous deposit that 

completely surrounds Dowa, and visual inspection of images 

west of the map area reveals that ejecta surrounds Pál as well. 

This deposit is characterized by an irregular surface, consisting 

of smooth areas that degrade into larger areas displaying pittedand-etched texture, revealing a surface that has been highly 

modified by eolian processes (fig. 23). This texture is formed 

partly by yardang-like features and wind-eroded depressions that 

are oriented northwest-southeast (radial to Pál); here the wind 

is preferentially eroding along the general ray structure of the 

ejecta contributed by Pál. This texture is also formed by clusters 

and chains of secondary craters, likely contributed by both 

Dowa and Pál, that also exhibit evidence for eolian modification. 

Secondary craters from both Dowa and Pál, forming chains 

and clusters, are also observed to extend across the topographic 

depression that defines the northern collapse zone of Waikato 

Vallis within the ridged plains, as well as the narrow channel that 

intersects the main canyon of Waikato Vallis.
14

Crater size-frequency distribution statistics and 

superposition relations are used to constrain the ages of the 

materials associated with craters Greg, Fitzroy, Avarua, Dowa, 

and Pál. Each of these craters has a similar preservation state 

and their geologic materials also contain similar numbers and 

size distributions of superposed impact craters. The secondary 

craters superposed on the ridged plains and Waikato Vallis 

that were produced by the Dowa and Pál impacts provide 

an important constraint on the ages of these features; crosscutting relations suggest that the Dowa and Pál impact events 

occurred in the Early Hesperian Epoch following ridged plains 

emplacement and modification. Crater statistics do not provide 

good constraints for assigning an age to the Fitzroy/Avarua 

assemblage; these craters are likely Late Noachian or Early 

Hesperian craters. Crater Greg clearly predates the formation 

of Reull Vallis and likely formed in the Late Noachian or Early 

Hesperian as well.

Geologic History

The geologic history of MTM –30247, –35247, and 

–40247 quadrangles east of the Hellas basin was determined 

from analysis of images (THEMIS, MOC, CTX, and 

HiRISE) and MOLA topographic data. Crater size-frequency 

distributions and cross-cutting and superposition relations were 

used to constrain the evolution of the region. The following 

sequence of geologic events was documented by identification 

of the various types of geologic materials, analysis of the 

geomorphic characteristics of those materials, and determination 

of their apparent stratigraphic relations. Although the following 

events are organized into specific time-stratigraphic sections, 

there is likely some overlap of events between sections that is 

not shown.

Early to Middle Noachian:

• Hellas impact event emplaced ejecta and uplifted 

blocks within highlands of Promethei Terra, forming 

mountainous material and basin-rim unit. Exposures 

occur as isolated knobs and massifs, as well as heavily 

cratered terrain, which contains remnants of larger crater 

rims from subsequent impacts (HNc1)

Late Noachian to Early Hesperian:

• Emplacement of ridged plains material throughout 

region by sedimentary and (or) volcanic processes

• Majority of wrinkle-ridge formation throughout map 

area occurs after emplacement of plains

• Collapse and erosion of unit HNpru forms Waikato 

Vallis; ridged plains material within and adjacent to 

Waikato Vallis underwent erosion by fluvial processes 

and forms unit HNprl. Sediments deposited on floor of 

canyon form unit HNWVf

• Water released from ridged plains material via Waikato 

Vallis flows south and is confined within Eridania 

Planitia; rim of crater Lipik, as well as other ancient 

craters and massifs of units Nm and Nh1, may have 

acted as barriers. Sediments deposited in basin form 

unit Npe

• Mottled plains material (unit Npm) emplaced in south 

part of map area consists of materials eroded from 

highland terrains interlayered with ejecta from highly 

degraded (unit HNc1) and moderately degraded (unit 

HNc2) impact craters

• Emplacement of lower (unit HNpsl) and upper (unit 

HNpsu) members of smooth plains material in southcentral part of map area

• Impact events form craters Greg, Fitzroy, and Avarua. 

Formation of these craters likely contemporaneous with 

shallow lake within Eridania Planitia as units HNGe and 

HNFAe superpose units HNpsu and Npe. Lobate ejecta 

suggests incorporation of volatiles; ejecta, rim, and wall 

materials of craters dissected by channels, suggesting 

erosion as water drained from these surfaces 

middle to Late Hesperian:

• Two large impact events form craters Dowa and Pál 

within ridged plains material in northern part of map 

area. Secondary crater fields from craters Dowa and Pál 

superpose northern depressed area and incised channel 

of Waikato Vallis and wrinkle ridges

• Lipik rim breached resulting in drainage of Eridania 

Planitia lake and erosion and downcutting into highland 

materials and units HNpsu and HNpsl to form 

Reull Vallis

• Sediments emplaced on floor of Reull Vallis form unit 

HRVfo, which is subsequently dissected by sinuous and 

braided channels as drainage receded

• Lower part of upper segment of Reull Vallis modified 

by collapse of plains, as well as fluvial erosion forming 

narrow canyon within floor of main canyon

mid-Hesperian to Amazonian:

• Impact craters of all morphologic types eroded; 

sediments emplaced on crater floors via fluvial erosion, 

eolian deposition, and (or) mass wasting of crater walls 

to form deposits of unit AHcf

• Volatile-rich sediments deposited on floor of Reull 

Vallis (unit AHRVfy) result in subsequent down-canyon 

movement and shear of deposit

• Impact craters with pristine morphologies form 

(unit AHc1)
15

Early to Middle Amazonian:

• Debris aprons (unit Ada) emplaced along walls of 

highland massifs, impact craters, and vallis walls via 

mass wasting

Late Amazonian:

• Volatile-rich sedimentary deposit emplaced (likely via 

airfall) across mid-latitudes within map area; some 

areas of deposit, especially on steeper slopes, became 

disaggregated and flowed down-slope, forming talus 

deposits along massifs of mountainous material or 

viscous flow features along walls of large craters

Acknowledgments

The authors would like to thank Tracy Gregg, Sharon 

Wilson-Purdy, and Ken Tanaka for their thoughtful and helpful 

reviews. This research was supported by grants from NASA’s 

Planetary Geology and Geophysics Program.

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Table 1. Crater size-frequency data and time-stratigraphic determinations for Reull Vallis region of Mars.[Units listed youngest to oldest as in Description of Map Units. N(1), N(2), N(5), and N(16) represent the cumulative number of craters >1, 2, 5, and 16 km in diameter/106 km2. Error = ± [(N1/2)/A] × 

106, where A = area in km2. – = no data; N() not calculated due to no superposing craters. Epoch ranges based upon crater counts using crater-density boundaries determined by Tanaka (1986) and Werner 

and Tanaka (2011); LA = Late Amazonian; MA = Middle Amazonian; EA = Early Amazonian; LH = Late Hesperian; EH = Early Hesperian; LN = Late Noachian; MN = Middle Noachian; and EN = Early Noachian. Epoch designations are based upon superposition relations and crater counts. See Description of Map Units for names of geologic units and unit subdivisions]

Unit

name

Unit

label

Area

(km2)

No. ≥1

(km)

No. ≥2

(km)

No. ≥5

(km)

No. ≥16

(km) N(1) N(2) N(5) N(16) Epoch range Epoch designation

Debris apron material Ada 2,428 1 1 1 0 412±412 412±412 412±412 – MN–LA EA–MA

Younger Reull Vallis floor material AHRVfy 280 0 0 0 0 – – – – EH–MA LH–MA

Older Reull Vallis floor material HRVfo 3,779 3 1 0 0 794±458 265±265 – – EH–LA LH

Waikato Vallis floor material HNWVf 14,853 52 21 8 3 3,501±486 1,414±309 539±190 202±117 EN-LH LN–EH

Ridged plains material, upper member HNpru 131,384 396 110 26 12 3,014±152 837±80 198±39 91±26 MN–LH LN–EH

Ridged plains material, lower member HNprl 17,815 58 25 8 3 3,256±428 1,403±281 449±159 168±97 EN–LH LN–EH

Smooth plains material, upper member HNpsu 51,780 82 27 14 5 1,584±175 521±100 270±72 97±43 MN–EA LN–EH

Smooth plains material, lower member HNpsl 24,788 9 7 3 1 363±121 282±107 121±70 40±40 LN–MA LN–EH

Mottled plains material Npm 21,416 62 26 13 2 2,895±368 1,214±238 607±168 93±66 MN–LH LN

Etched plains material Npe 35,997 61 22 9 4 1,695±217 611±130 250±83 111±56 MN–EA LN

Basin–rim unit Nh1 45,756 90 34 17 8 1,967±207 743±127 372±90 175±62 EN–EA EN–MN

Mountainous material Nm 69,875 247 94 43 16 3,535±225 1,345±139 615±94 229±57 EN–EH EN–MN

Crater floor material AHcf 5,706 6 1 1 0 1,052±429 175±175 175±175 – LN–LA EH–LA

Well-preserved crater material AHc3 27,117 60 24 6 1 2,334±301 933±191 233±95 39±39 LN–EA LH–LA

Moderately degraded crater material HNc2 9,556 23 7 2 0 2,407±502 733±277 209±148 – LN–EA LN–LH

Highly degraded crater material HNc1 25,255 110 30 11 6 4,356±415 1,188±217 436±131 238±97 EN–EH EN–EH

Ejecta material, Dowa/Pal HDPe 23,433 69 21 4 1 2,945±355 896±196 171±85 43±43 LN–LH EH

Crater material, Dowa HDc 20,347 1 1 0 0 147±85 49±49 – – EH–LA EH

Ejecta material, Greg HNGe 21,123 51 18 8 1 2,414±338 852±201 379±134 47±47 LN–LH LN–EH

Crater material, Greg HNGc 14,603 7 2 1 0 479±182 137±97 68±68 – EH–MA LN–EHEjecta material, Fitzroy/Avarua HNFAe 16,848 39 9 1 1 2,315±371 534±178 59±59 59±59 MN–LA LN–EH

Crater material, Fitzroy HNFc 13,497 1 0 0 0 74±74 – – – EH–LA LN–EH

Crater material, Avarua HNAc 14,276 7 2 1 1 490±185 140±99 70±70 70±70 MN–MA LN–EH

Crater and ejecta material, Dowa/Pal HDc + HDPe 23,433 70 22 4 1 2,987±357 939±200 171±85 43±43 LN–LH EH

Crater and ejecta material, Greg HNGc + HNGe 21,123 55 18 8 1 2,604±351 852±201 379±134 47±47 MN–EA LN–EH

Crater and ejecta material, 

Fitzroy/Avarua

HNFc + HNAc

+ HNFAe

18,572 40 9 1 1 2,154±341 485±162 54±54 54±54 MN–MA LN–EH
20

Table 2. Contact relations of geologic units in Reull Vallis region of Mars.

Unit

label Unit name

Contact relations1

Younger than Overlaps with2 Older than

Ada Debris apron material AHRVfy, HRVfo, HNWVf, HNpsu, HNpsl, HNprl,

Npm, Nh1, Nm, AHcf, HNc2, HNc1, HNGe

AHc3 –

AHRVfy Younger Reull Vallis floor material HNpsu, HNpsl, Nm AHRVfo Ada

HRVfo Older Reull Vallis floor material HNpsu, HNpsl, Nm, HNc1, HNFAe AHRVfy, Npe Ada, AHc3

HNWVf Waikato Vallis floor material HNpru, HNprl, Nm, HNc1 HNc2 Ada, AHc3HNpru Ridged plains material, upper member HNprl, Nm, HNc1 HNprl HN5f, AHcf, AHc3, HNc2, HNFAe, HDPe

HNprl Ridged plains material, lower member Nm, HNc1 HNc2 Ada, HNWVf, HNpru, AHc3

HNpsu Smooth plains material, upper member Nm, Nh1, HNc1 HNpsl, HNGe, HNFAe Ada, AHRVfy, HRVfo, Npe, AHc3, HNc2HNpsl Smooth plains material, lower member Nm, Nh1, HNc1 HNpsu, HNGe Ada, AHRVfy, HRVfo, Npe, AHc3, HNc2Npm Mottled plains material Nh1, Nm, HNc1 Npe Ada, AHc3, HNc2Npe Etched plains material HNpsu, HNpsl, Nh1, Nm HRVfo, Npm, HNGe, HNGc AHc3, HNc2Nh1 Basin-rim unit – Nm, HNc1 Ada, HRVfo, HNpsu, HNpsl, Npm, Npe, AHcf, AHc3, 

HNc2, HNGc, HNFAe

Nm Mountainous material – Nh1, HNc1 Ada, HRVfo, HNWVf, HNpsu, HNpsl, HNpru, HNprl, 

Npm, Npe, AHcf, AHc3, HNc2, HNGe, HNGc, 

HNFAe, HNAc, HDPe

AHcf Crater floor material HNpru, Nh1, Nm, AHc3, HNc2, HNc1, 

HNGc, HNFc, HDPe, HDc

– Ada

AHc3 Well-preserved crater material HNWVf, HNpsu, HNpsl, HNpru, HNprl, Npm, Npe,Nh1, 

Nm, HNc2, HNc1, HNGe, HNGc, HNFAe, HDPe

Ada AHcf

HNc2 Moderately degraded crater material HNpsu, HNpsl, HNpru, Npm, Npe, 

Nh1, Nm, HNGe, HNAc

HNWVf, HNprl Ada, AHRVfy, AHcf, AHc3

HNc1 Highly degraded crater material – Nh1, Nm Ada, HRVfo, HNWVf, HNpsu, HNpsl, HNpru, 

HNprl, Npm, AHcf, AHc3

HDPe Ejecta material, Dowa/Pal HNpru, Nm HNc2, HDc AHcf, AHc3HDc Crater material, Dowa – HDPe AHcf

HNGe Ejecta material, Greg Nh1, Nm, HNc1 HNpsu, HNpsl, Npm, Npe, HNGc Ada, AHcf, AHc3, HNc2

HNGc Crater material, Greg Nh1, Nm HNpsu, Npe, HNGe AHc3HNFAe Ejecta material, Fitzroy/Avarua HNpru, Nh1, Nm, HNc1 HNpsu, HNFc, HNAc HRVfo, AHc3HNFc Crater material, Fitzroy – HNFAe AHcf

HNAc Crater material, Avarua Nm HNFAe HNc2

1Contact relations for units that share in common one or more geologic contacts.2Exhibits one or more locations where contacts between units are topographically or stratigraphically equivalent.