Complex river terrace development in the Nenana Valley near Healy,

DALE F. RITTER Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901

ABSTRACT The valley of the Nenana River (Figs. 1 and 2), one of the major drainage-ways of the , has great importance in A complex sequence of river terraces in the Nenana Valley NARP because two of the oldest and most significant Early Man was examined to determine whether the surfaces were depositional sites are located in its foothill reach. Thus, the search for more and or erosional in origin. Of prime concern was the identification of older sites requires that considerable attention be given to detailing outwash derived from the Healy (early Wisconsin) and Riley Creek I (late Wisconsin) glaciations. Two terraces were formed during the Healy glacial cycle. The higher level represents the depositional surface of the Healy outwash. The lower level was formed during recession of the Healy ice when approximately 18 metres of that outwash was removed. The erosion was initiated when glacial Lake Moody, dammed behind the Healy moraine, spilled over the drainage divide and began excavation of the Nenana gorge. The surface of Riley Creek ][ outwash has been offset by faulting near the gorge mouth. Isolated remnants of that surface stand at different elevations, complicating the correlation of downvalley outwash with the Riley Creek I moraine. The evolution of the Nenana gorge is associated with spas- modic development of erosional terraces throughout post-Healy time as the Nenana River attempted to establish a new equilibrium condition. This erosional trend has been interrupted only by out- wash deposition during episodic expansion of the Riley Creek ice.

INTRODUCTION

River terraces are key elements in understanding local and regional geomorphic history. The sequence of terraces formed by the Nenana River was first mapped and described by Wahrhaftig (1958) in his classic report on the Quaternary geology of the Nenana Valley. Although the Nenana terraces probably represent the most complete record of Wisconsin fluvial and glaciofluvial events along the north flank of the Alaska Range, Wahrhaftig paid little atten- tion to their origin, as his pioneer study was concerned more with the age and distribution of deposits rather than their precise mode of development. Recently the Nenana terraces have taken on added significance because of the growing search for evidence of the first humans to occupy North America. As part of that venture, the National Geographic Society and the National Park Service jointly •conceived the North Alaska Range Project (NARP) to integrate the glacial geology, palynology, geomorphology, and archeology in the northern foothills region of the Alaska Range (Fig. I). and Zone A the foothills area.

Geological Society of America Bulletin, v. 93, p. 346-356, 7 figs., 2 tables, April 1982.

346

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Figure 2. Map showing terraces and deposits of Wisconsin age in the study area. Deposits and terraces younger than Riley Creek I are not shown.

bly one of the most complex to be found anywhere." This is perhaps a masterful understatement describing the remains of a geomorphic history so complicated that it defies total comprehension. This study deals with the origin of part of that sequence. Spe- cifically it treats the orgin of the early Wisconsin (Healy) terraces, the identification of gravel representing the initial influx of late Wisconsin (Riley Creek) outwash, and the origin of terraces formed between the early and late Wisconsin stades. As such, this paper is a small part of the total NARP effort and represents a refinement of Wahrhaftig's original observations.

Physical Setting

The Alaska Range is a 960-km-long, arcuate topographic feature extending west and southwest from the Canadian border to the Aleutian Range. Most of the area studied lies within the north- ern foothill belt of the Alaska Range (Fig. 1). In this area, the foothill zone is ~ 32 km wide in a north-south direction and consists of parallel to subparallel, east-trending ridges and valleys crossed by superposed north-flowing rivers emerging from the Alaska Range. The ridges are -915 to 1,525 m in altitude, and the valley floors are 300 to 760 m in altitude (Wahrhaftig, 1958). The very southern end of the study area is located about 8 km north of the entrance to Mount McKinley National Park. Mount McKinley (elevation 6,194 m), the dominant feature of the Alaska Range, lies south and west of the study area. The northern boundary of the study area approaches the southern limit of the Tanana Flats, or Lowlands, which begin near Ferry, Alaska (Fig. 1), and slope gently from the foothill belt north to the . Major streams flow 35 to 80 km across this zone before they join the Tanana River, a tributary of the . The entire study area is located on the Healy C-4, D-4, and D-5 and the Fairbanks A-5 quadrangle maps (U.S. Geological Survey 15' topographic series). The Nenana River heads in the Nenana Glacier on the south side of the Alaska Range but turns sharply to the north through a glacially scoured gap, where it flows into and through the Alaska Range. As the river enters the foothill zone, it occupies a deep gorge excavated during and after early Wisconsin time. The events asso- ciated with the development of the Nenana gorge are intimately related to the history of terrace development downstream from the gorge mouth. Underlying bedrock that is of significance in this study consists of several major units. South of Healy Creek (Figs. 1 and 2), the area is underlain by the Birch Creek Schist, probably Precambrian its Wisconsin history. As part of that goal, it became imperative to (possibly early Paleozoic) in age. Most of the formation consists of know which of the terrace gravels in the downstream portion of the highly contorted and foliated quartz-sericite schist, but locally it valley could be traced upvalley into moraines located in the moun- contains layers of quartzite, black carbonaceous schist, marble, and tains. These gravels, being glaciofluvial in origin, have significance sericite-calcite schist. It is characterized by ubiquitous quartz veins vis-à-vis the regional glacioclimatic sequence. that generally follow the local schistosity. In addition, numerous A major stumbling block to achieving these objectives is the mafic dikes, ranging from 1.5 to 5 m in thickness, intrude the schist. incredibly complex terrace sequence located near Healy, Alaska, North of the gorge, the region is underlain primarily by where 14 terrace levels were recognized by Wahrhaftig. In fact, this Tertiary (Oligocene-Miocene) coal-bearing formations and a thick sequence was characterized by Wahrhaftig (1958, p. 48) as "proba- Teritary conglomerate called the Nenana Gravel. The coal-bearing

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rocks are found in a series of synclinal basins that cross the north- TABLE 1. ESTIMATED AGES OF LATE WISCONSIN ern foothill zone. The group consists of conglomerates, sandstones, GLACIATIONS, NORTH FLANK OF THE ALASKA RANGE siltstones, shales, and coal. The rocks are faulted and folded, and local unconformable contacts are present. Glacial events Age range (yr B.P.) The Nenana Gravel, the dominant formation north of the gorge, consists of conglomerates and sandstones, along with minor Riley Creek 1 25,000-17,000 units of claystone and lignitic coal. Pebbles in the conglomerate Riley Creek II 15,000-13,500 layers normally range from 2 to 10 cm in diameter, but the particle Riley Creek III 12,800-11,800 size increases in the upper part of the formation where clasts up to Riley Creek IV 10,500- 9,500 45 cm are common. At the top of the formation, huge boulders, as much as 12 m in diameter, are present. Note: data from Ten Brink and Waythomas, 1979, personal commun. The Nenana Gravel complicates the geomorphology in the val- ley. The clasts in the gravel are similar in composition to those position. Wahrhaftig also recognized the presence of terrace levels included in the Quaternary terrace gravels, and they are commonly below his Carlo terrace, but he did not directly attribute them to a coated with a yellow-orange, iron oxide stain. In some situations, it glacial event. is not easy to distinguish poorly consolidated Nenana Gravel from Ten Brink and Waythomas recently defined a fourfold division the weathering zones produced on the younger terrace gravels. In of late Wisconsin glaciation, the Riley Creek I, II, III, and IV addition, the Nenana Gravel is rather friable and, therefore, sus- stades, based on the morainal sequence and age dates from a ceptible to lateral erosion by local rivers. Thus, it is often difficult to number of valleys in the Alaska Range (see Table 1). The Riley be certain that Quaternary gravels, presumed to be river deposits Creek glaciation of Wahrhaftig (1958) includes both the Riley having a mountain source, are not in reality Nenana Gravel that has Creek I and II stades. The moraine of Wahrhaftig's Carlo read- been slightly reworked by a laterally migrating Nenana River. vance is the Riley Creek III moraine of Ten Brink and Waythomas, Other bedrock units are present in the region and, in fact, are but the terraces mapped collectively as Carlo by Wahrhaftig have usually represented in the pebble assemblages of both the Nenana been subdivided by Ten Brink and Waythomas as Riley Creek II Gravel and the Quaternary terrace gravels. A more detailed account and III in age. The Riley Creek IV glaciation is younger than Wah- of the regional bedrock geology has been presented in a number of rhaftig's Carlo event, but terrace levels in the lower Nenana Valley studies by the U.S. Geological Survey (Capps, 1940; Wahrhaftig, cannot be definitively correlated with a Riley Creek IV stade. 1951, 1958, 1968, 1970a, 1970b, 1970c; Wahrhaftig and others, 1951). The reader is referred to those reports for details of the THE NENANA TERRACES geological setting. The major objective of this study was to correlate downvalley Review of Wisconsin History terrace remnants with the glacial sequence preserved near the entrance to Mount McKinley National Park. Any such correlation The Quaternary history of the Nenana Valley was first inter- involves two difficult interpretive ingredients: (1) to determine preted by Wahrhaftig (1958). Briefly summarized, Wahrhaftig's which terrace levels were erosional in origin and which were formed analysis of the Wisconsin history is as follows. The Healy glaciation by deposition of Healy or Riley Creek outwash and (2) to trace the occurred in early Wisconsin (possibly Illinoian) time, when the ice level determined to be the surface of Riley Creek I outwash into and extended down the Nenana Valley to the present site of the town of through the gorge, thereby connecting the downvalley terrace with Healy (Figs. 1 and 2). Two separate advances of Healy ice were the terminal zone of the late Wisconsin moraine. suggested by Wahrhaftig in order to explain the two Healy terrace Identification of glaciofluvial terraces was especially critical levels downstream from the moraine. At the culmination of the first near Healy, where 14 terrace levels were recognized by Wahrhaftig advance, the Healy glacier constructed a terminal moraine imme- (Figs. 3 and 4). Because of the limited number of distinct Wisconsin diately west of Healy and deposited very thick outwash, the surface glacial events, it seems certain that some of the terrace surfaces at of which subsequently became a well-defined terrace. Following the Healy are depositional in origin (result from glaciofluvial filling) initial advance, the Healy ice retreated, and the moraine was and some are erosional (fill cut or rock cut). removed by the Nenana River. In its second advance, the glacier Erosional terraces, often referred to as rock-cut or strath terminated at the same position as the first, constructed a complex terraces, are usually formed by rivers in temporary equilibrium. moraine and deposited more outwash. The moraine dammed the During this equilibrium phase, the river is presumed to be adjusted river and created a glacial lake (Moody) approximately 122 m deep. with regard to its load and discharge conditions and, therefore, The lake eventually drained to the northeast over a bedrock lip near migrates laterally across a stabilized valley bottom (Mackin, 1937, Garner (Figs. 1 and 2) and proceeded to downcut at that position, 1948). As lateral migration proceeds, the river truncates underlying thereby placing the river in its present location. bedrock while simultaneously depositing a thin veneer of point bar The Riley Creek glaciation of late Wisconsin age extended to gravel on top of the beveled bedrock surface. Thus, an erosional the present site of the McKinley Park Station, although later work terrace should be underlain by a thin sheet of channel sediment (Thorson and Hamilton, 1977) suggests that the ice may have spread evenly across a flat, laterally eroded rock surface. advanced farther downvalley. The Riley Creek glacier apparently Precisely how thick the gravel sheet of an erosional terrace can fluctuated over a distance of several miles before receding upvalley. be is a matter of interpretation because it varies with the character- After the Riley Creek ice had almost completely dissipated, a minor istics of each river. By definition, however, the gravel can be no glacial expansion, called the Carlo readvance, occurred. At the thicker than the depth to which the river can scour during maxi- culmination of this event, the ice front stood about 4 to 5 km north mum flow events. Wolman and Leopold (1957) suggest that natural of Carlo, or about 14 to 15 km south of the Riley Creek terminal alluvial channels are probably scoured to a depth 1.5 to 2 times the

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depth of flow attained during a flood. Measured depths of scour for Nenana. Because the Nenana has a broad, low, floodplain surface a variety of rivers (Leopold and others, 1964) suggest that, in the approximately 3 m above the channel floor, it is probable that most absence of unusual geological conditions, depth of scour during discharge above bankfull will be accommodated by an increase in floods is less than 9 m for rivers similar in size to the Nenana. stream width rather than depth. Mackin (1953) proposed a maximum thickness for gravel veneer on With the above in mind, terrace levels near Healy were initially rock-cut terraces in the Colorado piedmont to be about 5 m, designated as erosional or depositional on the basis of their gravel although the rivers he discussed were probably smaller than the thickness. Those terraces capped by gravel thicker than 6 m were tentatively considered to be depositional, and those having less than 6 m were tentatively classified as erosional (Table 2). These criteria are often unsatisfactory at great distance from the ice terminal zone because outwash commonly thins downvalley to thicknesses less than these arbitrary limits. In addition, there is every reason to believe that a body of outwash can itself be terraced by a laterally eroding river at some time after glaciofluvial deposition has ceased. In such a case, the terrace may be underlain by a very thick gravel mass, but the tread is erosional in origin. Such a terrace tread would have been cut by a river having a different load/discharge balance than that which originally deposited the gravel. Thus, because thickness is a fallible criterion of terrace origin, other types of geo- morphic evidence were also used to make an initial interpretation (Table 2). After tentative decisions had been made concerning which ter- race levels in the Healy area were depositional in origin, the terrace deposits were classified according to weathering characteristics. Weathering parameters seemed to be consistent, within reasonable limits, throughout the Nenana Valley. For example, depth of oxida- tion and disintegration of granite boulders in terrace deposits of a given age were similar regardless of whether they occurred in the downvalley or upstream segments (Chroback, 1980). Therefore, (A) after identifying a terrace as depositional, a relative age was first assigned on the basis of its topographic and geomorphic position with respect to the Healy moraine located west of Healy. A final relative age designation was based on the weathering of each de- posit. Correlation of the downvalley terraces through the gorge to the upstream terraces and Wisconsin moraines was accomplished on the basis of weathering combined with geomorphic relationships.

Terraces of Healy Age

The oldest and highest Wisconsin terraces begin immediately west of the town of Healy, where outwash emanates from the con- structional topography of the Healy moraine (Fig. 2). The large moraine marks the margin of Healy ice and apparently fills the pre-Healy, Nenana Valley. The Healy terrace levels and outwash underlying the terrace treads are easily traced down the Nenana Valley to Ferry, and they have been mapped by Wahrhaftig (1958, 1970c) northward for another 24 km. As noted in all previous investigations, elevation of the Healy gravel is different on the east and west sides of the present river valley, leading to distinction of a high and low Healy terrace. (B) The low Healy terrace is found only on the west side of the Figure 3. (A) Map of terrace levels near Healy, Alaska. valley. Between Dry Creek and Panguingue Creek (Figs. 1 and 2), it Numbers are those of Wahrhaftig (1958) with the highest number stands 18 to 24 m lower than the high Healy terrace. The high level representing the highest and oldest terrace. Levels indicated by (x) on the west side of the valley is a narrow terrace remnant stretching are parts of the Riley Creek I outwash surface as interpreted in this northward from Panguingue Creek for almost 3 km (Fig. 2). This report. H is the town of Healy, AK; and G is Garner, AK (modified level stands almost 485 m in elevation and certainly matches the from Wahrhaftig, 1958). (B) Aerial photo of the area shown in (A). high Healy surface on the east side of the valley. The downstream A = Healy-Riley Creek interstadial erosoional terraces; B = fan of edge of this small remnant marks the last location of two juxta- Riley Creek I age deposited by Dry Creek; C = Healy moraine; posed Healy levels. Farther north, only one Healy terrace level is D = mouth of Nenana gorge; E = erosional terrace of Healy age; present in the Nenana Valley, and it appears to be the downstream H = town of Healy; HC = Healy Creek. continuation of the high Healy terrace preserved on the east side of

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Location Thickness Proposed age ami origin Remarks

.7 km upstream of Dry Creek archeol. site > 30'(> 9 m) Healy outwash Sec. 11, T. 11 S., R. 8 W.; Healy D-5 quad., 140'(42 m) Healy outwash east side of river West side Nenana R. at gorge mouth 150'(45 m) Healy erosional Lower level cut into thick Healy outwash. Healy weathering. Higher Healy terrace 50' above this level East side Nenana R. across from mouth of 153'(46 m) Healy outwash Healy weathering Panguingue Cr. East side Nenana R. 800 m south of above 158'(47 m) Healy outwash NW1/4, Sec. 27, T. 11 S., R. 8 W. 30'(9 m) Healy erosional Lower Healy terrace; edge of pre-Healy Nenana Valley Sec. 19, T. 12 S., R. 8 W„ Healy D-5 quad. > 75'(> 23 m) Healy erosional Terrace cut into Healy moraine; well data thickness S1/2, Sec. 34, T. 11 S., R. 8 W. 24'-56'(7-17 m) Healy outwash High Healy terrace. Outwash pinching out at pre-Healy valley side Parks Highway roadcuts between Dry Creek 92-112'(28-34 m) Healy erosional Upper part of Healy fill missing; some fan and Panguingue Creek debris possible 3/4 km N. of Healy RR station along tracks 45'(I4 m) RC-I outwash T, surface of Wahrhaftig covered by thin fan debris from Dry Creek Sec. 3, T. 11 S., R. 8 W.; Healy D-5 quad.; 25'(8 m) RC-I outwash Topographic position below Healy terrace east side river Sec. 27, T. 10 S., R. 8 W.; Fairbanks quad.; 15'(5 m) RC-I outwash south of Ferry rr. bridge Intersection of Sheep Cr. and AK RR, 200 m 30'-35'(9-l 1 m) RC-I outwash Terrace gravel overlies Lake Moody clays and south of Moody is overlain by thick fan 1 km north McKinley Park entrance; west >30'Ç>9 m) RC-I outwash Terrace gravel overlies Lake Moody clays. side Nenana R. RC-I weathering

South side Healy Cr.; E. side Nenana at 51'( 15 m) RC-I outwash T8 level of Wahrhaftig mouth of gorge 200 m north of rr. tunnel at Garner; mile 356.6 30'(9 m) RC-I outwash Weathering and topographic position of RC-I Landslide exposure west side Nenana R. at 56'( 17 m) RC-I outwash Weathering RC-I on north side of fault at gorge mouth of gorge mouth. T|0 level of Wahrhaftig 1 km west of Healy 6'-12'(2-4 m) Healy/RC-I erosional Orange-stained sand and gravel at 12' depth interpreted as Nenana Gravel in this study. T,, level of Wahrhaftig

South side Healy Cr.; east side Nenana 8'(2 m) Healy/RC-I erosional Equivalent to Wahrhaftig T]0; weathering intermediate between Healy and RC-I

MILES-ALASKA RR Figure 4. Longitudinal profiles of terrace surfaces between Moody and Ferry, Modified from Wahrhaftig (1958).

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valley. Thus, it is important to stress that evidence for a low Healy Origin of Healy Terraces terrace exists only between the Nenana the gorge and the Ferry railroad crossing. At Ferry, the low Healy terrace on the west side Wahrhaftig (1958) suggested that the two Healy levels de- of the valley has apparently merged with the high Healy level on the scribed above probably require two phases of Healy glaciation, but east side of the valley (Fig. 4). Clearly, the high Healy terrace has a he entertained the possibility that they might have been formed steeper gradient than the low Healy surface, a fact pointed out by during the same episode of filling and cutting. No evidence in the Wahrhaftig (1958). present study demands separate episodes of glaciation as progeni- Healy outwash is extremely thick. On the east side of the val- tors of the high and low Healy terraces. If anything, the Healy ley, it was measured as 47 m thick directly across the valley from the terraces and their characteristics are more easily explained as the mouth of Panguingue Creek (Fig. 2; Table 2). This thickness seems result of one major Healy glacial cycle. to be fairly characteristic for east-side Healy outwash. On the west Distribution of Healy terraces is directly related to creation of side of the valley between Dry Creek and Panguingue Creek, the Lake Moody and the Nenana gorge, phenomena explicitly de- gravel under the low Healy surface averages about 31 m thick where scribed by Wahrhaftig (1958). The position of the Lake Moody beds exposed in cuts along the Parks Highway. There is reason to shows that the Nenana gorge postdates the Healy ice advance and believe, however, that some of that thickness may be caused by the filling of Lake Moody. Excellent evidence of this is presented by .indistinguishable.alluvial-fan debris deposited on top of the original Wahrhaftig (1958) and is probably best exemplified in the valley of terrace surface. Perhaps the most revealing gravel thickness was Bison Gulch, a minor tributary located west of the Nenana River measured where the first northern tributary of Panguingue Creek between Garner and Moody (Figs. 1 and 2). Here, the Lake Moody has incised into the high Healy outwash. Here the elevation of the beds are found 0.5 km west of the present Nenana River. Impor- high Healy surface matches almost exactly its counterpart on the tantly, as pointed out by Wahrhaftig (1958, p. 35), the lake beds are east side of the valley. The gravel, however, is considerably thinner separated from the present Nenana River in the gorge by a ridge of in the west side exposures, varying from 7 to 17 m thick (Table 2). It Birch Creek Schist. Obviously this remnant of Birch Creek repre- appears certain that the gravel thins rapidly toward the west, and sents the east side of the old Nenana Valley. In fact, the eastern thus, this zone probably marks the lateral margin of the old Nenana margin of the ancient Nenana Valley is defined by the distribution Valley. The surface of the high Healy terrace is 15 to 18 m higher of the Lake Moody beds. North of Moody station, the lake beds are than the low Healy terrace, which is underlain by only 28 m of in all cases located on the west side of the present Nenana River. As gravel where the Parks Highway crosses Panguingue Creek, a discussed above, the lake deposits are held in on their eastern side direct-line distance of only 760 m from the high Healy exposures by residual knobs of Birch Creek Schist, indicating that the present (Fig. 2). Thus, the combined thickness of the low and high Healy Nenana Valley postdates the beginning of deposition in Lake terrace deposits near Panguingue Creek is approximately equal to Moody and was carved through the bedrock that formed the east- the outwash thickness beneath the high Healy terrace on the east ern divide of the original valley. Thus, geomorphically it would be side of the valley (Fig. 5). difficult, if not impossible, for Healy outwash to exist in the gorge West and south of the town of Healy, several significant Healy because Healy outwash deposition preceded lake-bed deposition, terraces are preserved (Fig. 2). A pronounced, crescent-shaped ter- and gorge excavation occurred during and after lake-bed race remnant is located 3 km west of Healy. On the basis of weather- deposition. ing criteria, the material underlying this surface is definitely Healy The Nenana gorge is a portion of the valley with almost verti- in age, but the tread was produced by lateral erosion into the cal rock walls rising 60 to 90 m above the present river. The narrow moraine rather than representing the surface of aggraded outwash. gorge, only about 150 m wide, represents a distinct topographic Two kilometres south of Healy, two terrace levels, definitely Healy departure from the broader Nenana Valley both upstream and in.age, remain on the west side of the Nenana River (Fig. 2). The downstream. Everywhere in the gorge, the river is entrenched two levels apparently correlate with a similar sequence on the east deeply in the Birch Creek Schist, and whatever alluvium is present side of the river (south side of Healy Creek) where a small patch of occurs as river gravel capping small terrace remnants, or as alluvial- Healy ice shed outwash down the ancient Healy Creek upstream fan debris. from its confluence with the pre-Healy Nenana River. There are no unequivocal Healy outwash deposits in the gorge.

Figure 5. Schematic representation of the high and low Healy terraces along an east-west cross section of the Nenana Valley near Panguingue Creek. Solid line indicates position of modern topography along the valley sides. Section seen facing North.

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Wahrhaftig mapped one isolated patch of Healy outwash on the alluvial fans that forced the river to the eastern edge of the valley. It east side of the Nenana River approximately 1.6 km southeast of is difficult to understand how coarse-grained gravel requiring high Garner (Fig. 2; SE!4, Sec. 5, T. 13 S., R. 7 W.). This remnant stands fluvial energy for transport can be moved passively across flat-lying at an elevation slightly higher than 450 m. It is not clear how Lake lake silts and clays without eroding that surface. In addition, if a Moody clays could be deposited directly across the valley at an Healy outwash plain was developed across the lake-clay surface elevation greater than 500 m without covering the gravel. I assume, prior to alluvial-fan development, it must have extended from the therefore, that the gravel postdates Lake Moody and is probably southernmost position of Lake Moody clay deposits to Garner. Riley Creek in age. This outwash, therefore, should be preserved from near outwash may exist directly west of Garner as mapped by McKinley Park to Garner. Actually, no Healy outwash is found in Wahrhaftig (1970a). These gravels, standing above 500 m in eleva- that segment of the valley, and gravels overlying Moody clays are tion, occur in the original Nenana Valley and not in the gorge. They predominantly Riley Creek I in age (Wahrhaftig, 1958). Thus, all therefore represent material deposited prior to the excavation of the traces of Healy outwash had to be removed prior to Riley Creek gorge and have no bearing on the geomorphic history of that event. time, an unlikely occurrence. The time and mechanism of forming the Nenana gorge are 4. It is not possible to utilize the deposits assigned here to Riley essential factors in understanding the Wisconsin terraces downval- Creek as the proposed outwash plain existing prior to gorge forma- ley. Wahrhaftig's classic work (1958, p. 44) provides the basic tion, because Riley Creek I outwash can be traced entirely through ingredients for understanding this event. the gorge. This indicates that the gorge segment already existed before the Riley Creek 1 event, and therefore, could not be part of the alluvial sequence that triggered gorge erosion. Glacial Lake Moody drained northeastward over a bedrock lip near Garner. When the lake was completely filled with sediments, the Nenana River It seems probable that Lake Moody did not totally fill with began building an outwash plain across them. The river, which formerly had sediment. Instead, sediment accumulation partially filled the lake at been unable to erode the bedrock lip because it had not carried much the same time that the level of the overflow channel was being abrasive material, probably cascaded down a bedrock slope near Healy. lowered. However, once the outwash plain had been built, the river began to move more coarse material and to erode vigorously the bedrock barrier. At the Damming of Lake Moody and initial cutting of the Nenana same time, streams emerging from canyons on the west side of the glacial gorge were events leading to the creation of the low Healy terrace. gorge were building alluvial fans across the lake deposits and the outwash The sequence of events forming the two Healy terraces is inter- plain. As no comparable streams emerged from the east side of the gorge, preted as follows: the river was forced to flow against the bedrock wall of the gorge along the east side of the outwash plain. As down-cutting progressed, the river's, 1. The Hea.ly glacier advanced to a position west of the present course was superposed on the bedrock from Moody northward to Garner town of Healy and created a large moraine that choked the old and for a short distance near mile 350 (see Fig. 10). Elsewhere, the river was Nenana Valley. Some thin ice spilled over a low bedrock divide into able to cut its channel in the deposits of glacial Lake Moody. the present valley of Healy Creek, where it constructed a small stagnant-ice moraine that Wahrhaftig refers to as "kame moraine." A critical element of Wahrhaftig's hypothesis is that no down- Thick outwash underlying the high Healy terrace was deposited cutting of the gorge could occur prior to complete filling of the lake during the glacial maximum, both downstream from the main with sediment. Although Wahrhaftig's interpretation is viable, a moraine and from the spill-over moraine on the south side of Healy different interpretation is at least equally possible. There seems to Creek. be no overriding reason that a lake spilling over a bedrock thresh- 2. Recession of Flealy ice created a large lake (Lake Moody), old cannot accomplish erosion even if it is devoid of abrasive which overflowed at the lowest outlet elevation. The lake did not load at the point of overflow. For example, such a phenomenon outlet through the Healy moraine because the morainal topography was recognized by Gilbert (1890) where Lake Bonneville overflowed in the pre-Healy Nenana Valley is still intact and complete, indicat- through Red Rock Pass, eventually producing vertical entrench- ing that no breaching occurred. Clearly, the lake spilled over ment and lowering of the lake level by more than 110 m. through the bedrock sag near Garner and initiated the downcutting Admittedly, in that case, the initial downcutting occurred in allu- that culminated in the carving of the Nenana gorge. vium and proceeded through relatively nonresistant rocks. In the 3. Water spilling from Lake Moody into the valley of Healy Nenana Valley, entrenchment took place in the more resistant rocks Creek carried only the load it picked up by erosion of the spill-over of the Birch Creek Schist. Even so, it is possible that erosion of the channel. As a result, the water lost little energy in transporting load threshold occurred simultaneously with deposition of the Lake and possessed considerable erosive power as it moved downvalley. Moody clays and without the use of abrasive load. The arguments The spill-over water was capable of eroding the outwash at the for this interpretation are as follows: mouth of the Nenana Gorge, forming the lower Healy surfaces at 1. The location of the overflow channel coincides with an that position. Farther downstream, the spillover lake water carved, intense fracture zone of undetermined width. This sheared zone was by lateral erosion, the crescent-shaped terrace on the east side of the bound to be less resistant than normal Birch Creek Schist and thus main Healy moraine (Fig. 2) and removed 15 to 18 m of gravel from susceptible to accelerated hydraulic erosion. It is even likely that the the Healy outwash downvalley from the moraine. Thus, the low position of the north-flowing tributary into which Lake Moocly Healy terrace on the west side of the Nenana Valley represents an waters were released was controlled by the fracture zone. erosional surface cut into the original fill that was deposited during 2. The lake must have released considerable amounts of water, the Healy glacial maximum (Fig. 5). The lower Healy surface, that is, the meltwater discharge entering the south end of the lake therefore, stands ~ 18 m below the high Healy terrace level near the would have to be balanced with that leaving the overflow channel. moraine. Interestingly, the lower level gradually merges down- 3. It is not clear what the age of the outwash layer is that covers stream with the higher level, because the relatively sediment-free the Lake Moody clays. I assume Wahrhaftig is suggesting that these water draining Lake Moody eroded at a lower gradient than the are Healy outwash gravels that overlie the clays and underlie the slope on the outwash surface.

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The absolute time involved in this sequence of events is min- ravine several hundred metres south of the Healy bridge. The ravine imal. This argument is supported by the fact that there is no tangi- was cut by and is occupied by an extremely small creek, which was ble difference in the weathering characteristics developed on the capable of such notable entrenchment because of the weakness in gravels beneath the Healy erosional and depositional levels. The the bedrock along the fault. The bedrock and overlying gravel on major influx of outwash that formed the high Healy terrace the north side of the ravine appear to be upthrown ~3 m. occurred during the glacial maximum, and the erosion creating the A second fault zone is proposed at the mouth of the gorge (Fig. low Healy terrace occurred during recession of the Healy ice. 2). This fault zone is not recognizable as a surface lineament, but Actual filling of the lake was probably accomplished quickly, instead in revealed by the following observations: depending on how much meltwater was released from the receding 1. The trace of the zone underlies two major landslides that and stagnating ice mass. Overflow and erosion began as soon as the were identified by Wahrhaftig and Black (1958) and Wahrhaftig lake filled to the level of the sag in the bedrock divide. (1958, 1970a). One slide is located near the north side of the Alas- kan Railroad tunnel through the Birch Creek Schist 1 km north of Terrace of Riley Creek I Age Garner. The second involves the sliding of Riley Creek gravel along the tracks in Section 29, T. 12 S., R. 7 W., about 1.6 km south of One of the most difficult goals to achieve in this study was to Healy (Fig. 2). identify the outwash surface of the Riley Creek I stade. Determin- 2. At the northeast end of the landslide involving the Riley ing which of the 14 levels at Healy was the surface of the Riley Creek gravel, a distinct zone of alteration is present in the Birch Creek I outwash and tracing that level upstream through the gorge Creek Schist underlying the gravel. The zone contains a central core proved to be a challenging task. in which the schist is totally altered to a bright yellow and light gray In the valley segment north of Healy, the surface of the Riley mass of secondary enrichment. The secondary mineralization Creek I outwash is probably Wahrhaftig's T9 terrace level (Fig. 3 A). spreads out along planes of schistosity, gradually decreasing in Outwash gravel exposed along the Alaska RR % km north of the magnitude until it disappears into normal Birch Creek Schist mate- Healy station house is 14 m thick. The surface of the outwash at rial about 100 m to the northeast. The precipitates in the yellow and that locality is equivalent with the T9 level immediately to the south, gray mass have been analyzed by plasma spectrometry, X-ray dif- but the outwash is overlain by a thin layer of alluvial-fan debris. fraction, and scanning electron microscopy and identified as the This debris marks the southern edge of a large fan built by Dry minerals coquinbite [Fe2(S04)3 • 9H2O] and epsomite (MgS04 • Creek and designated by Thorson and Hamilton (1977) as being 7H2O) (W. Hood and P. Robinson, 1979, personal commun.). This Riley Creek I in age (Fig. 2). All terrace levels above the T9 level secondary mineralization is interpreted as resulting from hydro- west of Healy are truncated by this fan. This indicates that Riley thermal fluids rising along the proposed fault zone. Creek fan development did not begin until the outwash deposition 3. In the landslide-exposed gravel overlying the Birch Creek that aggraded the valley to the T9 level had ended. Observations of Schist near the proposed fault, well below the weathing zone, clasts alluvial-fan deposits elsewhere in the Nenana Valley suggest that are conspicuously coated with CaC03, epsomite, and iron oxide, tributary streams begin to develop fans in the interval immediately and the matrix of the deposit is sporadically enriched in CaCOj. following cessation of outwash deposition and prior to incision of The coatings are not the result of weathering processes. In addi- the trunk river at the onset of the next glacial advance (Ritter and tion, near the top of the alteration zone in the Birch Creek Schist, a Ten Brink, 1980). 5-m-wide gap exists in the bedrock, which might be interpreted as The depositional terrace of Riley Creek I-age stands 37 to 54 m an opening caused by a block moving under gravity toward the above the modern river north of the gorge. Between Healy and entrenched river. However, in the walls of the gap, iron- and Ferry, the outwash surface is more completely preserved on the epsomite-stained cobbles and boulders of the overlying Riley Creek west side of the Nenana Valley (Fig. 2). Exposures north of Healy outwash have been squeezed into the Birch Creek material. The are rare, however, because much of the outwash has been covered boulders, which actually penetrate the altered schist along the folia- by younger alluvium, and tributary entrenchment has not been tion planes (Fig. 6), could be removed only with rigorous hammer- great enough to reveal the glaciofluvial deposits. Nonetheless, the ing and digging. It appears that these rounded clasts were forced outwash is exposed in large bluffs on the east side of the valley into the bedrock under intense pressure, probably as they were south of Ferry, where it thickness averages about 7 m. caught in the differential movements within the shear zone. Between the Healy railroad cut and the gorge, the precise level 4. Schistosity in the Birch Creek Schist above the railroad of the Riley Creek I outwash is difficult to ascertain because a tunnel north of Garner changes orientation at the proposed shear number of geomorphic factors combine to hinder a simple correla- zone. tion. The intricacies of the situation are best illustrated in Wahrhaf- 5. Projected along strike to the northeast, the proposed fault tig's longitudinal profiles presented here as Figure 4. In that zone passes very close to a power plant located on the east side of diagram, the zone between Mile 359.5 and Mile 357.5 represents the the Nenana River near the Healy bridge. A well drilled at the plant region from the Healy railroad cut to the mouth of the gorge. Note struck thermal water (71 °F) at ~73 m. The water is very high in that in this reach there is not only a confusing array of terrace iron, and nonpotable. Projected southwest, the fault trend crosses levels, but also some of the terraces have anomalously high the Parks highway near Paul's Roadhouse. A well drilled here gradients. produced "sour water," which is also not potable. Much of the terrace chaos existing in this reach stems from The proposed fault is the most critical element in reconstruct- two faults that trend across the Nenana Valley in the zone between ing the Nenana Valley history during Riley Creek time. Three ter- the Healy bridge and the gorge. The trace of one fault is clearly race remnants exemplity the significance of the fault. As discussed 3 visible in aerial photos as a lineament located about 300 m south of above, the terrace surface /4 km north of the Healy railroad station and paralleling the old Healy road.This fault, reported by Thorson was mapped by Wahrhaftig (1958) as T9 (Fig. 3A). The terrace level (1979), was followed eastward along strike into a narrow, deep at the landslide exposure (Sec. 29, T. 12 S., R. 7 W.), and a level

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present configuration the remnants of that surface cannot be corre- lated on the basis of elevation. For example, the Tio level and Ts level differ in elevation by at least 12 m on opposite sides of the proposed fault. In Figure 3A, the remnants of the Riley Creek I outwash surface are indicated by (x) behind the terrace number assigned in Wahrhaftig's (1958) study. Other evidence suggests that the three levels are all Riley Creek I in age. As discussed above, the T9 level north of Healy is almost certainly Riley Creek I in age, based on its geomorphic relationship with the large fan built by Dry Creek (Thorson and Hamilton, 1977). The gravel beneath the Tjo level at the landslide exposure has weathering characteristics similar in every respect to those on other deposits that ar e definitely Riley Creek I in age. The Tg level is the only post-Healv surface underlain by thick gravel on the east side of the gorge mouth (Table 2). In addition, the deposit has typical Riley Creek I weathering, arid its surface can be traced upvalley through the gorge to Moody (Fig. 7) and from there directly into the Riley Creek I moraine at McKinley Park Station. Thus, the presence of faults at the gorge mouth (Fig. 2) readily explains how a number of thick terrace gravels having surfaces at different elevations can be the remains of a single depositional event. The faults also account for the anomalously high gradients on terraces downstream from the gorge mouth. Additionally, the Birch Creek Schist was probably weakened along the fault zone, thereby providing an avenue for the spillover of Lake Moody and a cogent reason for the position of the modern Nenana River. Refuting the faults makes it impossible to relate the terraces north of the gorge to the glacial sequence upvalley. Wahrhaftig's Tg level (Fig. 3A) is higher and older than the level of the Riley Creek II outwash (Thorson and Hamilton, 1977). Therefore, without faulting, three episodes of valley filling must have occurred between the Healy and Riley Creek II stades. However, during that interval, only one glacial event (Riley Creek I) can be documented.

Healy-Riley Creek Erosional Terraces

Figure 6. Boulders of Riley Creek I outwash squeezed into As showr, on Figures 3 and 4, a number of narrow terrace altered Birch Creek Schist along foliation planes. Located adjacent levels exist above the level of Riley Creek I outwash and below the to the at the mouth of the Nenana gorge along the level of Healy outwash. Much of the confusion surrounding the proposed fault zone. terrace sequence near Healy is related to these levels. West of Healy, the intermediate levels continue northward located on the east side of the Nenana gorge (SW'/i, Sec. 28, T. 12 until they are abruptly truncated by the large fan of Riley Creek I S., R. 7 W.), were designated by Wahrhaftig (1958) as TIQ and Tg, re- age (Thorson and Hamilton, 1977) built by Dry Creek (Figs. 2 and spectively (Figs. 2 and 3A). The elevation of the T9 surface is about 3B). The alluvial deposits beneath the terrace levels are rarely 424 m. The Tio level stands at 459 m, and the Tg level at -445 m. exposed; however, trenching along the scarp of T| 1 about 1 km west All elevations were determined by a Brunton compass survey using of Healy indicated that the gravel capping the terrace was -3.6 m bench marks along the Alaska Railroad tracks for base line eleva- thick. At the northern edge of the Dry Creek fan, the intermeditate tion control (U.S. Department of Commerce, 1973). terrace levels reappear 2 km south of Panguingue Creek and extend Each of the three terrace levels is underlain by gravel having northward from there until they are once again cut off by a fan of considerable thickness (Table 2), and therefore, the tread of each Riley Creek I age formed by Panguingue Creek (Fig. 2). It is inter- terrace probably represents the surface of a valley fill; that is, the esting to note, however, that elevation differences between the terraces are depositional in origin. If correct, this interpretation intermediate levels are much smaller north of the Dry Creek fan demands three distinct episodes of valley filling, separated by peri- than they are near Healy. As pointed out by Wahrhaftig (1958), ods of rapid downcutting, to create terraces Tio, T9, and Tg. levels converge downvalley until they apparently merge with the Recognition of the fault discussed above allows a greatly sim- Riley Creek I level (Fig. 4), or they have been removed by valley plifed interpretation. I suggest that the three terrace remnants (Tio, widening north of Panguingue Creek prior to Riley Creek time T9, Tg) were originally parts of the same surface, which represented (Fig. 2). Thus, north of Panguingue Creek, they are difficult to the level attained as the Nenana Valley aggraded with outwash from distinguish, if indeed they exist. the Riley Creek I glacier. The original outwash surface, however, South of Healy Creek, levels T12 and Tn (Fig. 3A) are was displaced by faulting after the Riley Creek I stade, and in their unquestionably rock-cut surfaces, having only scattered pockets of

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gravel resting on beveled Birch Creek Schist. The alluvium capping may increase drastically away from the scarp which, as in other Wahrhaftig's Tio level (Fig. 3A) is exposed, showing that 2.5 m of cases (Moss and Bonini, 1961; Ritter, 1967), would require a depo- rounded gravel truncates underlying Birch Creek Schist. According sitional origin for such a terrace. to the weathering characteristics of this gravel, the deposit should The age of the intermediate terrace development depends on have a relative age intermediate between Healy and Riley Creek I time limits placed on episodes of glaciation. Because intermediate (Chroback, 1980). terraces stand lower than the low Healy terrace and higher than the On the basis of the above observations, terrace levels Tio to surface of Riley Creek I outwash, they are designated here as inter- T|2 (Fig. 3A) probably represent continuation of the lateral plana- stadial, having formed between the Healy and Riley Creek stades. tion that began during waning phases of the Healy glaciation. The Wahrhaftig (1958) considers the intermediate levels to be part of the exception is the Tio level, which has been designated in this study as Riley Creek event. He does, however, suggest that the terraces are the surface of Riley Creek I outwash. Each level represents a still- erosional in origin (Wahrhaftig, 1958, p. 49-50), but that each stand in carving of the Nenana gorge as the river attempted to represents a short pause in the retreat of the Riley Creek glacier. If establish a gradient consistent with its post-Healy valley. Periods of one accepts my interpretation of the Riley Creek I outwash surface, equilibrium, represented by rock-cut surfaces, were interrupted by terraces above that level must predate the Riley Creek I maximum episodes of accelerated downcutting. Precisely what initiated the rather than originating in the retreatal phase of that ice. The inter- downcutting is conjectural, but it might have been caused by tecton- pretation presented here suggests that the intermediate levels are a ics, breaching of more resistant zones in the gorge segment, or continuation of the erosional phase initiated in late Healy time. attainment of fluvial thresholds. It is well documented (see Each terrace level represents a pause in vertical entrenchment, con- Schumm, 1977) that a series of terraces can be generated without trolled by conditions within the developing gorge and by the rever- external stimuli such as climatic change or tectonics. Whether the sion of-the system to one of valley-filling at the culmination of the intermediate levels near Healy represent a similar function of intrin- Riley Creek I glaciation. It is also known, however, that erosional sic threshold behavior is hypothetical at this time, but it should also terraces exist in the Nenana Valley at elevations lower than the not be discounted. Riley Creek I outwash. Thus, the tendency to form erosional terra- A note of caution should be raised at this juncture. The sugges- ces has continued since Healy time and has been interrupted only tion that the intermediate levels are underlain by thin gravels is periodically by episodes of glacio-fluvial valley filling. Presumably, based on limited observation along the terrace scarps. No data exist the process will continue until the gorge and the downstream valley to indicate whether thicknesses determined at the terrace edges are segment reach a stable fluvial equilibrium or as long as local fault- constant underneath the entire surface of each terrace. Thickness ing periodically upsets the trend toward equilibrium.

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SUMMARY AND CONCLUSIONS Gilbert, G. K., 1890, Lake Bonneville: U.S. Geological Survey Monograph 1, 438 p. Leopold, L., Wolman, M. G., and Miller, J., 1964, Fluvial processes in An alternative hypothesis for the origin of the terrace sequence geomorphology: San Francisco, W.H. Freeman Company. at Healy has been presented. Its key elements include the following. Mackin, J. H., 1937, Erosional history of the Big Horn Basin, Wyoming: 1. The high Healy terrace is the surface of the outwash plain Geological Society of America Bulletin, v. 48, p. 813-893. constructed in front of the Healy moraine. The low Healy terrace 1948, Concept of the graded river: Geological Society of America Bul- was formed by erosion during the retreat of Healy ice when Lake letin, v. 59, p. 463-512. 1953, Stream planation near Colorado Springs, Colorado: Geological Moody waters spilled over the bedrock saddle near Garner and Society of America Bulletin, v. 64, p. 705-710. initiated excavation of the Nenana gorge. Moss, J. H., and Bonini, W., 1961, Seismic evidence supporting a new 2. The outwash surface of the Riley Creek I stade is identified interpretation of the Cody terrace near Cody, Wyoming: Geological as the only post-Healy level underlain by gravel having considerable Society of America Bulletin, v. 72, p. 547-556. Ritter, D. F., 1967, Terrace development along the front of the Beartooth thickness. The gravel has consistent weathering characteristics and Mountains, southern Montana: Geological Society of America Bul- can be traced upstream to the Riley Creek I moraine at McKinlev letin, v. 78, p. 467-484. Park. Near the gorge mouth, the surface has been offset by faulting Ritter, D. F., and Ten Brink, N. W., 1980, Alluvial fan development in the into isolated remnants with different elevations. Nenana Valley, Alaska, and implications for site discovery: Geological Society of America Abstracts with Programs, v. 12, no. 7, p. 510. 3. Levels below the Healy terraces and above the Riley Creek I Schumm, S. A., 1977, The fluvial system: New York, John Wiley & Sons. outwash are erosional in origin. They are considered to be intersta- Thorson, R. M., 1979, Recurrent late Quaternary faulting near Healy, dial in age and represent a continuation of the erosional phase Alaska: Short notes on Alaskan geology—1978: Alaska Division Geo- initiated in late Healy time, which was interrupted by the episode of logical and Geophysical Survey, Geological Report 61, p. 10-14. filling during the Riley Creek I glaciation. Younger periods of ero- Thorson, R. M., and Hamilton, T. D., 1977, Geology of the Dry Creek site; sional terrace formation were disrupted by valley-filling during the A stratified early man site in interior Alaska: Quaternary Research, v. 7, p. 149-176. Riley Creek II and Riley Creek III glaciations. U.S. Department of Commerce, 1973, Vertical control data between Cant- The terrace sequence at Healy exemplifies the need to under- well and Healy, Alaska: National Oceanic and Atmospheric Adminis- stand the regional geology in order to make viable interpretations tration, National Geodetic Survey, Rockville, Maryland. of geomorphic history. Failure to build on earlier observations Wahrhaftig, C., 1951, Geology and coal deposits of the western part of the Nenana coal field, Alaska, in Barnes, F. F., and others, Coal investiga- or to integrate the relationships between key tectonic, glacial, la- tions in south-central Alaska, 1944-1946: U.S. Geological Survey Bul- custrine, and geomorphic elements in the entire Nenana River basin letin 963-E, p. 169-186. would make the analysis of local geomorphology unintelligible. 1958, Quaternary geology of the Nenana River Valley and adjacent parts of the Alaska Range: U.S. Geological Survey Professional Paper 293-A, 68 p. ACKNOWLEDGMENTS 1968, Schists of the central Alaska Range: U.S. Geological Survey Bulletin 12 54-E, 22 p. Funds were provided for this study by the National Geo- 1970a, Gee logic map of the Healy D-4 quadrangle, Alaska: U.S. Geo- graphic Society and the National Park Service through a grant to logical Survey, Map GQ-806. Norman W. Ten Brink. I am especially indebted to Dr. Ten Brink, 1970b, Geologic map of the Healy D-5 quadrangle, Alaska: U.S. Geo- logical Survey, Map GQ-807. who coordinated the NARP effort, for many helpful discussions 1970c, Geologic map of the Fairbanks A-5 quadrangle, Alaska: U.S. and his review of an early draft of the paper. Very able assistance in Geological Survey, Map GQ-811. the field was provided by D. Chroback and A. Werner. An early Wahrhaftig, C., and Black, R. F., 1958, Engineering geology along part of draft of the manuscript was reviewed by D. Chroback, C. Way- the Alaska Railroad: U.S. Geological Survey Professional Paper 293-B, 50 p. thomas, A. Werner, and C. O. Frank. The manuscript was critically Wahrhaftig, C., Hickcox, C. A., and Freedman, J., 1951, Coal deposits on reviewed by R. Thorson and D. Hopkins. Their comments were Healy and Lignite Creeks, Nenana coal field, Alaska, in Barnes, F., and very beneficial and greatly appreciated. others, Ccal investigations in south-central Alaska, 1944-1946: U.S. Geological Survey Bulletin 963-E, p. 141-165. REFERENCES CITED Wolman, M. G., and Leopold, L., 1957, River flood plains: Some observa- tions on their formation: U.S. Geological Survey Professional Paper 282-C, p. 87-109. Capps, S. R., 1940, Geology of the Alaska railroad region: U.S. Geological Survey Bulletin 907, 201 p. Chroback, D. A., 1980, Evaluation of various weathering and sedimento- logic analyses for the differentiation of river terraces in the Nenana MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 12, 1981 Valley, Alaska [M.S. thesis]: Carbondale, Illinois, Southern Illinois REVISED MANUSCRIPT RECEIVED JUNE 1, 1981 University, 111 p. MANUSCRIPT ACCEPTED JUNE 4, 1981

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