geosciences

Article Edifice of Fluvial Terrace Flights, Stacks and Rows

Wolfgang Schirmer

Former Department of Geology, Heinrich Heine University Düsseldorf, Wolkenstein 24, D-91320 Ebermannstadt, Germany; [email protected]; Tel.: +49-9194-724772



Received: 31 August 2020; Accepted: 8 December 2020; Published: 15 December 2020


Abstract: The paper presents a review of the architecture and structures of river deposits in valleys. A new terminology for some features is included in this review. It presents principles of the fluvial systems with morphological river terraces and fluments (new term for terrace bodies), different stages of the morphological terraces, the texture—the arrangement—of fluments in the form of terrace flights, terrace stacks and terrace rows, and the (inner) structure of a single flument. The contact between the valley fill and the bedrock is named by the new term “pelma”. Special topics deal with flument overlaps and insight into the deepest valley fill down to the bedrock. A comparison with other terms of the fluvial inventory is annexed.

Keywords: river terrace; flument (terrace body); pelma (contact flument/bedrock); terrace flight; terrace row; flument stack; flument overlap; valley underground

1. Introduction This paper highlights frequent as well as rare situations derived from constructing terrace flights, terrace rows and flument stacks. River terrace flights, stacks and rows are well known and described in detail, e.g., [1–20]. In supplement to these papers, the following explanations separate morphological river terraces and fluments as terrace bodies, highlight texture (arrangement) of terrace flights, terrace stacks and terrace rows, and highlight problems of flument overlap and of the study of the deepest valley fill down to the bedrock. It emphasizes the (inner) structure of the fluments as tools for recognizing single flument portions. Fluments and terraces always are indicated by a name and not numbered, as already stressed by Howard [21,22]. As the focus in this text is given to the architecture of the elements of the valley, only sometimes their genesis, e.g., [23–27], and age determination, e.g., [28,29], are mentioned marginally. In the later part of the paper, selected examples besides the general explanations relate to the Upper Main region in south-eastern Germany (Figure1). The selection in Central Europe or sometimes in the Rhine–Main River area gives the results a regional character. In comparison to other visited or studied areas in the world, though, e.g., North America or the Orient [30–32], many common features turned out to fit into the given schemes.

Geosciences 2020, 10, 501; doi:10.3390/geosciences10120501 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 501 2 of 22 Geosciences 2020, 10, x FOR PEER REVIEW 2 of 23

Figure 1. Location map of the UpperUpper MainMain areaarea/Bavaria./Bavaria. Inset: Map of Germany. Black rectanglerectangle == location of this map. Violet: Uplift Uplift area area of the the Rhenish Shield. Green Green area: area: Upper Upper Rhine Rhine graben. graben.

2. Materials Materials and and Methods Methods The followingfollowing report report results results from from extended extended and and long long fieldwork fieldwork along riversalong inrivers the Alpine in the foreland, Alpine foreland,the central the upland central and upland the northern and the lowland northern of Central lowland Europe, of Central North Europe, America, North and Mesopotamia. America, and Mesopotamia.The investigation includes: Mapping in scale 1: 5000, 1: 25,000, separation of morphological terraces,The andinvestigation investigation includes: of their Mapping pedological in properties,scale 1: 5000, including 1: 25,000, pedochemical separation properties. of morphological Study of terraces,the vertical and structure, investigation from of thetheir highest pedological fluvial properties, deposits downincluding to the pedochemical very valley properties. underground Study at ofthe the bedrock, vertical is structure, done case from by casethe highest using grain fluvial size deposits separation down of to gravel, the very sand valley and underground fines, chemical at 14 theproperties bedrock, (carbonate, is done case organic by case carbon, using phosphorus, grain size heavy separation metals), of datinggravel, bysandC, and dendrochronology, fines, chemical propertiespollen analysis, (carbonate, and archaeological organic carbon, and phosphorus, historical heavy materials. metals), The dating Main by River 14C, dendrochronology, valley in southern pollenGermany, analysis, to which and the archaeological focus is frequently and givenhistorical here, materials. gave the mostThe completeMain River insight. valley From in southern all these Germany,studies resulted to which the the general focus insights is frequently into the given fluvial here, valley gave edifices the most presented complete in insight. the following. From all these studies resulted the general insights into the fluvial valley edifices presented in the following. 3. Principles of Flument Systems

3.3.1. Principles Flument and of Flument Terrace Systems

3.1. FlumentPenck [ and33] triedTerrace to separate the morphological “terrace” from the sedimentological “gravel”, but used the term terrace for both facts. In this way it is often used until today. Boesch [34] points to the Penck [33] tried to separate the morphological “terrace” from the sedimentological “gravel”, but fact that the accumulation body and its topping terrace are mostly not of the same age. Leopold et al. [35] used the term terrace for both facts. In this way it is often used until today. Boesch [34] points to the (p. 460) state that the sedimentary deposit of a terrace “should more properly be referred to as a fill, fact that the accumulation body and its topping terrace are mostly not of the same age. Leopold et al. alluvial fill, or alluvial deposit, in order to differentiate it from the topographic form”. [35] (p. 460) state that the sedimentary deposit of a terrace “should more properly be referred to as a Thus, here a distinction is drawn between the sedimentary body and its morphological surface, fill, alluvial fill, or alluvial deposit, in order to differentiate it from the topographic form”. called terrace. The river deposit is called flument (from Latin flumen = river, stream), a term newly Thus, here a distinction is drawn between the sedimentary body and its morphological surface, introduced [14] (p. 135). As shown in Figure2, the structure of the flument in its best preservation shows: called terrace. The river deposit is called flument (from Latin flumen = river, stream), a term newly basal lag layer, channel deposit, floodplain deposit and floodplain soil. This is a frequent example of introduced [14] (p. 135). As shown in Figure 2, the structure of the flument in its best preservation the flument structure among many other possibilities. The flument is topped morphologically by the shows: basal lag layer, channel deposit, floodplain deposit and floodplain soil. This is a frequent river terrace that is a mere surface feature. example of the flument structure among many other possibilities. The flument is topped morphologically by the river terrace that is a mere surface feature.

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Figure 2. Flument structures. A complete flument is composed of all members of the fluviatile series. Figure 2. Flument structures. A complete flument is composed of all members of the fluviatile series. TheFigure skeleton 2. Flument gravel isstructures. marked by A blackcomplete pebbles flument in all is figures composed of this of allpaper. members of the fluviatile series. The skeleton gravel is marked by black pebbles in all figures of this paper. The skeleton gravel is marked by black pebbles in all figures of this paper. In case the flument rests upon the bedrock the contact between the flument and the bedrock is In case the flument rests upon the bedrock the contact between the flument and the bedrock is called Inpelma case (fromthe flument Greek restspélma upon = sole), the a bedrock term introduced the contact recently between [15] the (p. flument 70) (Figure and 3).the The bedrock basal is called pelma (from Greek pélma = sole), a term introduced recently [15] (p. 70) (Figure3). The basal bedrockcalled pelmacontact (from is the Greekground pélma pelma, = sole), and the a term lateral introduced bedrock contact recently is [15]the wall(p. 70) pelma. (Figure Ground 3). The pelma basal bedrock contact is the ground pelma, and the lateral bedrock contact is the wall pelma. Ground pelma hasbedrock a certain contact affinity is the to ground the term pelma, “strath”: and the it lateraldoes not bedrock conform contact to strath is the wallsensu, pelma. Bucher’s Ground original pelma has a certain affinity to the term “strath”: it does not conform to strath sensu, Bucher’s original definitionhas a certain [36]. Theaffinity term to is thediscussed term “strath”: by various it doesredefinitions, not conform e.g., [21,26,37]to strath (seesensu, Chapter Bucher’s 7). original definition [36]. The term is discussed by various redefinitions, e.g., [21,26,37] (see Section7). definition [36]. The term is discussed by various redefinitions, e.g., [21,26,37] (see Chapter 7).

Figure 3. Ortho-terrace and pelma: A complete flument is bounded on top by the ortho-terrace as Figure 3. Ortho‐terrace and pelma: A complete flument is bounded on top by the ortho‐terrace as tread, at the basal bedrock contact by the ground pelma, and at the lateral bedrock contact by the wall tread,Figure at the3. Orthobasal ‐bedrockterrace andcontact pelma: by the A groundcomplete pelma, flument and is at bounded the lateral on bedrock top by contactthe ortho by‐ theterrace wall as pelma. (The flument exemplarily is figured as V-flument). pelma.tread, (The at the flument basal bedrock exemplarily contact is figuredby the ground as V‐flument). pelma, and at the lateral bedrock contact by the wall pelma. (The flument exemplarily is figured as V‐flument).

Geosciences 2020, 10, x FOR PEER REVIEW 4 of 23 Geosciences 2020, 10, 501 4 of 22 3.2. Types of Fluments

3.2. TypesThe ofinner Fluments structure of the flument simplified shows two main sedimentary types, the vertical aggradation type and the lateral accretion type (Figure 2)—though it must be remarked that these The inner structure of the flument simplified shows two main sedimentary types, the vertical sedimentary types are two of many possibilities. The first type mostly results from braided reaches, aggradation type and the lateral accretion type (Figure2)—though it must be remarked that these the latter from the meandering parts within a river course. A flument of the lateral accretion type is sedimentary types are two of many possibilities. The first type mostly results from braided reaches, called L‐flument (according to Lateral), that of the vertical aggradation type, V‐flument (according to the latter from the meandering parts within a river course. A flument of the lateral accretion type is Vertical) [9] (p. 25). The L‐flument at its base is characterized by skeleton gravel (open work gravel). called L-flument (according to Lateral), that of the vertical aggradation type, V-flument (according to Vertical)3.3. Stages [9 ]of (p. River 25). Terraces The L-flument at its base is characterized by skeleton gravel (open work gravel). 3.3. StagesThe geometry of River Terraces of river terraces varies significantly with age. The surface of a flument is called a terrace as soon as the flument has been cut by the river, forming a lower situated younger river plain, The geometry of river terraces varies significantly with age. The surface of a flument is called leaving a downward step (front riser) toward the new river plain, and leaving an upward step (back a terrace as soon as the flument has been cut by the river, forming a lower situated younger river riser) toward the slope of the valley edge. In a case where the flument reaches its mature stage or has plain, leaving a downward step (front riser) toward the new river plain, and leaving an upward step already fulfilled its mature stage, the topping terrace (tread) is called an ortho‐terrace (Figure 3). The (back riser) toward the slope of the valley edge. In a case where the flument reaches its mature stage or ortho‐terrace is as old as the last river deposits upon the flument. Soil formation may continue upon has already fulfilled its mature stage, the topping terrace (tread) is called an ortho-terrace (Figure3). the ortho‐terrace. The ortho-terrace is as old as the last river deposits upon the flument. Soil formation may continue An age of an ortho‐terrace may cover a long period. The frequency of flooding in the floodplain upon the ortho-terrace. is normally decreasing with the upgrowth of the flood plain. Thus, the intermittent periods between An age of an ortho-terrace may cover a long period. The frequency of flooding in the floodplain is the floods become longer. These periods are the chance for soil formation within the floodplain. The normally decreasing with the upgrowth of the flood plain. Thus, the intermittent periods between the longer the breaks between flood events, the stronger the soil formation on the flood deposits. floods become longer. These periods are the chance for soil formation within the floodplain. The longer Consequently, soil formation is part of the decreasing floodplain sedimentation (threshold of the the breaks between flood events, the stronger the soil formation on the flood deposits. Consequently, critical power in the stream [18] (p. 10)). A further participation of soils in floodplain deposits is the soil formation is part of the decreasing floodplain sedimentation (threshold of the critical power in intermingling of relocated soil material called fluvisoliment [9] (p. 27). Additionally, synpedological the stream [18] (p. 10)). A further participation of soils in floodplain deposits is the intermingling sediment deposition occurs. of relocated soil material called fluvisoliment [9] (p. 27). Additionally, synpedological sediment Upon older fluments, the ortho‐terrace slowly gets eroded. The erosional surface still may deposition occurs. appear as a terrace, but is an inclined plane that cuts the flument diagonally (Figure 4). This terrace Upon older fluments, the ortho-terrace slowly gets eroded. The erosional surface still may appear is younger than the ortho‐terrace and becomes younger and younger through time. It is called here a as a terrace, but is an inclined plane that cuts the flument diagonally (Figure4). This terrace is dia‐terrace—from the diagonal erosional cut through the flument. younger than the ortho-terrace and becomes younger and younger through time. It is called here a dia-terrace—from the diagonal erosional cut through the flument.

Figure 4. Changing geometry of terraces. Dia-terrace: a complete flument when diagonally cut (redFigure lines) 4. generatesChanging dia-terraces.geometry of Shownterraces. are Dia three‐terrace: random a complete stages (1–3) flument of dia-terraces, when diagonally leaving cut behind (red morelines) or generates less volume dia‐ ofterraces. the previous Shown flument. are three random stages (1–3) of dia‐terraces, leaving behind more or less volume of the previous flument. In Figure4, the dia-terrace shows di fferent stages (1–3) of erosion drawn in red. The upper stage (1) allows one to determine the position of the ortho-terrace by the terrace remnant preserved at In Figure 4, the dia‐terrace shows different stages (1–3) of erosion drawn in red. The upper stage the outer edge of the previous terrace. Additionally, all members of the flument remain preserved. (1) allows one to determine the position of the ortho‐terrace by the terrace remnant preserved at the The middle stage (2) leaves behind remnants of the floodplain edge channel. By this, experiences of outer edge of the previous terrace. Additionally, all members of the flument remain preserved. The floodplain thicknesses of the investigation area should allow one to calculate the former level of the middle stage (2) leaves behind remnants of the floodplain edge channel. By this, experiences of ortho-terrace. The lower stage (3) can only indicate the terrace pelma. Moreover, this stage (3) prevents floodplain thicknesses of the investigation area should allow one to calculate the former level of the one’s identifying an L- or V-flument.

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Geosciencesortho‐terrace.2020, 10 ,The 501 lower stage (3) can only indicate the terrace pelma. Moreover, this stage5 of 22(3) prevents one’s identifying an L‐ or V‐flument. IfIf the the erosion erosion has has reached reached the the pelma pelma leaving leaving only only a a lag lag layer layer above above the the bedrock, bedrock, it it is is called called a a pelma-terracepelma‐terrace (Figure (Figure5 ).5). The The pelma pelma terrace terrace may may have have a a back back riser riser (Figure (Figure5b) 5b) or or may may be be isolated isolated (Figure(Figure5c). 5c). The The lag lag layer layer of of a a pelma pelma terrace terrace is is mostly mostly not not more more than than an an eluvial eluvial loose loose litter litter of of pebbles. pebbles.

Figure 5. Pelma-terrace: a complete flument (a) eroded down (green line) to the basal facies leaving onlyFigure the 5. lag Pelma layer‐terrace: and the a groundcomplete pelma flument (contact (a) eroded of the down basal (green facies withline) to the the bedrock), basal facies (b) withleaving a backwall,only the (lagc) crowning layer and a hilltop.the ground Normally, pelma the(contact pelma-terrace of the basal deposit facies is awith loose the litter bedrock), of pebbles. (b) with a backwall, (c) crowning a hilltop. Normally, the pelma‐terrace deposit is a loose litter of pebbles. As a result, younger fluments of an incised valley can be followed and compared by the elevation of their ortho-terraces,As a result, younger older fluments fluments by of theiran incised dia-terraces. valley can Sometimes, be followed they and tend compared more to by be parallelizedthe elevation byof theirtheir ground ortho‐terraces, pelma/strath. older Ortho-,fluments dia-, by andtheir pelma-terraces dia‐terraces. Sometimes, may occur asthey paired tend or more unpaired to be terracesparallelized [38]. by their ground pelma/strath. Ortho‐, dia‐, and pelma‐terraces may occur as paired or unpaired terraces [38]. 4. Texture (Arrangement) of Fluments 4. TextureWithin (Arrangement) a sequence of fluments of Fluments or terraces, they may be cut in the bedrock or in older fluments: cut-inWithin fluments a sequence or cut-in terraces.of fluments If the or base,terraces, walls, they and may terrace be cut of in a flumentthe bedrock are cutor in into older a preceding fluments: flument,cut‐in fluments it is called or acut nested‐in terraces. flument If/ terrace.the base, walls, and terrace of a flument are cut into a preceding flument,In addition, it is called if a cut-ina nested terrace flument/terrace. cuts a flument with its bedrock beneath, or cuts the bare bedrock, it is calledIn addition, a strath terrace,if a cut‐in after terrace the strath, cuts a which flument is thewith name its bedrock for the contactbeneath, flument or cuts upon the bare the bedrockbedrock, orit theis called bare fluviala strath cut terrace, bedrock after (([ the18] strath, p. 8), ([ which26] p. is 32), the [ 36name,39,40 for]); the cf. contact Section flument 7.3. upon the bedrock or theFluments bare fluvial may formcut bedrock rows, one (([18] flument p. 8), deposited([26] p. 32), beside [36,39,40]); a preceding cf. chapter one: 7.3. row fluments with row terracesFluments [9]. may form rows, one flument deposited beside a preceding one: row fluments with row terracesFluments [9]. may form stacks by overlapping and/or completely superimposing on a preceding flument:Fluments overlap may flument, form stacked stacks by flument. overlapping and/or completely superimposing on a preceding flument:Preferentially, overlap flument, there occur stacked three flument. major flument textures within valleys: flument/terrace flights (staircases),Preferentially, flument /thereterrace occur rows, three and major flument flument stacks. textures Actually, within all valleys: forms may flument/terrace be mixed within flights the(staircases), valley edifice. flument/terrace Whether staircase, rows, and row, flument or stack, stacks. all are Actually, controlled all forms by tectonic may be uplift, mixed subsidence, within the orvalley equilibrium. edifice. InWhether addition, staircase, the trend row, to more or stack, or less all flument are controlled deposition by tectonic is controlled uplift, by subsidence, the manifold or climaticalequilibrium. alterations In addition, of glacial the trend and interglacial to more or periods less flument during deposition the Quaternary, is controlled e.g., [23 by,28 the,29 ].manifold climatical alterations of glacial and interglacial periods during the Quaternary, e.g., [23,28,29].

4.1. Flument Flights Respectively Terrace Flights

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4.1.Geosciences Flument 2020 Flights, 10, x FOR Respectively PEER REVIEW Terrace Flights 6 of 23 These are the result of rhythmical cut-and-fill processes of fluvial incision and fluvial These are the result of rhythmical cut‐and‐fill processes of fluvial incision and fluvial deposition deposition [41,42]. Terrace flights occur in areas of subsided water level, caused either by subsided sea [41,42]. Terrace flights occur in areas of subsided water level, caused either by subsided sea level or level or by land uplift. The latter is the case shown here. The highest situated terrace of a flight is the byGeosciences land uplift. 2020, 10The, x FOR latter PEER is REVIEW the case shown here. The highest situated terrace of a flight6 isof the23 oldest; oldest;the lowest the lowest is the is youngest the youngest terrace. terrace. This is This the is normal the normal expectation. expectation. However, However, it need it not need be not so, be as so, as shownThese later. are Examples the result forof rhythmical terrace/flument cut‐and‐ flightsfill processes are given of fluvial from incision the Middle and fluvial Rhine deposition River within the shown[41,42]. later. Terrace Examples flights occurfor terrace/flument in areas of subsided flights water are level, given caused from either the Middle by subsided Rhine sea River level withinor the uplift area of the Rhenish Shield since the Oligocene (Figure6; compare also the fine changes in [ 4]). upliftby land area uplift. of the The Rhenish latter is Shield the case since shown the here. Oligocene The highest (Figure situated 6; compare terrace of also a flight the fineis the changes oldest; in [4]). WithinWithinthe this lowest this flight, flight, is the most mostyoungest fluments fluments terrace. are are This cut-in cut is‐in the fluments. fluments. normal expectation. The Holocene Holocene However, floodplain floodplain it need fluments fluments not be so, are areas nested nested in in thethe Lateshown Late Pleistocene Pleistocenelater. Examples fluments. fluments. for terrace/flument A A next next terrace terrace flights flight flight are is isgiven given given from from from the the theMiddle Main Main Rhine River River River (Figure(Figure within7 7).). the ThisThis flightflight is conspicuouslyis upliftconspicuously area of interrupted the Rhenishinterrupted byShield a thickby since a thick flument the Oligocene flument stack (Figure (Sectionstack (Chapter6; compare4.2). Additionally, 4.2). also theAdditionally, fine changes herein severalhereinin [4]). several fluments appearflumentsWithin to be thisappear nested. flight, to most be nested. fluments are cut‐in fluments. The Holocene floodplain fluments are nested in the Late Pleistocene fluments. A next terrace flight is given from the Main River (Figure 7). This flight is conspicuously interrupted by a thick flument stack (Chapter 4.2). Additionally, herein several fluments appear to be nested.

Figure 6. Scheme: Flument/terrace flight of the Middle Rhine River within the Rhenish Shield. FigureFigure 6. 6. Scheme: Scheme: Flument/terrace flight flight of ofthe the Middle Middle Rhine Rhine River River within within the Rhenish the Rhenish Shield. EifelShield. = Eifel = Eifelmountainsmountains= mountains adjoining adjoining adjoining thethe RhineRhine the valley valley Rhine to tothe valleythe west. west. Vertical to Vertical the scale west. scale exaggerated. Vertical exaggerated. scaleLocation: exaggerated.Location: The uplifted The uplifted Location: TheRhenish upliftedRhenish Shield Shield Rhenish (violet(violet Shield areaarea in (violetin Figure Figure area 1). 1). in Figure1).

Figure 7. Scheme of the flument/terrace flight of the Main River and adjoining rivers in Central Figure 7. Scheme of the flument/terrace flight of the Main River and adjoining rivers in Central Europe. Europe. Not to scale. For the age of the Holocene terraces, see Figure 9. Location: Whole Main River. Not to scale. For the age of the Holocene terraces, see Figure 9. Location: Whole Main River. Figure 7. Scheme of the flument/terrace flight of the Main River and adjoining rivers in Central 4.2.Europe. Flument Not Stacks to scale. For the age of the Holocene terraces, see Figure 9. Location: Whole Main River.

4.2. Flument Stacks

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Flument stacks are the result of superposition of fluments. Within the stack, in contrast to the terrace flight, the lowest flument is the oldest, and the uppermost flument is the youngest (Figure 8 and Main Formation in Figure 7) [43]. Figure 7 shows a change from flument flight to flument stack and back to flight. Its interpretation is: in the Early Pleistocene (~1.7 Ma [44], p. 333), the Upper Rhine Graben (Figure 1) started to subside more strongly. Thus, the Main River had to incise deeply to level with the receiving water of the Rhine. When the downward movement of the Upper Rhine Graben tapered off by ca. 1 Ma [43], the Main River with its clayey‐sandy hinterland could form the rhythmicalGeosciences 2020 aggradation, 10, 501 of the 30–60 m thick stack of the Main Formation. By further uplift7 of of 22 Southern Central Europe (~0.5 Ma [44]), the stack was cut to continue the terrace flight until today. It is remarkable that within the fluvial stack several fluments tend to overlap laterally, owing to 4.2. Flument Stacks the upward widening of the preformed valley that is used by the stack formation (Figure 8). TheFlument fluvial stacks stack areof the the Main result River of superposition (Figure 7), dating of fluments. to roughly Within 1–0.5 the Ma stack, [43], seems in contrast to grow to up the ratherterrace continuously. flight, the lowest However, flument each is the break oldest, between and the the uppermost six preserved flument fluments is the youngest shows a (Figure hiatus,8 indicatingand Main a Formation standstill inwith Figure soil7 formation)[ 43]. Figure and7 certainshows aerosion. change Concerning from flument the flight glacial to flumenthistory, these stack hiatusesand back indicate to flight. the Its interpretationclimatic change is: in between the Early glacial Pleistocene and (~1.7 interglacial Ma [44], periods, p. 333), the including Upper Rhine the transitionalGraben (Figure period1) started between to subsideboth. Actually, more strongly. during these Thus, transitional the Main River periods had tothe incise bulk deeplyof erosion to level and awith great the deal receiving of the accumulation water of the Rhine.happened. When the downward movement of the Upper Rhine Graben taperedOn otheff by other ca. 1 Mahand, [43 ],there the Main are often River withcases its where clayey-sandy fluvial hinterlandstacks contain could long form‐term the rhythmical hiatuses involvingaggradation several of the glacial 30–60 periods, m thick as stack demonstrated of the Main later Formation. in Chapter By further5.3. uplift of Southern Central Europe (~0.5 Ma [44]), the stack was cut to continue the terrace flight until today.

Figure 8. Flument stack. Scheme of the fluvial stack in the Main River area, called the Main Formation. Figure 8. Flument stack. Scheme of the fluvial stack in the Main River area, called the Main Formation. It consists of maximum six fluments: A, B, C, D1, D2, and E. Flument A, B, D1, D2, and E are fluments It consists of maximum six fluments: A, B, C, D1, D2, and E. Flument A, B, D1, D2, and E are fluments of vertically aggrading rivers. The small flument C presents a lateral accretional river weathered by a of vertically aggrading rivers. The small flument C presents a lateral accretional river weathered by a Luvisol (red brown). Flument E embraces much slope debris supplied by a gully, cp. [45]. Each flument shows at least remnants of floodplain channel fills (green): A (reworked in B); B; D1 (reworked in D2); D2, and E (largely preserved). Above the back swamp of E (green) are silty-sandy deluvial coverbeds with two paleo-Luvisols (red brown) and the Luvisol at the surface. The Upper Triassic Burgsandstein is a quartz sandstone interfered with red shale. These shales, softer than the quartz sandstone, provide the possibility for lateral overlapping of the river deposits. (The overlapping extent is drawn here small owing to the page size. The scale of the heights is exaggerated.). Geosciences 2020, 10, 501 8 of 22

It is remarkable that within the fluvial stack several fluments tend to overlap laterally, owing to the upward widening of the preformed valley that is used by the stack formation (Figure8). GeosciencesThe fluvial 2020, 10 stack, x FOR of PEER the REVIEW Main River (Figure7), dating to roughly 1–0.5 Ma [ 43], seems to grow8 of 23 up rather continuously. However, each break between the six preserved fluments shows a hiatus, indicating Luvisol (red brown). Flument E embraces much slope debris supplied by a gully, cp. [45]. Each a standstillflument with shows soil at formation least remnants and certainof floodplain erosion. channel Concerning fills (green): the A glacial (reworked history, in B); these B; D1 hiatuses indicate(reworked the climatic in D2); change D2, and between E (largely glacial preserved). and interglacial Above the back periods, swamp including of E (green) the are transitional silty‐sandy period betweendeluvial both. coverbeds Actually, with during two thesepaleo‐Luvisols transitional (red brown) periods and the the bulk Luvisol of erosion at the surface. and a The great Upper deal of the accumulationTriassic happened.Burgsandstein is a quartz sandstone interfered with red shale. These shales, softer than the Onquartz the other sandstone, hand, thereprovide are the often possibility cases where for lateral fluvial overlapping stacks contain of the long-term river deposits. hiatuses (The involving severaloverlapping glacial periods, extent as is demonstrated drawn here small later owing in Section to the 5.3 page. size. The scale of the heights is exaggerated.). 4.3. Flument Rows and, Respectively, Terrace Rows 4.3. Flument Rows and, Respectively, Terrace Rows In the case of the missing incision of a river and continuing rhythmical formation of fluvial deposits,In flumentsthe case of and the terraces missing canincision form of a a lateral river row,and continuing called, respectively, rhythmical row formation fluments of fluvial and row terracesdeposits, [9] fluments (pp. 12, and 28). terraces There arecan aform lot ofa lateral papers row, dealing called, with respectively, row terraces, row fluments e.g., [6,16 and]. Herein, row theterraces terraces [9] lie (pp. side 12, by 28). side, There at least are a with lot of its papers surfaces dealing on the with same row level. terraces, Mostly, e.g., though,[6,16]. Herein, the flument the terraces lie side by side, at least with its surfaces on the same level. Mostly, though, the flument bases bases also hold a similar level, which is the case among the Holocene floodplain fluments. Moreover, also hold a similar level, which is the case among the Holocene floodplain fluments. Moreover, each each of the younger fluments can form nested fluments within the terrace row (Figure9). Separation of of the younger fluments can form nested fluments within the terrace row (Figure 9). Separation of the individual fluments is possible morphologically by their floodplain edge channels, pedologically the individual fluments is possible morphologically by their floodplain edge channels, pedologically by their floodplain soils, and chronologically by their age. The amount of Holocene fluments within by their floodplain soils, and chronologically by their age. The amount of Holocene fluments within thethe terrace terrace row row at at the the valley valley bottom bottom is is rather rather constantconstant in Central Europe Europe [6,11]. [6,11].

FigureFigure 9. 9.Terrace Terrace/flument/flument row. row. Scheme Scheme along along the the Main Main River. River. From From the the Low Low Terrace Terrace 2 (LT 2 (LT 2) on,2) on, the the river river terraces of the valley floor form a terrace row. Occasionally, they get flooded (yellow), and terraces of the valley floor form a terrace row. Occasionally, they get flooded (yellow), and therefore therefore were called floodplain terraces. H = Holocene. were called floodplain terraces. H = Holocene.

5. Application5. Application of of the the Principles, Principles, and and SpecialSpecial Problems

5.1.5.1. Fluvial Fluvial Overlap Overlap and and Stratigraphical Stratigraphical Consequences Consequences WhereWhere flument flument flights, flights, flument flument rows, rows, and and flument flument stacks stacks occur occur together together in one in one and and the samethe same fluvial edifice,fluvial the edifice, building the of building a stratigraphical of a stratigraphical system may system get di mayfficult. get In difficult. the following, In the somefollowing, examples some are presentedexamples showing are presented interaction showing of flument interaction staircases, of flument stacks, staircases, and rows: stacks, and rows:

5.2.5.2. Lateral Lateral Overlap Overlap Confusing Confusing the the Assignment Assignment ofof Terraces or Fluments AA normal normal river river deposit, deposit, called called flument, flument, isis limited on top top with with a a more more or or less less even even morphological morphological terraceterrace plain, plain, an an ortho-terrace ortho‐terrace (Figure (Figure3 ).3). The The way way toto becomingbecoming a terrace terrace involves involves that that the the river river plain plain andand flument flument were were again again cut cut by by the the river river to formto form a deeper-lying a deeper‐lying younger younger river river plain, plain, leaving leaving aside aside parts parts of the older plain. of the older plain. If a river aggradation rises higher than the preceding river incision, the aggradation can overlap the incision form, provided that there is lateral space enough for overlapping (Figure 10a). Various overlapping stages are discussed by Blum & Törnqvist [19].

Geosciences 2020, 10, 501 9 of 22

If a river aggradation rises higher than the preceding river incision, the aggradation can overlap the incision form, provided that there is lateral space enough for overlapping (Figure 10a).

VariousGeosciences overlapping 2020, 10, x stagesFOR PEER are REVIEW discussed by Blum & Törnqvist [19]. 9 of 23

Figure 10. Flument overlap. (a) An overlapping flument causes two pelmas, a ground pelma and

overlap pelma. (b) An erosional plane separates the two pelmas and the deeper part of the flument fromFigure its higher 10. Flument part. (c )overlap. A wrong (a) interpretationAn overlapping of flument the separated causes two fluments, pelmas, actuallya ground presentingpelma and one flument.overlap (d) pelma. An easier (b) An interpretation, erosional plane using separates the flument the two pelmas structure and in the case deeper both part flument of the proportionsflument from its higher part. (c) A wrong interpretation of the separated fluments, actually presenting one show, respectively, the typical basal and upper facies of a flument. flument. (d) An easier interpretation, using the flument structure in case both flument proportions The lateralshow, respectively, overlap results the typical in the basal formation and upper offacies two of dia flument.fferent base levels (pelmas)—a lower pelma starting withThe lateral the first overlap emplacement results in the of theformation flument, of two and different a second base and levels higher (pelmas)—a pelma starting lower pelma with the lateralstarting overlap with of thethe river.first emplacement Thus, one and of the the flument, same flument and a second has two and base higher levels: pelma the starting true ground with the pelma and thelateral overlap overlap pelma of the (Figure river. Thus, 10a). one The and overlap the same pelma flument lies has higher two thanbase thelevels: true the ground true ground pelma of the riverpelma aggradation. and the overlap This pelma flument (Figure geometry 10a). The may overlap cause pelma geological lies higher misinterpretation than the true ground in thepelma case of partialof erosion the river (Figure aggradation. 10b). This The erosionalflument geometry plane cuts may the cause Flument geological A in misinterpretation a way that both in flument the case relics of Flumentof partial A appearerosion (Figure to represent 10b). The two erosional different plane fluments cuts the of aFlument stair case—an A in a way apparent that both older flument flument 1 andrelics an apparent of Flument younger A appear flument to represent 2 (Figure two 10differentc). Consequently, fluments of a the stair overlap case—an Flument apparent A mayolder give flument 1 and an apparent younger flument 2 (Figure 10c). Consequently, the overlap Flument A rise to counting the overlapping flument as two fluments, respectively, to consider the overlap pelma may give rise to counting the overlapping flument as two fluments, respectively, to consider the as an own ground pelma (Figure 10c). That means that constructing a flument flight of a valley may overlap pelma as an own ground pelma (Figure 10c). That means that constructing a flument flight involveof a considering valley may twoinvolve pelmas considering as belonging two pelmas to two as fluments, belonging unaware to two fluments, of the fact unaware that the of the two fact pelmas couldthat represent the two one pelmas flument could only. represent Studying one flument the case only. of FigureStudying 10 theb, the case fact of Figure that both 10b, fluments the fact that belong togetherboth could fluments be underlinedbelong together by thecould flument be underlined structure: by the the flument development structure: of the the development deeper lying of flumentthe with basaldeeper facies lying (block flument layer with [46 basal] (Figure facies8)), (block skeleton layer gravel, [46] and(Figure the 8)), development skeleton gravel), of the higherand the lying flumentdevelopment with finer of grained the higher clastics lying and flument/or initial with floodplainfiner grained facies clastics (Figure and/or 10 initiald). floodplain facies (Figure 10d). 5.3. Lateral Overlap Confusing the Assignment of Individual Single Fluments 5.3. Lateral Overlap Confusing the Assignment of Individual Single Fluments The fluvial stack of the Main River, the Main Formation (Figure7), comes up to 30–60 m thickness, 30 m upriver,The fluvial 60 m downriver.stack of the Main It consists River, ofthe at Main least Formation six fluments. (Figure All 7), fluments comes up of to the 30–60 stack m tend thickness, 30 m upriver, 60 m downriver. It consists of at least six fluments. All fluments of the stack to overlap laterally. The overlap is shown schematically in Figure8, and as a drawing true to life tend to overlap laterally. The overlap is shown schematically in Figure 8, and as a drawing true to in Figurelife in 11 Figure. 11.

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GeosciencesGeosciences2020, 102020, 501, 10, x FOR PEER REVIEW 10 of 2310 of 22

Figure 11. Flument overlap in the Main Formation. Flument D2 overlaps laterally more than 18 Figuremeters; 11. FlumentFlument E overlap overlaps in 37 the m. Main Formation. Flument D2 overlaps laterally more than 18 meters; FlumentFigure E overlaps 11. Flument 37 m. overlap in the Main Formation. Flument D2 overlaps laterally more than 18 Figuremeters; 11 Flument shows Etwo overlaps overlaps: 37 m. the lower overlap of flument D2, and the upper one of flument E. FigureBoth overlaps 11 shows developed two overlaps: from lateral the lower erosion, overlap cp. [26], of flument by the D2,Main and River the against upper onehard of Triassic flument E. Bothsandstone, overlapsFigure using 11 developed shows there, twohowever, fromoverlaps: laterala weaker the lower erosion, shale overlap interlayer cp. [of26 flument], by for the prograding. D2, Main and the River Ifupper the against riverone of of hard flument this Triassic level sandstone,wouldE. Both cross using overlaps mere there, developedclaystone, however, fromthe a weaker progresslateral erosion, shale of overlap interlayer cp. [26], would by for the be prograding. Main easier River and against If perhaps the river hard much ofTriassic this more level wouldextendedsandstone, cross mere(Figure using claystone, 12). there, Figure however, the 12a progress is a a weaker theoretical of overlap shale figure interlayer would demonstrating be for easier prograding. and that perhaps If the the upper muchriver offluments more this extendedlevel of a would cross mere claystone, the progress of overlap would be easier and perhaps much more (Figureflument 12). stack Figure may 12 bea isable a theoreticalto deposit widely figure separated demonstrating flument that portions. the upper Thus, fluments as shown of in a Figure flument 12b,extended it is hardly (Figure possible 12). Figure to assign 12a issome a theoretical fluments figure (green) demonstrating as a flument that of the upperstack (violet)fluments or of as a an stack may be able to deposit widely separated flument portions. Thus, as shown in Figure 12b, it is incisionflument flument stack mayof the be terrace able to flight deposit (green) widely that separated follows flumentafter the portions. pile‐up ofThus, the fluments.as shown inIt getsFigure even hardly possible to assign some fluments (green) as a flument of the stack (violet) or as an incision more12b, difficult it is hardly to assignpossible deposits to assign of some the oppositefluments (green)side of as the a flumentvalley (redof the in stack Figure (violet) 12b). or The as an best flument of the terrace flight (green) that follows after the pile-up of the fluments. It gets even more possibilityincision flumentto assign of the those terrace isolated flight (green)fluments that wouldfollows beafter to the date pile ‐themup of thedirectly. fluments. Dating It gets by even their difficult to assign deposits of the opposite side of the valley (red in Figure 12b). The best possibility to lithologicalmore difficult composition to assign (e.g.,deposits pebble of the analysis, opposite heavy side of mineral the valley analysis) (red in isFigure useless, 12b). because The best the assignpossibility those isolated to assign fluments those would isolated be fluments to date them would directly. be to Datingdate them by theirdirectly. lithological Dating by composition their fluments of the flument flight (green) involve mostly reworked material from the flument stack lithological composition (e.g., pebble analysis, heavy mineral analysis) is useless, because the (e.g.,(violet). pebble analysis, heavy mineral analysis) is useless, because the fluments of the flument flight (green)fluments involve of mostly the flument reworked flight material (green) involve from the mostly flument reworked stack (violet).material from the flument stack (violet).

Figure 12. (a) A theoretical flument stack: 1 the oldest, 5 the youngest flument of the stack. (b) The stack

has been eroded forming a flument flight (green) with four cut-in terraces (6–9) and leaving behind

onlyFigure remnants 12. (a of) A the theoretical stack 1–5 flument (dark violet). stack: 1 On the the oldest, left side 5 the of youngest the valley flument occur isolated,of the stack. so-called (b) The lost Figure 12. (a) A theoretical flument stack: 1 the oldest, 5 the youngest flument of the stack. (b) The flumentsstack has (red), been which eroded are forming obviously a flument unpaired, flight and (green) the assignment with four of cut which‐in terraces is not clear,(6–9) and whether leaving they stack has been eroded forming a flument flight (green) with four cut‐in terraces (6–9) and leaving belong to the stack (violet) or to the cut-in terrace flight (green) or represent additional fluments. Geosciences 2020, 10, x FOR PEER REVIEW 11 of 23

behind only remnants of the stack 1–5 (dark violet). On the left side of the valley occur isolated, so‐ called lost fluments (red), which are obviously unpaired, and the assignment of which is not clear, whether they belong to the stack (violet) or to the cut‐in terrace flight (green) or represent additional Geosciencesfluments.2020 , 10, 501 11 of 22

5.4. Lateral Overlap Owing to Shift of the Thalweg 5.4. Lateral Overlap Owing to Shift of the Thalweg The Main River during its evolution changed its thalweg from east towards west. This was The Main River during its evolution changed its thalweg from east towards west. This was triggered by the onset of enhanced periglacial slope activity during the Middle Pleistocene. triggered by the onset of enhanced periglacial slope activity during the Middle Pleistocene. At first, the river was close‐joint running along the foot of the steep Jurassic escarpment of the At first, the river was close-joint running along the foot of the steep Jurassic escarpment of the Franconian Alb (Figure 1) for some hundred thousand years (Figure 13, eastern flank of the valley). Franconian Alb (Figure1) for some hundred thousand years (Figure 13, eastern flank of the valley). Then, external feeding of periglacial debris from slopes and tributaries of the escarpment increased Then, external feeding of periglacial debris from slopes and tributaries of the escarpment increased so so eminently that the river was gradually pushed to the western side. This side was bound by Lower eminently that the river was gradually pushed to the western side. This side was bound by Lower Jurassic clayey deposits. Thus, the Main River easily undermined the western side by gradual shifting Jurassic clayey deposits. Thus, the Main River easily undermined the western side by gradual shifting of its thalweg toward the west. of its thalweg toward the west.

FigureFigure 13. 13. SchemeScheme of of the the valley’s valley’s shift from easteast toto west:west: RedRed line: line: Pelma Pelma of of the the eastern eastern valley valley filled filled by bythe the Main Main Formation. Formation. Green: Green: Fluments Fluments of of the the westernwestern valley with overlap of of the the Grundfeld Grundfeld Flument Flument overover the the truncated truncated eastern eastern valley valley filled filled by by the the Main Main Formation. Formation. Brown Brown rectangle: rectangle: Detail Detail of of the the overlap overlap situationsituation in in the the Grundfeld Grundfeld gravel gravel pit pit in in Figure Figure 14. 14 .

ByBy the the shift shift of of the the thalweg, thalweg, the the river river ceased ceased downworking downworking into into the the loose loose gravel gravel of of the the Main Main Formation,Formation, leaving leaving a a socle socle of of the the Main Main Formation Formation of of 1 1km km in in width width and and more more than than 10 10 m m in in thickness. thickness. InIn its its new new thalweg, thalweg, it it cut cut vertically vertically deeper deeper into into a a clayey clayey bedrock. bedrock. Now, Now, nestled nestled in in its its new new valley valley bed, bed, thethe river river in in places places filled filled up up its its aggradation aggradation somewhat somewhat higher higher than than the the truncated truncated Main Main Formation, Formation, overlappingoverlapping the the truncated truncated surface surface of of its its old old thalweg. thalweg. Figure Figure 13 13 as as scheme, scheme, and and Figure Figure 14 14 as as drawing drawing truetrue to to life, life, show thethe overlappingoverlapping Grundfeld Grundfeld Flument Flument over over the the socle socle parts parts of the of Mainthe Main Formation Formation prior priorto being to being covered covered by deluvial by deluvial slope debris. slope Thus, debris. two Thus, morphological two morphological terraces lie interraces tight superposition lie in tight superpositionin the eastern riverin the bed: eastern the truncation river bed: terrace the oftruncation the Main terrace Formation of andthe theMain Grundfeld Formation Terrace and uponthe Grundfeldit as a thin Terrace overlap upon flument. it as Certainly, a thin overlap it is a flument. rare combination: Certainly, anit is overlap a rare flumentcombination: upon an a cut-in-fill overlap flumentterrace. upon Coming a cut back‐in‐ tofill Section terrace. 4.2 Coming, the Grundfeld back to Chapter flument 4.2, (assigned the Grundfeld to the Marine flument Isotopic (assigned Stage to 8) thehere Marine forms Isotopic a stack togetherStage 8) here with forms the Main a stack Formation together below, with the presenting Main Formation an unconformity below, presenting of roughly an300 unconformity ka years between of roughly them. 300 ka years between them. AA comment comment on on Figure Figure 14: 14 :The The Grundfeld Grundfeld Flument Flument that that overlaps overlaps here here with with its its uppermost uppermost part part consistsconsists of of three three subunits subunits (1–3). (1–3). All All of of them them merge merge from from upslope upslope local local gravel gravel (triangles) (triangles) downslope downslope intointo Main Main gravel gravel (green, (green, gravel gravel == pebblepebble signature, signature, sand sand == dots).dots). The The three three subunits subunits are are separated separated by by soil horizons (red colored, see legend). The subunits 2 and 3 could present substages of the Marine Isotopic Stage (MIS) 7.

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soil horizons (red colored, see legend). The subunits 2 and 3 could present substages of the Marine Geosciences 2020, 10, 501 12 of 22 Isotopic Stage (MIS) 7.

Figure 14. Grundfeld-Strassäcker gravel pit. Location: see quadrangle in Figure 13. Overlap of the GrundfeldFigure 14. Grundfeld Flument, with‐Strassäcker subunits gravel 1–3, upon pit. Location: the truncated see quadrangle Main Formation. in Figure The 13. colors Overlap of subunits of the 1–3Grundfeld vary in Flument, order to with contrast subunits the three 1–3, subunits,upon the truncated and local Main gravel Formation. and Main The gravel colors (green) of subunits within the1–3 subunits.vary in order to contrast the three subunits, and local gravel and Main gravel (green) within the subunits. 6. Flument Inventory of the Valley Bottom and Its Underground

6.1.6. Flument Terrace PatternsInventory of the of Valleythe Valley Bottom Bottom and Its Underground

6.1. TerraceThe lowest Patterns surface of the of Valley the valley Bottom is called the valley bottom. The valley fill beneath is called the valley underground. The terrace pattern (Figure 15) is the configuration of the nowadays preserved The lowest surface of the valley is called the valley bottom. The valley fill beneath is called the fluments of the valley bottom. The four main terrace patterns (surface patterns) are recognized as valley underground. The terrace pattern (Figure 15) is the configuration of the nowadays preserved occurring frequently along most rivers in Central Europe and other areas, e.g., [3,47–49]. Naturally, fluments of the valley bottom. The four main terrace patterns (surface patterns) are recognized as those patterns also existed during all former interglacials. However, these former interglacial periods occurring frequently along most rivers in Central Europe and other areas, e.g., [3,47–49]. Naturally, are preserved by their fluments only and not by their terraces. those patterns also existed during all former interglacials. However, these former interglacial periods 6.2.are preserved Terrace Texture by their of the fluments Valley Underground only and not by their terraces. The valley underground is a treasure full of secrets. Unravelling them is difficult because of its frequent position under groundwater. In rare cases, construction pits allow full insight. On the Upper Main River sometimes the gravel pits were pumped out to gain the gravel, nearly completely in contrast to exploitation of gravel and sand from those pits filled with ground water. Within areas of minor tectonical uplift, the valley underground consists of the following units (Figures9 and 16): the Holocene fluments are presented as row terraces. Sometimes, Late Pleistocene fluments are adjoining. Underneath occur socles of Late Pleistocene fluments down to the pelma. The terraces of the valley bottom can form step terraces, form row terraces, form stack fluments with overlap, or they can present nested terraces (Figure 16). Mostly a mixture of all occurs. The term step Terrace (Figure 16a) refers to a flight of steps at the surface; additionally, the bases are often stepped. The term row Terraces (Figure 16b) refers to a level row of different terraces at the surface; additionally, Geosciences 2020, 10, 501 13 of 22 the bases may be level among themselves, or they have a nested texture, as visible in Figure9 with the H5 and H6 terraces. Stack Fluments (Figure 16c) refer to a stacked texture, the uppermost flument of which forms a level surface; the bases of the fluments underneath may be level, incised, or have a ± nested or other texture. In contrast to these foregoing textures, a nested texture (Figure 16d) refers to the base of the flument and highlights that the flument base lies higher than the base of the flument underneath; the surfaces of a nested texture may form a terrace row, a stepped terrace flight (Figure 16d), or a level surface (Figure 16c). The reason for the different textures mostly is either tectonic movements or alterations of the river regime during the glacial–interglacial changes. In areas of stronger uplift, the fluments form step terraces, where the Holocene terraces form the pelma of the valley fill, e.g., on the Danube River [6] (p. 46). In contrast, areas of subsidence present textures much more differentiated and richer in the amount of fluments above the pelma, e.g., molasse basins [50], or in graben structures. In the Upper Rhine Graben (Figure1), the thickness of the Quaternary comes up to 500 m subdivided by many lithofacies units [44]. Additionally, the Pliocene Geosciences 2020, 10, x FOR PEER REVIEW 13 of 23 underneath comes up to 500 m [51].

Figure 15. Terrace patterns in the valley bottom. Figure 15. Terrace patterns in the valley bottom.

6.2. Terrace Texture of the Valley Underground The valley underground is a treasure full of secrets. Unravelling them is difficult because of its frequent position under groundwater. In rare cases, construction pits allow full insight. On the Upper Main River sometimes the gravel pits were pumped out to gain the gravel, nearly completely in contrast to exploitation of gravel and sand from those pits filled with ground water. Within areas of minor tectonical uplift, the valley underground consists of the following units (Figures 9 and 16): the Holocene fluments are presented as row terraces. Sometimes, Late Pleistocene fluments are adjoining. Underneath occur socles of Late Pleistocene fluments down to the pelma. The terraces of the valley bottom can form step terraces, form row terraces, form stack fluments with overlap, or they can present nested terraces (Figure 16). Mostly a mixture of all occurs. The term step Terrace (Figure 16a) refers to a flight of steps at the surface; additionally, the bases are often stepped. The term row Terraces (Figure 16b) refers to a level row of different terraces at the surface; additionally, the bases may be level among themselves, or they have a nested texture, as visible in Figure 9 with the H5 and H6 terraces. Stack Fluments (Figure 16c) refer to a stacked texture, the uppermost flument of which forms a ± level surface; the bases of the fluments underneath may be level, incised, or have a nested or other texture. In contrast to these foregoing textures, a nested

Geosciences 2020, 10, x FOR PEER REVIEW 14 of 23 Geosciences 2020, 10, x FOR PEER REVIEW 14 of 23 texture (Figure 16d) refers to the base of the flument and highlights that the flument base lies higher texturethan the (Figure base of 16d) the refersflument to underneath;the base of the the flument surfaces and of highlightsa nested texture that the may flument form abase terrace lies higherrow, a thanstepped the terracebase of flight the flument (Figure underneath; 16d), or a level the surfacesurfaces (Figure of a nested 16c). texture may form a terrace row, a steppedThe terrace reason flight for the (Figure different 16d), textures or a level mostly surface is either(Figure tectonic 16c). movements or alterations of the riverThe regime reason during for the glacial–interglacialdifferent textures mostly changes. is either tectonic movements or alterations of the river Inregime areas duringof stronger the glacial–interglacialuplift, the fluments changes. form step terraces, where the Holocene terraces form the pelmaIn ofareas the ofvalley stronger fill, e.g., uplift, on the the fluments Danube Riverform step[6] (p. terraces, 46). In wherecontrast, the areas Holocene of subsidence terraces form present the pelmatextures of muchthe valley more fill, differentiated e.g., on the Danube and richer River in [6] the (p. amount 46). In contrast,of fluments areas above of subsidence the pelma, present e.g., texturesmolasse basinsmuch [50],more or differentiated in graben structures. and richer In thein theUpper amount Rhine of Graben fluments (Figure above 1), thethe thicknesspelma, e.g., of molassethe Quaternary basins [50], comes or inup graben to 500 structures. m subdivided In the by Upper many Rhine lithofacies Graben units (Figure [44]. 1), Additionally, the thickness the of GeosciencesthePliocene Quaternary 2020underneath, 10, 501comes comes up toup 500 to 500m subdividedm [51]. by many lithofacies units [44]. Additionally,14 ofthe 22 Pliocene underneath comes up to 500 m [51].

Figure 16. Principal flument textures of minor tectonical movements of the valley bottom in Central FigureEurope. 16. 16. PrincipalPrincipal flument flument textures textures of minor of minor tectonical tectonical movements movements of the ofvalley the bottom valley bottomin Central in CentralEurope. Europe. 6.2.1. Varied Relief Sculpture of the Pelma in the Valley Underground 6.2.1. Varied Relief Sculpture of the Pelma inin thethe ValleyValley UndergroundUnderground Normally, the sculpture of the pelma or strath varies from even to wavy. Wegmann et al. (Apennines)Normally, [52] the and sculpture Burbank ofet al. the (USA) pelma [26] or record strath of varies altitude from variation even to of wavy.wavy. the strath/pelma Wegmann Wegmann upet et al. al.to (Apennines)3 m. Sometimes [[52]52] there and Burbankoccur scours et al. from (USA) locally [26] [26] record older erosional of altitude events, variation as shown of the in strathstrath/pelma Figure/pelma 17 by up the to 3Flument m. Sometimes 1; it is a there presumably occur scours MIS 5 from erosion. locally older erosional events, as shown in Figure 17 by the Flument 1; it is a presumably MIS 5 erosion.erosion.

Figure 17. Three stacked fluments in the valley underground below a Last Glacial terrace. Sketch of the construction pit of the sewage Unterzettlitz close to Bad Staffelstein/Bavaria (Location in Figure1).

Length about 100 m. Height enlarged about eight times. Flument 1 shows a warm-climate L-flument

(presumably MIS 5). Flument 2 presents the Reundorf Flument (70–c. 30–20 ka). Flument 3 represents a late Pleniglacial flument (20–11.7 ka) (Schönbrunn or Ebing Flument).

Moreover, hillocks of the pelma occur as shown in Figure8 below the Zettlitz Terrace. The hillock rises so high that the basal erosion of the Zettlitz Flument could completely scour away the socle of the Reundorf Flument. In rare cases—if harder rocks adjoin much weaker ones—such hillocks tower above the valley terraces forming islands in the valley bottom. Geosciences 2020, 10, 501 15 of 22

6.2.2. Scouring Out the Valley Underground In the Upper Main area, where the water is pumped out for excavation of gravel, the whole structure of the valley underground can be studied. Normally, the pelma of the valley underground is formed by the Late Pleistocene deposits, in Figure9 the older parts of the Reundorf Flument. Where the Reundorf Terrace itself is cut by younger fluments, these younger fluments show nested texture (Figure9) in a way that the older parts of the Reundorf flument remain always as socle below the younger fluments. Such an example is shown by the Zettlitz Flument that overlies the socle of the Reundorf Flument. In the case of Figure 18, the older part of the Reundorf flument encompasses at its base remnants of mammoth tusks with a 14C age of 23.415 475 a BP, roughly calibrated to 28 ka BP. Above occurs ± a layer of fines with drop soils, and even higher follow the fine grained interstadial Abtswiesen Beds [53] (p. 51) with a thin peat layer dated to 20.525 410 a BP, calibrated to roughly 25 ka BP. ± These Abtswiesen Beds cut unconformably ice wedge casts (Figure 19) that extend down through the pelma into the bedrock—here represented by Upper Triassic red shales called Feuerletten (fire shale).

GeosciencesIn places, 2020 between, 10, x FOR the PEER basal REVIEW Reundorf Flument and the Zettlitz Flument flument remnants may16 be of left23 of older Holocene age dated by wood.

Figure 18. Stacked fluments in the valley underground below a Holocene terrace. Sketch of an area of several gravel pits in the Zettlitz Terrace in Lichtenfels-Trieb/Bavaria. The Reundorf Flument is Figure 18. Stacked fluments in the valley underground below a Holocene terrace. Sketch of an area of preserved by its socle containing the interstadial Abtswiesen Beds. These Pleniglacial deposits are several gravel pits in the Zettlitz Terrace in Lichtenfels‐Trieb/Bavaria. The Reundorf Flument is unconformably overlain by the roughly 2000 years old Zettlitz Flument that is completed by the Early preserved by its socle containing the interstadial Abtswiesen Beds. These Pleniglacial deposits are Medieval Hochstadt Soil horizon [54] (p. 52), since then overlain by younger flood deposits. unconformably overlain by the roughly 2000 years old Zettlitz Flument that is completed by the Early Medieval Hochstadt Soil horizon [54] (p. 52), since then overlain by younger flood deposits.

Figure 19. The lateral accretion gravel (L‐gravel) of the Holocene Zettlitz Flument overlies unconformably the vertical aggradation gravel (V‐gravel) of the Late Pleistocene Reundorf Flument that includes an ice wedge cast (Photo: W. Schirmer, 12 October 1972).

Geosciences 2020, 10, x FOR PEER REVIEW 16 of 23

Figure 18. Stacked fluments in the valley underground below a Holocene terrace. Sketch of an area of several gravel pits in the Zettlitz Terrace in Lichtenfels‐Trieb/Bavaria. The Reundorf Flument is preserved by its socle containing the interstadial Abtswiesen Beds. These Pleniglacial deposits are Geosciences 2020, 10, 501 16 of 22 unconformably overlain by the roughly 2000 years old Zettlitz Flument that is completed by the Early Medieval Hochstadt Soil horizon [54] (p. 52), since then overlain by younger flood deposits.

Figure 19.FigureThe 19. lateral The lateral accretion accretion gravel gravel (L-gravel)(L‐gravel) of of the the Holocene Holocene Zettlitz Zettlitz Flument Flument overlies overlies unconformably the vertical aggradation gravel (V‐gravel) of the Late Pleistocene Reundorf Flument unconformablythat includes the vertical an ice wedge aggradation cast (Photo: gravel W. Schirmer, (V-gravel) 12 October of the1972). Late Pleistocene Reundorf Flument that includes an ice wedge cast (Photo: W. Schirmer, 12 October 1972).

6.2.3. Holocene Fluments and Terrace Row

TheGeosciences differentiation 2020, 10, x FOR of PEER the REVIEW Holocene flument row [9,11] is possible by: 17 of 23

1. 6.2.3.mapping Holocene the morphological Fluments and Terrace terrace Row row (see Figure 15), starting with mapping from aerial photos, followedThe differentiation by field mapping of the andHolocene (hand)coring flument row at particular[9,11] is possible points; by: 2. identification of their surface soils, best to do by small pits, less good by corings. These soils 1. mapping the morphological terrace row (see Figure 15), starting with mapping from aerial vary from older to younger terraces from strongly or mature soils to the incipient soil of the photos, followed by field mapping and (hand)coring at particular points; 2.youngest identification terrace. of In their the surface temperate soils, best climate, to do theby small soil catenapits, less ranges good by from corings. Luvisol These over soils Cambisol vary to Leptosol.from older This to younger soil development terraces from passesstrongly o orff maturequicker soils the to lower the incipient the carbonate soil of the content youngest of the floodplainterrace. deposits.In the temperate Thus, climate, in river the areas soil catena with highranges carbonate from Luvisol content over Cambisol (e.g., Alpine-connected to Leptosol. rivers),This the soil di developmentfferentiation passes of floodplain off quicker soils the allowslower the optimal carbonate diff erentiationcontent of the of thefloodplain individual floodplaindeposits. terraces Thus, in [11 river] (p. areas 37); with high carbonate content (e.g., Alpine‐connected rivers), the 3. gatheringdifferentiation datable of material floodplain within soils the allows fluments: optimal wood differentiation remnants, of especially the individual rannen floodplain (tree trunks), terraces [11] (p. 37); pollen, etc. Remarkable are rannen (Figure 20) that give exact dendrochronological death times 3. gathering datable material within the fluments: wood remnants, especially rannen (tree trunks), for their imbedding moment. These death times testify to the regularly flooding that destroys the pollen, etc. Remarkable are rannen (Figure 20) that give exact dendrochronological death times riverinefor their forest imbedding and accumulates moment. These the Zettlitz death times flument testify (H4 to inthe Figure regularly9). Asflooding in all that seven destroys Holocene flumentsthe riverine and terraces,forest and accumulates this Zettlitz the flument Zettlitz flument is distributed (H4 in Figure in all 9). valleys As in all ofseven Central Holocene Europe, indicatingfluments enhanced and terraces, fluvial this activity. Zettlitz On flument the Main is distributed River it dates in all from valleys 226 of BC Central to 130 ADEurope, [55]. indicating enhanced fluvial activity. On the Main River it dates from 226 BC to 130 AD [55].

FigureFigure 20. Rannen20. Rannen (fossil (fossil tree tree trunks) trunks) inin thethe Zettlitz Flument Flument (226 (226 BC–130 BC–130 AD) AD) in a in gravel a gravel pit at pit at Lichtenfels-TriebLichtenfels‐Trieb/Bavaria./Bavaria. Stratigraphical Stratigraphical position position see see Figure Figure 18 18 (Photo: (Photo: W. W. Schirmer Schirmer 19 19 September September 1971). 1971).

7. Discussion The discussion focuses terms and processes of the valley inventory in comparison to other usage (Figure 21).

Geosciences 2020, 10, 501 17 of 22

7. Discussion Geosciences 2020The, discussion10, x FOR PEER focuses REVIEW terms and processes of the valley inventory in comparison to other usage18 of 23 (Figure 21).

Figure 21. Terraces and fluments discussed in this paper. (a) Fluvial rock terrace; (b) Cut-in-fill terrace; (c) Pelma terrace; (d) Cut-in flument; (e) Nested flument; (f) Stacked flument. Figure 21. Terraces and fluments discussed in this paper. (a) Fluvial rock terrace; (b) Cut‐in‐fill terrace; (7.1.c) Pelma Definitions terrace; of Fluvial(d) Cut Terms‐in flument; Used in (e this) Nested Paper flument; (f) Stacked flument.

7.1.1. Fluvial Erosional Forms: (Morphological) Terraces 7.1. Definitions of Fluvial Terms used in this Paper Cut-in terrace. Synonyms: Stream-cut terrace, e.g., [36], erosional terrace, degradational terrace 7.1.1.(e.g., Fluvial [26] Erosional (p. 32)). Using Forms: “erosional” (Morphological) and “degradational Terraces terrace” should mark whether the degradation was formed fluvially or by general wearing down. Most common erosional terrace type. Cut‐inSpecial terrace Conditions:. Synonyms: Stream‐cut terrace, e.g., [36], erosional terrace, degradational terrace (e.g., [26]Erosion (p. 32)). host Using is pure bedrock:“erosional“ and “degradational terrace“ should mark whether the degradationa. Fluvial was formed rock terrace fluviallyor Fluvial or by rock-cutgeneral terrace:wearing A down. terrace Most within common the bedrock, erosional softer orterrace harder, type. without fluvial deposits. Sometimes difficult to decide whether it originates from fluvial erosion, Specialor fromConditions: another erosional process in the context of wearing down, or a mixture of both. Erosion host is pure bedrock: a. Fluvial rock terrace or Fluvial rock‐cut terrace: A terrace within the bedrock, softer or harder, without fluvial deposits. Sometimes difficult to decide whether it originates from fluvial erosion, or from another erosional process in the context of wearing down, or a mixture of both. Comparison: Corresponds to Rock terrace sensu Bucher [36] and to strath terrace (pars) without any flument (alluvial cover) e.g., [40]. See Chapter 7.2. Erosion host is pure flument (see below 2.): b. Cut‐in‐fill terrace, e.g., [17,56]. Synonym: Fill‐cut terrace (e.g., Bull [18] (p. 8)). Comment: Erosional terraces in fluments should be checked carefully whether they actually are not cut‐in

Geosciences 2020, 10, 501 18 of 22

Comparison: Corresponds to Rock terrace sensu Bucher [36] and to strath terrace (pars) without any flument (alluvial cover) e.g., [40]. See Section 7.2. Erosion host is pure flument (see below 2.): b. Cut-in-fill terrace, e.g., [17,56]. Synonym: Fill-cut terrace (e.g., Bull [18] (p. 8)). Comment: Erosional terraces in fluments should be checked carefully whether they actually are not cut-in fluments. Compare: (e.g., Hargitai [57], 2240). Remark on Figure2b in Hargitai: It should not have a line separating the fill-cut flument (“Terrace“) from the hosting flument; [56]. Erosion host is flument plus bedrock: c. Pelma terrace: A terrace cut into the flument base almost down to the pelma carrying an eluvial relictic flument litter. This relictic flument litter may be developed by fluvial degradation and/or by denudational erosion.

7.1.2. Fluvial Depositional Forms: Fluments V-flument: Vertically aggradational flument. Most common flument type of the braided rivers. L-flument: Laterally accretional flument. Most common flument type of meandering rivers. Both types of flument terminate above with a flument terrace that changes from the well-preserved ortho-terrace by erosion or denudation to the oblique dia-terrace and eventually to the pelma-terrace that traces the pelma with last relics of the eluvial flument. Compare: “fill terrace“ (e.g., Bucher [18,36,57]), aggradational terrace or constructional terrace e.g., [26]: they correspond to V- or L-flument plus their terrace (tread).

7.1.3. Fluvial Forms with Cyclic Alternating Erosion and Deposition d. Cut-in flument: A flument cut in older fluments or/and bedrock. Most common flument texture in valleys. e. Nested flument, fill-in flument: A normal flument inset in an older flument. Actually, it is an incised flument within a small flument staircase, with its tread below that of the host flument and its base above the base of the host flument. Compare: “Nested fill terrace”, e.g., [17], or “Cut-and-fill terrace” that correspond to nested flument. f. Stacked flument: Overlapping flument and/or superimposed flument (Figure8). Actually, a superimposed flument always should start with overlapping.

7.2. Comment to Strath and Strath Terrace It is not easy to place the term strath terrace in the category of the other terraces or fluments. Therefore, the following remarks apply to this term: Its definition has been altered several times since Bucher (1932) [36]. “The controls on the formation of bedrock straths are still debated” (Burbank & Anderson 2012 [26], p.33). Bucher’s strath is a flat, elongated, and especially wide valley surface with both degradational stream portions and marginally non-fluvial, probably weathering portions. He neither mentions the position of the surface in bedrock nor occurrence of stream deposits. However, both bedrock surface and stream deposits were coupled with Bucher’s term strath later [21,25,29,37,39,40,58]. The following facts were developed from Bucher’s scanty statements:

1. The contact between bedrock and overlying deposits of any kind is called strath. The bedrock forming the bed of a contemporary channel is called active strath (Wohl [59] p. 1). 2. The name “strath terrace” is given to any terrace involving the strath: both to an abandoned surface of the bedrock as ledge in a fluvial context, and to a strath covered with fluvial deposits, sometimes degradational, sometimes aggradational, sometimes very thin, sometimes up to 5 m in thickness [57]. In the end, terraces and fluments of all those types were called “strath terrace”. Geosciences 2020, 10, 501 19 of 22

They occur within a staircase, sitting on bedrock that is bare or covered by aggradational deposits, e.g., [18,52].

Howard’s fillstrath terrace [21,60] (see 7.2), far beyond these definitions, was meanwhile dropped. Comment: Whether a flument is cut into the bedrock or not is not important for the flument. Fluments may have an underlay that merges from bedrock into older fluments (Figure 11). The kind of underlay of a flument should not affect the type and name of the flument (plus its morphological terrace). A classification of fluments should characterize the fluments themselves and their context in the fluvial valley edifice. Consequently, strath terraces cannot be grouped and compared with the other mentioned “terraces” or fluments. The major importance of strath terraces is to determine incision rates of the stream [29] (p.1131}, [40,52,61]. Hancock & Andersen [29] (p. 1131) state: “The beveled strath and thin alluvial cover distinguish strath from fill terraces, which have no underlying bedrock platform and can be many tens of meters thick”. Comment: 1. For the fluvial beveling of strath by an eroding stream, the thickness of the overlying flument is unimportant. The thin alluvial cover on strath terraces is manifold stated [18,22]. It is a consequence of the deeply eroded (degraded) fill of the highest elevated fluments of a terrace flight. The Main Formation of 30 m thickness as stack of fluments (“fill terrace”) rests on a strath/ground pelma. 2. Each flument (“fill terrace”), of course, sits on underlying bedrock. In the case of a flument stack, it is only the lowermost flument that rests on bedrock. A strath terrace with sparse relics of an eluvial flument corresponds to the ground-pelma terrace (Figure5). A strath terrace with thinner or thicker aggradational cover is a normal flument terrace. The use of the name “strath terrace” for a flument overlying the bedrock is a frequently applied case in American literature. Yet, Easterbrook [17] does not involve it into his system of terraces at all. Thus, “strath terrace” is not a term characterizing a flument itself, but characterizing the spatial position of a fluvial phenomenon in regard to the bedrock within a valley [40,52]. The only common feature of strath terrace and flument terrace is that both are cut-in terraces.

7.3. Glossary of Fluvial Terms Used in the Paper See Table S1 in supplementary file. It includes the term Main Formation [62].

8. Conclusions The edifice of fluvial deposits exhibits a large variety of forms and textures, in addition to a large variety of preservation conditions. The older the fluments are, the more relictic is their architecture. An important point of vertical subdivision is the recognition of unconformities between different fluments. For this, it is essential to study good and large outcrops and to follow their excavation progress over long periods of time. The principles mentioned first in Section3 should be applicable to all fluvial investigations, even to pre-Neozoic ones [63]. The knowledge of the inner structure of a flument is important for the reconstruction of fluviatile series by flument remnants. Likewise, an ortho-terrace can be reconstructed by knowledge of the flument structure. By building a flument flight, smaller or larger terrace stacks can be an obstacle. Flument overlap is certainly a frequent case, but is difficult to recognize. A special case is the shift of a valley. It may highly confuse the construction of a terrace flight, as shown in Section 5.3. The occurrence of a flument flight side by side to an earlier fluvial stack is surely a rare case. However, in smaller sections or during shorter periods, it may happen in many valleys. The flument architecture of the valley underground varies depending on the local tectonical situation. The cases presented here, including some variations, are frequent in many valley undergrounds all over the world. Geosciences 2020, 10, 501 20 of 22

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3263/10/12/501/s1, Table S1: Glossary of fluvial terms used in the paper. Funding: This research received no external funding. Acknowledgments: Thanks to the two unknown reviewers for their valuable advice and constructive criticism. For providing literature, I thank Gudrun Klein, FAU Erlangen. I am grateful for the editorial improvement of the English style. Conflicts of Interest: The author declares no conflict of interest.

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