Dear Dr. Miller,

the terms is not widely used anymore and mostly in papers on sedimentary geology described in a more detailed way from creep to slide.

In general it is a mass of loose material derived from the soil and the rocks immediately underneath. The processes include creeping, washing and sliding triggered by precipitation, swelling and contracting of mineral matter etc.  The mass wasting processes lead to a dumping of the material at the foot of the hill slopes, the  inclination of which can change from gentle to steep. Neither the length of transport nor the angle of the hill slopes are used as discriminators. I suggest to be more specific and perform a more subtle distinction of the processes and jump this general terms and make use of a wide range of terms used to describe mass wasting processes in geomorphology and sedimentology.

There are some papers like:

Botha, G.A., Fedoroff, N., 1995. Palaeosols in Late Quaternary

colluvium, northern KwaZulu-Natal, South Africa. Journal of

African Earth Sciences 21, 291–311.

Lehre, A.K., 1987. Rates of soil creep on colluvium-mantled hillslopes

in north-central California. In: Erosion and Sedimentation in the

Pacific Rim, Proc. Corvallis Symp., vol. 165. IAHS Publ., pp. 91–

100.

that may be helpful.

Best regards

H.G.Dill

I would say that it's based on the depositional mechanism - colluvium by gravity (e.g. mass wasting, slope processes) while alluvium is by fluvial action.

There's an interesting difference between the German and English definition / use of colluvium. I find this reference helpful:

Kleber, A. (2006): 'Kolluvium' does not equal 'colluvium'. Zeitschrift für Geomorphologie N.F. 50 (4), p. 541-542. 

One point you can consider additionaly is the distance of transport. It is always rather short for colluvium (at the slope scale) and longer for alluvium (catchment scale).

Dear Dr. Miller,

the terms is not widely used anymore and mostly in papers on sedimentary geology described in a more detailed way from creep to slide.

In general it is a mass of loose material derived from the soil and the rocks immediately underneath. The processes include creeping, washing and sliding triggered by precipitation, swelling and contracting of mineral matter etc.  The mass wasting processes lead to a dumping of the material at the foot of the hill slopes, the  inclination of which can change from gentle to steep. Neither the length of transport nor the angle of the hill slopes are used as discriminators. I suggest to be more specific and perform a more subtle distinction of the processes and jump this general terms and make use of a wide range of terms used to describe mass wasting processes in geomorphology and sedimentology.

There are some papers like:

Botha, G.A., Fedoroff, N., 1995. Palaeosols in Late Quaternary

colluvium, northern KwaZulu-Natal, South Africa. Journal of

African Earth Sciences 21, 291–311.

Lehre, A.K., 1987. Rates of soil creep on colluvium-mantled hillslopes

in north-central California. In: Erosion and Sedimentation in the

Pacific Rim, Proc. Corvallis Symp., vol. 165. IAHS Publ., pp. 91–

100.

that may be helpful.

Best regards

H.G.Dill

for a field geologist & prosmector there is a simple rule: alluvium is deposited in (by) rivers, you find colluvium on hillslopes ansd is the effect of gravity transport. The definitions are geomorphological though.

Lithologically - sedmimentological, you can end up with the same.

Basically, colluvium can be defined as the sedimentary material which has been transported across and deposited on slopes as a result of mass movement processes and soil wash. It is frequently derived from the erosion of weathered bedrock (eluvium) and its deposition on low angle surfaces, and can be differentiated from material which is deposited primarily by fluvial agency (alluvium).

Further details can be found from the following:

Goudie, A.S. (2004). Encyclopedia of Geomorphology, London: Routledge.

Thomas, M.F. (1994) Geomorphology in the Tropics: A Study of Weathering and Denudation in Low Latitudes, Chichester: Wiley.

Bertram, P., Hetu, B., Texier, J.P. and van Steijn, H. (1997) Fabric characteristics of subaerial slope deposits, Sedimentology 44, 1–16.

Hi Bradley:

The US Geological Survey (USGS) still uses the term colluvium quite a bit, especially in the description of units in trenches that are dug for paleoseismic assessment. I believe it is a definition that isn't going to the geology dust-bin just yet....and I believe the USGS to be a relatively knowledgeable and unbiased authority. But then I work for the USGS!

Here is the official USGS definition of colluvium (an unconsolidated, mass-wasting material): A general term applied to any loose, heterogeneous, and incoherent mass of soil material and/or rock fragments deposited by rainwash, sheetwash, or slow, continuous downslope creep, usually collecting at the base of gentle slopes or hillsides.

http://mrdata.usgs.gov/geology/state/sgmc-lith.php?code=1.5

And here is the USGS definition for alluvium (an unconsolidated material): A general term for clay, silt, sand, gravel or similar unconsolidated detrital material, deposited during comparatively recent geologic time by a stream or other body of running water, as a sorted or semi-sorted sediment...

http://mrdata.usgs.gov/geology/state/sgmc-lith.php?code=1.1

Hope this helps.

Dear Colleagues,

Thank you for the great contributions.

Now, my question is: will you still maintain your definition if we consider the Martian surface as well? I.e. currently no (flowing) water on the surface (?), still there are deposits that seem to be fluvial (not necessary just in valles). Much of talus material (intentionally avoiding the categories above) on slopes, of course with strong (?) eolian influence. Clearly, yet neither sampling, nor field work possible, so e.g., the composition/grain size cannot be used in the definition. IMHO the terms should be somehow robust enough to become applicable to the features observable on Mars as well.

I am looking forward to reading your comments on that. Thanks in advance and kind regards, Balázs

Dear Balazs,

interresting on what you are working, but we have to distinguish sediment from the land form.If you don't have field observation (sediment analyses, soil profile or outcrop structure) it is hard to say something. Your talus slope are landforms and not sediment (colluvium or not). Furthermore some landform looks similar, i.e some glacial landform (protalus landform) shows similar form with lobe of debris flow but processes are different.

The problem of coluvium is that this term is use in many discipline in different meanings (geomorphology,soil sciences, quaternary geology). Each disciplines will use different diagnostic criteria leading to confusion in the mind of the other scientist. For geomorphologist the landscape position "at the base of hillslope" is important, for soil scientist the time and spatial scale are different, new sediment coming on top of the soil after rainfall is colluvium, for quaternary geologist the sediment structure, fabric, lithology, shape of fragment is important and colluvium is link to quaternary time scale, for archeologist colluvium means neolithic land clearing...

Some authors will avoid colluvium for the above reasons and use specific terms as Slope deposit and classify Colluvium in a specific case of Slope Deposit.

Dear Jérôme,

Thanks for the prompt answer, exactly that is my problem and that is why wanted to tackle the geomorphic vs. sedimentological issue. You have pointed out that there is some confusion in the use of the terms even in the terrestrial disciplines, so the confusion is even greater in the Martian literature, especially if we do allow that photogeology or remote sensing geology exist. If we allow that there are many implications that may (or may not) work in the terrestrial context (but many people do as they would work), but less probably on the Mars.

On the other hand I do not believe that these terms should be related to time, so in theory the terms are or will be applicable in the Martian context as well (as soon as we have some "field" evidence, e.g., from rovers), regardless on the timescale of the processes.

My conclusion then seems to be supported by your answer (my interpretation!), that many researchers use these terms erroneously in the Martian literature.

Thank you again and kind regards, Balázs

The barrage is a term associated with streams of water, either permanent or ephemeral, the deposit is normalente stratified, in ascending order, thicker bottom and thinner upward, all with some degree of roundness, the more distal than the deposit grains tend to sphericity, the reservoir matrix is typically sand.

The colluvium is a fallen gravity in conjunction with rainwater material in countries where rainfall is scarce is the deposit is as messy, but is called scree .. between blocks of rock matrix is ground and / or materilales finer ...

Dear Dr. Miller,

I would use the previously mentioned source: Goudie, A.S. (2004). Encyclopedia of Geomorphology, London: Routledge.

In my opinion, the distance of transport is important, although this may ultimately be subjective.

Also, I would examine the local slope to help determine whether gravity or flowing water is most likely the key process involved.

In my opinion, colluvium is NOT an obsolete term and is still useful.

Finally, I would still use the term in Martian geology.  While fluvial processes are not presently occurring on Mars, deposits formed in the past still need to be described.

Regards,

Don Hooper

It is more of an unsorted, unorganized, usually angular material that goes down the slope under the influence of gravity. More often such a mass (colluvium) will accumulate in the shape of a wedge (cone) and therefore known as a colluvial wedge. The transport direction is down the slope. 

From a generalized mapping perspective, in Alberta, colluvium is defined as poorly sorted material that has reached its present position as a result of direct, gravity-induced movement. It commonly occurs as slope and slump deposits along valley slopes and floors and includes pre-existing bedrock, till, glaciolacustrine, glaciofluvial and eolian sediments.

Hope that you find this definition useful.  

As long as I know colluvium is defined as a almost in situ sediment moved in response to gravity. Granulometry may vary from clay to blocks, may be mono or multy granulometric.

Alluvium is a water transported and deposited sediment in response to soil/rock erosion. Eolic and glacial transported are not called like that although the result is almost the same.

Dear Jérôme (and others)

I find "colluvium" a very useful summary term for all deposits that occur on hill slopes. It contrasts with "alluvium", which occurs on valley floors. I disagree that colluvium is a special case of slope deposit - the two are essentially the same. It includes deposits on hill slopes from creep, landslides and other gravity-driven processes, and surface wash where the deposit is on a hill slope. Once colluvial materials reach the valley floor and are incorporated by alluvial processes they become alluvium. 

Dear Dr. Miller.

I agree with guys above. Very complete answers.

In my humble opinion, Colluvium refers to deposits of materials (rocks, fragments, and soil) that were moved or transported by gravity action.

In pedology this term is very common, and is necessary to stablish soil genesis.

I attached a couple of papers, I guess they could be useful for your questions

Greetings from Costa Rica.

Dear Dr Pain,

Ok colluvium are for you synonymous of slope deposit, but as pointed by Dr Bradley Miller, the definition changes between disciplines (quaternary geology, geomorphology, soil science) and countries.

I agree with you that until the deposit are on hillslopes even footslopes and mainly gravity driven ,the deposit are colluvium, it is clear for coarse sediments but not for fine (silt to clay). But the problem is that we define colluvium according to landscape position, what will be for you the criterias of recognition on a soil pit and for fine sediment? Without using grain size analyses or other techniques like geochemy to find lihtic discontinuities it is hard.

Furthermore, for soil scientist (agropedologist), colluvium is also all new material comming at the surface of the soil after plouging and rainfall event. It isexpressed in the soil system of classification WRB by the "Novic" qualifier. WRB as also the "colluvic" qualifier wich refers to Colluvic material defininig more or less your definition of Colluvium.

Best regards

Dear colleagues,

the problem becomes more and more evident that we are going to cross the boundary between geogenic and anthropogenic as we consider human processes as part of the story (e.g. ploughing and dumping). It opens up a new road into technosoils, dumps etc. In my opinion the term should only be used as a discriminator between natural processes of transport controlled by running water (alluvial-fluvial) and those controlled by gravity with little or no impact by water. For a more detailed description, we can avail ourselves of a wide range of technical terms in both compartments such as creep, bedload transport etc.

Best regards

H.G.Dill

To my mind, and in agreement with several of the responses, colluvium is a sedimentary deposit created by mass wasting (dominantly gravitational) processes.  Similarly, alluvium is the material product of fluvial (running water) processes.  It is not a definition based on the sedimentological properties. The sedimentological characteristics of colluvium can be very helpful in identifying colluvium and distinguishing it from loess, till, or alluvium, but the sedimentary properties depend on the texture of the original source material that can be highly variable.  For example, mass wasting (e.g., slumping) of a loess deposit may result in a well sorted silt, but the deposit will still be colluvium.  Conversely, other processes besides mass wasting, may result in a gravelly, poorly sorted material; e.g., glacial deposition of till. Designating a material as colluvium, therefore, specifies the basic process of formation and should be reserved for cases where that process is understood. Otherwise, we should use a non-genetic description of the sedimentoilogy.  For example, if a gravelly deposit is of unknown origin, we call it a diamicton.  A well-sorted silt may be loess, alluvium, colluvium, till, etc. 

I'm also in a lot of agreement with previous answers here, especially those emphasising that these are interpretative terms. For my money, the prevailing definitions are those outlined several times above - colluvium is mobilised by gravity alone; alluvium is mobilised by shear resulting from broadly newtonian, channelized flow over a surface, i.e., riverine processes broadly defined. Things like debris flows are going to be necessarily ambiguous under such a scheme, and could be viewed as both in different contexts. I see two good reasons why these terms are not widely used:

1. There's a lot of ambiguity over where to draw the line between the two (especially IMO in the world of non-newtonian, steep, channelized flows), so why use a term that could easily be misunderstood?

2. Probably more importantly, these are big garbage can terms covering a whole bunch of actual process interpretations, but yet they remain fundamentally interpretative terms. So if you want to interpret, why use a less specific term when you could be process specific one? ...but if you want to dispassionately describe a sediment (i.e., most cases?), you shouldn't be using these terms at all.

So to my mind the use cases are largely field-based and colloquial (AKA, "I can't be bothered to walk all the way over to that other hillside to check out what's actually on it, but it's clearly covered in loose stuff"), or a few rare cases where you actually want a description lumping together hillslope and/or river bed deposits, without actually forcing a formal, precise interpretation.

Thank you everyone for the helpful comments! Following up on this discussion, Jérôme Juilleret and I have initiated a survey to measure the range of perspectives out there. To help us better understand how researchers differentiate these two materials and to look for common ground between different perspectives, we are asking your help by completing the survey linked to below. One early respondent described the survey as “a lot of fun.” We hope you enjoy it too!

Colluvium versus Alluvium Survey

http://goo.gl/forms/fV8RBWKjis  

Please also forward this survey to anyone you know working with these materials. Thank you for your time and input.

scale matters!  to what scale the term colluvium is appropriate?

Thank you Dr. Constantini for raising this point about scale! To help the discussion, could you elaborate on what you mean by scale? Are you referring to the scale at which the diagnostic properties are identified, the extent over which the deposit occurs, or something else?

colluvial areas, where slope sediments are deposited can be usually mapped at detailed or semi detailed scales, At broader scales these areas are included in larger forms, like debris accumulation forms

Los coluvios (en zonas tropicales) son cualquier tipo de material, caido desde las laderas rocosas, principalmente por la acción de la gravedad, el agua y en combinación con los suelos residuales. Este posee una selección de materiales, por el transporte, su ángulo de estabilidad varía entre 28 y 35 grados, con vegetación podría aumentar. El otros ambientes solo se da la caida gravitacional y recibe el nombre de canchal.

The colluvium (in the tropics) are all materials, fallen from the rocky slopes, mainly by the action of gravity, water and combined with the residual soils. This has a choice of materials for transportation, stability angle varies between 28 and 35 degrees, could increase vegetated. The only other environments gravitational fall is given and called Canchal.

Dear Edoardo,

beyond the 2D mapping scale isue, there is the 3D scale issue. I mean the scale in an outcrop (geologist and geomorphologist) or a soil profile (pedologist) to define material.

Geologist (I include geomorphologist) look at colluvium with other eyes than pedologist.

For them the A and B horizon will be merge with the C horizon on which they diagnose the material. Geologist will diagnose colluvium according most of the time on coarse element because they are rock fingerprint from the source (saprolite). Of course, also matrix (silt, clay and sand) will be used in a more detailled way in detailled studies in quaternary research but I am speaking from a geological mapper point of view. Also quatenarist knows that in geological survey, surficial formations are badly mapped.  Consequently, they will diagnose at a 50 cm to several meters scale.

For soil scientist. Looking at a soil profile, we will be able to diagnose a colluvial deposits below 50 cm depth, 5 cm sees to be a lower limit. It is the Novic qualifier of the WRB. Novic (nv) (from Latin novus, new): having a layer, ≥ 5 cm and < 50 cm thick, overlying a buried soil that is classified with preference according to the ‘Rules for classifying soils’.

However , applying a Novic qualifier (colluvium

I’m currently working on engineering geology mapping of slope deposits aimed at improving shallow landslide susceptibility assessment. In my perspective slope deposits reasonably correspond to what most of you define as colluvium: loose deposits located on hillslopes, derived from the upsolpe and close underneath soils and rocks, by creeping, surface washing, landsliding and weathering processes.

In most geological maps, at least in Italy (but probably also in other countries…), colluvium corresponds to a specific legend item among Quaternary deposits. This fact implicitly implies that colluvium polygons are only discontinuously mapped, mostly where large regions of the underlying “substratum” cannot be recognized during geological survey. Instead, in mid-latitudes regions, a more or less thick colluvium layer widely covers hillslopes except for some landforms and very steep areas. Lithology of the substratum, morphometric conditions, land cover, climate, etc., control the colluvium extension and properties.

For these reasons, in my opinion, when performing colluvium-oriented mapping, the “alternative” term slope deposit may be used in order to discriminate from the concept of colluvium as a specific geological mapping legend item (i.e. the “traditional” geological mapping approach).

Hope this may help discussion.

Dear Brad,

Perhaps this opinion arrives too late or is repeated, but I had the opportunity to study a colluvial system that is just 20 cm long, and it helped to understand better the genetical process. Colluvium, is the word describing the suite of deposits formed by colluvial fans and aprons, which are formed dominantly by gravitational forces over tractive forces. As a result of this difference, potential energy is higher for large blocks than smaller and fine grains are found in proximal reaches, while the largest ones advance further down, All colluvial fans show this, independent of their scale; large blocks are found distally. On the other hand, when transport depends on the shear stress, that could be the drag of a fluid of variable density, larger clasts are left in proximal areas as soon as the shear stress of the fluid decreases and smaller grains advance to the distal areas, as observed in alluvial systems (downstream fining).

This difference is well portrayed by Blikra and Nemec (Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record; DOI: 10.1046/j.1365-3091.1998.00200.x). And, due to the fact this process is not dependent on scales, it can be observed in very small scales as decimeters as I described for molards (Milana, 2016; Molards and Their Relation to Landslides Involving Permafrost Failure, DOI: 10.1002/ppp.1878).

I know I repeated some concepts already portrayed, but ity was needed to round the entire idea of colluvium as the result of a specific depositional system.

Best regards, Juan Pablo

Dear Leonardo and Juan, thanks for your contribution.

@Juan, your Colluvium definition (gravitationnal forces only) is the American view of Colluvium however some authors attributes also runoff as driving processes. In this case proximal deposits are blocks and distal the fine material.

Our study is to better understand trough discipline (geology, geomorphology and soil sciences) what we consider as colluvium. 

Please can you fill our survey and send it to your colleagues: https://www.researchgate.net/deref/http%3A%2F%2Fgoo.gl%2Fforms%2FfV8RBWKjis

Cheers