How do geologist know what they are looking at really is what they say it is?

I'm glad you mentioned mud.So clastic sedimentary rocks begin as other rocks and before they can exist there needs to be time enough to first weather and erode other rocks to make the small rock and mineral fragments that have been eroded and transported to a depositional center that edge mentions in Message 6.That brings up a couple other points.First it seems from what edge said we need two processes. We need weathering and erosion and then the pieces parts need to be transported to some spot where the stop and accumulate. To end up as a layer in the geological column of a given location the pieces parts need to stay in that location long enough for the individual pieces parts to turn back into another big rock.And back towards mud. It seems that the size of pieces parts also plays some part in determining what the final clastic rock will become.Is that correct and if so what types of clastic rocks are made from the different sized pieces parts?How can someone tell the various resulting rocks apart?

That is where data collection comes into play. We literally map rock bodies in three dimensions. We can actually see their shapes in space. We can see the paleotopography of a valley, the piles of glacial till, the lake beds and the great fans of river delta silts and sand channels.I once modeled a hill and realized that it was really a lake back in the Miocene. In fact, I'm certain that it had hot springs feeding it. Others tried to model it as a big featureless blob, but I showed that the whole environment made sense and controlled mineralization.Hey, it isn't easy and there's a lot of uncertainty that some people can't deal with, but as Pressie says, it all works.

Well, I have to say that learning from books and lectures is one thing, but there is nothing like seeing the field relationships.

That sums it up pretty well. Clastic rock definition is mostly based on grain size. That's were it gets really technical. In order of decreasing grain size:Gravel ---> conglomerate
Sand ----> sandstone
Silt ----> siltstone
Mud ----> mudstone
Clay ----> claystoneYeah, I know, it's tough.We can make it more difficult by giving them modifiers like 'calcareous' or 'organic'; or we can combine the terms to things like 'sandy carbonaceous mudstone' (which may not actually exist, but you get the idea).Hey, people spend careers studying this stuff.And we haven't gotten into carbonates or igneous rocks, etc. ... The most definitive way would be microscopically, but in the field one gets a feel for it. Basically, if you can see the grains you are dealing with siltstone or coarser.

I think so but as usual, a few questions. I've seen things described as mudstone and siltstone but also as shale.Particle size is pretty clear but what produces the different sized particles. Why does something end up as silt or mud or clay?Where does shale fit in? What is it and why is it different? And then there is slate???????

Shale has lots of thin layers.

Either chemical weathering to smaller mineral grains, or abrasion during transport. Shale is the traditional term for a mudstone. Here is the Wentworth scale for determining rock type: The different gravel sizes become conglomerates.Sands are self-explanatory.Muds include clay and silt in this diagram. In my work, I reserved claystone for very soft material that had no silt-sized grains in it. Slate is a low-grad metamorphic rock derived from mudstones. They usually have a very strong cleavage (finely space parallel partings).

Yes, shale is defined as a mudstone with a lot of irregular partings due to bedding.

Okay but jargon alert. Help please. What does irregular partings mean and what does bedding mean?

Well one factor is the proportion of chemical weathering on the one hand, and mechanical weathering and erosion on the other. Obviously mechanical weathering and erosion can produce chunks of pretty much any size.Chemical weathering, on the other hand ... consider a nice typical igneous rock such as granite. It will contain quartz, mica, amphibole, and two kinds of feldspar. Broadly speaking, the mica, amphibole and feldspar will be turned by chemical weathering into clay, which doesn't hold together particularly well, and which gives you fine particles: mud, in effect. The quartz, on the other hand, is indestructible, and gives us sand.(Because sand and clay particles are different sizes, they will be transported differently, which is why you end up with some places that are all sand and some which are all mud.)One consequence of this is that when there's been a lot of mechanical weathering involved in the production of sand the sand is arkose: that is, the grains contain an appreciable amount of feldspar; whereas when chemical weathering predominates the feldspar is converted to clay and the resulting sand is practically pure quartz.

Oh, I missed this. Slate is a metamorphic rock, and comes apart into layers because it has undergone foliation. To explain this, consider that the silicate minerals in a rock have shapes: they can come in strings or sheets. So when a rock is heated and compressed, these minerals get pushed so that they lie perpendicular to the direction of the compression.Have you read my book? All this stuff is in there; though I see the point of discussing these issues on this thread.

So far we have discussed two kinds of rock that both are found to create layers, shale and slate. The later has undergone metamorphose and it was that process through crystal alignment that produced the layering.Are the layering found is shale produced a different way?

Yes indeed, that's just ordinary layering as found in sedimentary rock. Though it occurs to me that perhaps the fact that clay is a sheet silicates may also have something to do with the layering in shale as well: the sheet silicates would mean that clay often comes in flakes, which would tend to settle parallel to the ground and then would be forced even more into that orientation by compaction.Clay particles, magnified:

That makes sense.So you are telling us that sometimes more than one process contributes to create a final sample? Is that correct?And would the form of things like mica and asbestos have similar origins?

Hi edge...Too busy to really be involved, but I have been lurking Anyway, I was thinking that since the topic is "How do geologists know..." the topics of grain shape (or roundness) and sorting should be covered, since those are major factors that help determine deposition environment.Also you mentioned chemical and mechanical weathering in the context of the production of the sediment; but don't those processes also occur during and after lithification? And don't those characteristics give us clues as to the depositional environment?HBD