Geology- working up from basic principles.

I've read a fair few discussions on geology, and whether what we see is the result of eons of slow deposition or a rapid global flood a few thousand years ago, and I've also seen the various arguements used to undermine conventional geology and promote flood geology.Many discussions seem to get nowhere, often based on misunderstandings or ignorance of how geology works and how geologists come to te conclusions that they do. They also seem to focus on single examples, for example a polystrate fossil or an alleged footprint.To combat this, I'd like to have a discussion which starts out with some of geology's basic principles, and once these have been satisfactorally dealt with, we will work up to discussing how certain rocks can form and what they can tell us about their environment, and then on to how we can read sequences of rocks to gain an understanding of change in conditions through time and space.The first thing I would like to deal with is the law of superposition. This states that sedimentary layers form in a time progressive sequence with the oldest layers at the bottom and the youngest on the top. To view this simply, picture stacking books one on top of the other- the book you put down first will by at the bottom of the pile, and the book you put down last will be at the top. Can we agree this principle is sound?

Hi Matt,While I am promoting your topic you may want to look at a thread we had running a while back.
Exploring the Grand Canyon, from the bottom up.No dates were being used in this thread, only discussion of processes in the formation of the layers of the Grand Canyon.

Thread moved here from the Proposed New Topics forum.

Thanks for promoting this Unfortunately I know barely anything about the Grand Canyon, so I found I couldn't really make much of a contribution to that thread, although it does make a great read.

Sound? fairly obvious, actually, assuming gravity is the primary force involved.However there are those that will wag their finger and claim that the present is not the key to past.Go read the debate between Razd and Simple on the age of the earth.Simple claims that at one time trees grew rings daily or weekly. So I suppose in the past there could have been a time when gravity was reversed, the speed of light was instantaneous, and leprechauns overturn the layers at night.

No objections yet then. Good. some more principles:The principle of Cross cutting relationships
If a feature such as an igneous intrusion (rock formed by magma cooling below ground) or a fault cuts through layers of rock, it must be younger than the rock it cuts.The principle of inclusions and components
If a rock has parts of another incorporated into it, the incorporated parts must be older than the rock. For example if a layer of sandstone contains pebbles or boulders, they must be older than the layer they are contained in.
This can also be seen in igneous intrusions, where they contain pieces of the rock that surrounds them (that are often thermally altered). These pieces are called xenoliths.Questions?

No questions. All very clear, The Matt. Keep it up.

Next, the principle of original horizontality.
Originally it was believed that all strata was deposited in horizontal layers and if they appear tilted, it is because they have been disturbed by tectonic movements. Think of this as more of a guideline. Most strata seems to be formed more or less horizontally, however material in some circumstances can be laid down at an angle of up to 30 degrees. The point to take away here is that strata seen with an extreme tilt or folded into anticlines and/or synclines must have been tilted after deposition and lithification (turning to rock). More on how we tell how rock is tilted later when I deal with way-up structures.

Looking at the picture in you message, I think we need some more explanations here. How can we determine that what we see was folded after the fact and not laid down in a rippled manner?

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Aslan is not a Tame Lion

Good question. There are a number of things we could look at to determine whether we are looking at a fold or a current ripple:Firstly we could look at the geometry of the features. While the ones pictured may bear a superficial resemblance to current ripples, many do not, for example these and these. Many of the tilts we observe greatly exceed the angle of repose for the substance in question. The angle of repose refers to the maximum slope angle that can be achived before friction is overcome and the slope fails. For dry sand, this is approximately 30 degrees from horizontal, and this is a high figure for unconsolidated sediment. From this we can know that if we see a sedimentary layer at an angle of say 60 degrees, it cannot have been laid down at that angle in flowing water.Secondly, we could look at the rocks involved. Lavas could be incorporated in to such features, as can evapourites, both of which cannot be formed by quickly moving moving water. Rocks could also show evidence of exposure to air at the surface, such as mud cracks or fossil rootlets or give us clues about which way up they were during their deposition (I will deal with these in more detail soon).Thirdly, we need to understand how ripples form. Various different types of ripples can form depending on the exact conditions present, however those caused by a flow (as opposed to an oscillation, with no net flow) are all characterised by erosion of material from one side, and deposition on the other, so the internal structure of the ripple differs substantially from the wavy form we see at the surface. See the lower diagram here. Symmetrical ripples (upper diagram) as you can see are characterised by layers deposited on either side that are not continuous and overlap in the middle. If we saw this kind of internal structure within a feature, we may consider it current deposited, but otherwise it seems unlikely.
We also need to know what kind of conditions can produce huge ripples. We have observed them in places where massive floods have occured, such as the channeled scablands of the US. These floods were very rapid and highly erosive, with very high water speeds, and as such would not allow the deposition of sands or fine clays which can take hours/days to completely settle out even in still water. The presence of such fine material in a large fold would rule out being formed by currents.

And ripple marks occur on the top of a stratum. That's one way to tell which way is up. Of course the top of one bed is the bottom of another, so it can be hard to tell sometimes. But on a wide enough scale each bed approximates a planar structure, and it's orientation can be measured. And usually dip at deposition is not more than a degree or two. With the exceptions you mentioned.OK, I'll buy all the principles so far. Please continue!

I really suspect you're pulling your info from some website and not giving due credit to that site.I suspect it is Radiometric Dating and the Geological Time Scale, a page I do thing highly of.Adminnemooseus

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That's quite an accusation. The principles I've dealt with so far are the very basics of geology, so it is natural that we would both start with them. Currently I'm writing almost entirely from my own knowledge, although I consulted Knighton's 'fluvial processes and landforms' breifly to jog my memory on ripple formation. I assure you if I lift any material from a website or book it will be duely cited.{Note: See here and upthread for the apology and discussion of the issue. End of discussion here. - Adminnemooseus}Edited by Adminnemooseus, : See above.Edited by Adminnemooseus, : Fixed link in previous edit. I had linked to the wrong topic.

I, for one, have no reason to doubt this. Keep up the step by step. The advantage of having this in a discussion thread, rather than as a set article, is that we non-geologists can ask for clarification on specific points as you go along, as Jar has already done above. So I'm bumping up the thread to encourage you, and because, like many here, I need the lesson!And a warm welcome to EvC.

Thanks for the vote of confidence bluegenes Moving on:I've stated so far that rocks are laid down oldest at the bottom and youngest at the top, but I've also mentioned that folds are able to disturb strata and tilt them subsequent to deposition. A good question to ask here would be "how would we know if layers have been overturned entirely?"One method I have already mentioned- this is the principle of inclusions and components. If a sedimentary layer contains eroded fragments of another, the layer must post date it. If we see a layer containing eroded bits of the one above it we know they have been overturned.There are also a number of other features, collectively referred to as way-up structures. These include:Mud /desiccation cracks.
When wet mud dries it often produces a cracked pattern on its surface with small fissures penetrating down in to it, as shown here. These can become filled with another sediment, often sand, and be preserved, as seen here showing us that at the time of deposition the cracks must have been pointing down. Any kind of fissure would do the trick, such as neptunian dykes (a blanket term which seems to encompass any kind of fissure infilled with sediment that formed in a marine environment) Link (subscription required unfortunately). Likewise Ice wedge and sand wedge casts (periglacial, i.e. permafrost features) could be used Link (roughly one third of the way down the page)Geopetal structures
These form when an organism with an internal cavity becomes partially infilled with mud soon after death, leaving the surface of this mud roughly horizontal. The rest of the cavity is usually mineralised when the sediment is lithified (turned to stone). This does more than tell us up from down- it actually acts as a crude spirit level, and measuring them can give us an approximate angle at which the sediment was deposited. See hereTool/groove marks
These are a small erosional features formed by flowing water on the surface of the sediment. They form where stones, shells, or indeed anything dragged by water leave small furrows and/or dents where they are dragged or bounced along the soft sediment. See here (roughly half way down).Fossils & trace fossils
Fossils such as rootlets can tell us down from up as naturally roots must have grown into the ground. Trace fossils (features left by an organism, but not actually being part of it) can be used too, such as footprints or burrow traces.Ripples are a possibility too, as Bdfoster already mentioned.This list could go on and on but I think I have covered enough for now. The link I provided earlier seems to be the best online resource for sedimentary structures I’ve found, but even this isn’t 100% comprehensive.Anything unclear so far?