Geology Question

While travelling in central Pennsylvania on highways that had been cut through various mountains I noticed a common trait of the strata in the sedimentary rock. There are often alternating layers of various colored rock that form a pretty regular pattern. A narrow dark band on top of a wider light band on top of another narrow dark band on top of another wider light band etc. This pattern repeats for many layers and is suprisingly, at least to me, uniform. Given that each layer took thousands of years to form, what was going on in the environment that caused the pattern to repeat at such a regular basis over such a long period of time?

While you’re at it, please explain what I think was called “The Great Inconsistency” that was pointed out to me on a week-long river tour of the Grand Canyon (very highly recommended BTW). Its two adjacent layers of rock differing in age by a billion (2B?) years or something like that. I think the guide said it was simply errosion of everything that had been deposited in the mean time but that it was a matter of some debate among geoligists.

Enlighten me Dopers.

The simplest explaination is that you’re looking at a varved deposit. This is when you’ve got a river or stream running into a relatively still body of water, and (typically) the thick layers will represent a spring run-off, which is when most sedimentation happens, and the dark layers represent the much finer, much lower volume sedimentation that happens during the rest of the year.

Another possibility is that you might be looking at a turbidite. These are caused by underwater landslides, which deposit the thick layers and then the thin darker layers are the background sedimentation that was going on during the interim between slides. So the thick layers were actually deposited almost instantly, whereas the thin dark layers may represent hundreds-to-thousands of years of sedimentation.

That’s just a couple of possibilities-- a package of sedimentary rocks that are all the exact same rock type is generally the exception, not the rule, so there are a lot of other possibilities. Those two are the ones I’d say off the top of my head that produce more-or-less uniform alternating layers of thick light and thin dark rocks. If there are any hardcore sedimentology nerds who are on this board I’m sure they could blather on for pages about this stuff (and berate me for oversimplifiying what I have said).

As for the Great Unconformity, this is an angular unconformity, which represents a period where deposition of sediments shut off and there was erosion and structural deformation before deposition restarted. Read all about unconformities of all types:

Geologists used to make a lot more of the so-called “Great Unconformity” because there are a lot of places where you have older Paleozoic rocks (from the early appearance of identifiable fossils) resting on top of pre-Cambrian rocks (which is a term for the time before identifiable fossils, and which spans about 87% of earth history). It was thought that this represented some sort of world-wide event that shut off deposition and/or sped erosion everywhere. But this was back when the only method of dating rocks they had was by observing the relative positions of rocks with certain kinds of fossils and so they had no idea how old pre-Cambrian rocks actually were (or that they did represent such a huge portion of earth history). So with modern methods to precisely date the rocks underneath the various expressions of the Great Unconformity, we can see that there are differences of hundreds of millions to billions of years between them and so it doesn’t really make sense to think of them all as a single event.

They still call it the Great Unconformity at the Grand Canyon because that’s where it was first described, and it does still represent a heck of a lot of missing time (about 1.2 billion years in some places).

It could be the progression and regression of the sea- alternating layers of sand dunes and swamp.

The Great Inconsistency formed by having really old bedrock wiped clean and having much younger strata deposited on top. Then the river cut through everything, showing all those different layers.

You seem to be assuming that each layer was turned into rock separately, before the next layer was deposited, which is almost certainly not the case. As GreasyJack implies, it’s vastly more likely they were all laid down as sediment over a relatively short period, then all subsequently turned into rock as geological conditions in the area changed.

Whether or not they turned to rock separately, it is possible for sedimentary formations to display astonishingly regular patterns across quite a long geological period.

Quantoxhead in Somerset, England is like that - the formation there alternates between foot-thick bands of dark, flaky slate/shale, interspersed with six-inch layers of more uniform, pale stone that breaks up into polygonal bricks - all of which has been warped and tilted so that when you walk along the beach, you’re constinually going back and forward in geological time, without going up or down very much.
There’s a picture at the bottom of thispage and another one here (neither do it any kind of justice, but are the best I can find)

My assumption, on first visit was that the bands of many thin shale/slate layers must have represented maybe a few decades or centuries of of ordinary deposition, punctuated by thick and uniform deposits laid down by singular exceptional flood events now and again.

Turns out that the formation spans approximately two million years from top to bottom - and the thick bands are limestone that took quite a while to be deposited - it’s a relatively brief geological span, but still seems like a surprisingly long time for a highly consistent and cyclical pattern of deposition to have lasted.

Cyclothems, is the technical term for such sea/land transgressions in the Carbo/Permian.

If the OP can be more specific about where these outcrops were seen, the Penn. Geological Survey actually has a quite nice website for finding out specifics.

I’ve see this near State College so maybe I’ll contact their geology dept. Another cool thing I’ve seen in this area is this layering effect go from horizontal to vertical in a matter of 20’-30’. Its just bent over like it was a soft clay. Maybe it was but the forces at work still had to be amazing. I should have paid attention in science class all those years ago.