Sunday, May 15, 2011

Angular Unconformities, Part 3: Lyell on how the rocks were tilted

Continuing in Chapter 5 of Lyell's Elements of Geology here he is discussing the expectation that folded strata would be likely to break under the tension rather than simply fold:
It would be natural to expect the fracture of solid rocks to take place chiefly where the bending of the strata has been sharpest, and such rending may produce ravines giving access to running water and exposing the surface to atmospheric waste. The entire absence, however, of such cracks at points where the strain must have been greatest, as at a, Fig. 63, is often very remarkable, and not always easy of explanation. We must imagine that many strata of limestone, chert, and other rocks which are now brittle, were pliant when bent into their present position. They may have owed their flexibility in part to the fluid matter which they contained in their minute pores, as before described p. 62 and in part to the permeation of sea-water while they were yet submerged. [my emphasis]
Here I just have to comment on his assumption that the rocks must have been damp in order to bend without breaking, which makes perfect sense, just as modeling clay will bend while it's still wet, but break when it has hardened. Some years ago at EvC someone objected to my making this obvious point so I'm happy to see Lyell had the same expectation I had, that rocks must have some water in them in order to bend. Of course even damp clay will break under extreme tension so there must be a limit to the folding tension that could probably be measured.

This is also a welcome observation because it fits with the expectation that the strata were laid down in the Flood. At the very least, since geology also affirms that horizontal strata had to have been formed in water, if the rocks are still damp this does contradict the idea of extreme slowness and the long time span that is supposed to have been needed for the formation of the folds. Lyell himself could have made this deduction from his own observations, but he was under the spell of Hutton's old-earth theory by this time and continues to insist on it despite its being contradicted by his own facts.

Next I'm quoting him on how tilted or upright strata that appear to have been broken off were originally folded, the folds then having been "denuded" -- which was his word for what we would now call "eroded."
We have already explained, Fig. 69, that stratified rocks have usually their strata bent into parallel folds forming anticlinal and synclinal axes, a group of several of these folds having often been subjected to a common movement, and having acquired a uniform strike or direction. In some disturbed regions these folds have been doubled back upon themselves in such a manner that it is often difficult for an experienced geologist to determine correctly the relative age of the beds by superposition. Thus, if we meet with the strata seen in the section, Fig. 72, we should naturally suppose that there were twelve distinct beds, or sets of beds, No. 1 being the newest, and No. 12 the oldest of the series. But this section may perhaps exhibit merely six [87]beds, which have been folded in the manner seen in Fig. 73, so that each of them is twice repeated, the position of one half being reversed, and part of No. 1, originally the uppermost, having now become the lowest of the series.

These phenomena are observable on a magnificent scale in certain regions in Switzerland, in precipices often more than 2000 feet in perpendicular height, and there are flexures not inferior in dimensions in the Pyrenees. The upper part of the curves seen in this diagram, Fig. 73, and expressed in fainter lines, has been removed by what is called denudation, to be afterwards explained.
I had also wondered about this possibility but I'd tended to think that in the case of angular unconformities the strata had been broken and forced upright in one direction, then sheared by the friction between them and the horizontal strata above which formed a sort of ceiling. Instead the strata may have been folded and the bent section sheared off in the same way.

In that case the order of the strata as Lyell describes them, those on one side of the fold reversing the order of those on the other, would be the evidence. But where the strata are identical, as in the case of Siccar Point, that might not be easy to determine. At Siccar Point, however, the formation does continue far enough to show a section where the strata were folded, as in Lyell's Fig. 78, making the evidence in that case.

Also Siccar Point demonstrates my own expectation that the layer right above the tilted section would be a different kind of rock, which it seems to me would allow for easier sliding between them when pressure is exerted against the lower strata. At Siccar Point it's red sandstone in the horizontal strata above and greywacke in the tilted strata beneath. Most of Lyell's comments and illustrations also indicate that there were once more strata above that have eroded away, which also fits the model of angular unconformities I have in mind. Here's the whole model spelled out:
  • The strata were ALL in place before the tilting occurred, the entire geologic column.

  • The rocks were still damp which explains their folding rather than breaking, so it was not long after the Flood that the tilting occurred.

  • The folding occurred in a lower section of the strata under pressure from the side or beneath, either volcanic or tectonic.

  • The force was dissipated at a point where the upper strata were resistant because of their great weight. That is, at a point where the weight of the upper strata was about equal to the force from beneath as it reached that level.

  • The immediately superior layer was of a different type of rock, most probably a harder sediment, than the immediately lower one, allowing for slippage between the sections.

  • The eroded area usually seen between the lower tilted and upper horizontal sections was produced by the abrasion between them as the lower buckled and slid beneath the upper.

  • Where only a few layers or even only one remain over the tilted part of the rock the higher layers were most likely removed by remaining waters of the Flood.


  1. While I'll admit most of geologic structures cannot be observed at the point(s) of occurrence, same can be said for most of past-historical events that where considered to mundane to recorded in words, film, or video. I will agree that the model doesn't match most observable features, though a model is to be a simplified interpretation of the concept, I can think of any model that is 100% accurate. You do make a fair point that most unconformities are associated with fluvial movement, and to be honest I am unaware of any other means.
    Having wet strata for bending does make sense, but what about the water that was present in that case? There's a fair amount of exposed and now brittle rock, with your hypothesis though the water displace observed around the world is associated the reduction of ice bodies. And as a question what is your opinion on gradual pressure applied to a "hard" material, metals when pressurized too quickly will snap and be brittle but under gradual long presence of pressure is more elastic, and becomes even more so with heat; with that example do you think the same concept can be applied to brittle rock with low water values?

  2. Perhaps I'm getting tired but I'm not understanding your comment here.


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