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PaulK, i see your point, but i think the correction is quite trivial....perhaps i should re-phrase it as relating to the same relative or proportional point in the same layer?
That solves one problem, but it does potentially raise other issues (for instance, when did deposition start ? is it occurring at the same rate ?). The point is that you must add other assumptions, beyond superposition to identify the order when you include horizontal displacement.
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Nonetheless, the point is that vertically-successive strata can be formed by a simultaneous, dynamic mechanism, by HORIZONTAL progression- and said strata, if analysed "blind", would conventionally be interpreted as forming one layer after another: the lowest first, then the next, then the next, and so on, according to the normal rendering of the Principle of Superposition; whilst the truth of the matter would be that the stratum were all formed at the same time: with the "youngest" part being at the horizontal "start", and not at the vertical "start" (bottom)...
I have to disagree. The vertical relationships all obey the Law of Superposition. It's just that the time involved is small. It would be more true to say that the deposition of the strata overlap and assigning a precise age to the whole stratum could be misleading. I'll also add that the sort of scenarios you seem to be proposing look to me to be small scale - both in terms of geographical area and time - and so not likely to be a big problem for geology.
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Also, if people want to appeal to a particle-by-particle deposition for the dynamic scenario as still validating the general Principle of Superposition, then i'd have to remind them (and myself!) that the principle is concerned with the inter-stratum chronology - true? Thus, this appeal would reduce the necessary validity of the PoSn to only having intra-stratum relevance, instead of the inter-stratum relevance it is intended to have.
I don't think it does. That is, the particle-by-particle argument works in the same way whether the particles are part of the same layer or not. In every case the particle on top needs to arrive after the one on the bottom. The restriction that the argument does impose is that we are limited to strict vertical sections - however these sections can include any number of layers.
However, your idea of horizontal levels marking the same time runs into even bigger trouble if it is used between different layers. If the older layer is eroded unevenly, later sediment may be deposited into holes and gullies, arriving at the same horizontal level as sediment from the older layer.
So, I think all that can be said is that we need some caution when considering the horizontal dimension. In a strict vertical sense superposition works, unless the newer particles have some way of getting under the older particles already deposited.
Edited by PaulK, : Fixed broken sentence
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