Young earth explanations for Angular Unconformities

Adding some more explanation in case Faith reads this exchange between you and Tanypteryx, another difference between trash compactors and tectonic forces is that rock isn't significantly compressible the way trash is. Granite density varies by less than 5%, sandstone by 25%, shale by 15%, slate by less than 5%. Tectonic forces on rock move, fold and bend rock rather than compressing it very much.It's also worth noting that tectonic forces pushing up from below cannot exert significantly greater pressure on strata than already exists because that would require an additional force pushing down. But the only force pushing down is gravity and no additional downward force can be brought to bear on the layers of strata, not unless more sedimentary layers are deposited on the top strata. It's like lifting a cheerleader up into the air - the cheerleader won't be compressed by the force (only slightly greater than her weight) pushing her into the air unless a new force starts pushing her down from above. Clarifying what I think you mean with your scenario of a horizontal trash compactor, very visible shearing should be apparent between the upper uncompressed portion and the bottom compressed portion.--Percy

I didn't understand this, but let me venture a guess: do you mean that the lower rocks were eroded a long, long time ago, then shortly after that erosion the upper layers were deposited upon them, then the upper layers were buried, then they were exposed along with the lower layers that now make up a small cliff face, then weathering removed some upper layer rock by mechanical erosion. If that's not it would appreciate some clarification. Not sure which images are meant. I might not get this, either, but anyway, here's the image: My guess is that you're trying to explain that the weathering effects that caused the way the overlying sandstone and the underlying granite appear today was not what caused the embedded granite boulders to become mechanically weathered into spheroidal shapes.Also, about this part: About the depression Faith refers to that's about 1/3 of the way from the left side of the image and at the boundary between the sandstone and granite? That looks like it's really there to me. It looks like one of the granite boulders detached from the granite face and fell to the ground.Faith's comment also highlights again her belief that holes and depressions in rock faces represent real cavities buried in the strata that have been revealed by erosion, rather than that they're just caused by pieces of rock breaking off and falling to the ground.--Percy

I was wondering if that was the case - so what's the difference? I don't think this could be the right image because I'm pretty sure Siccar Point is not artificial: So again, I'm not sure which image you mean. Are we getting the Siccar Point and the "spheroidal weathering" cases confused? Just to be clear, now we're talking about this image again: I guess I don't know how to interpret those "boulders" if they weren't really boulders. I assumed they were granite boulders that were ground into spheroid shapes by the action of weather and by being scraped along fast-moving river bottoms. How did those shapes actually form? I did read this explanation from you: But why would there be fractures along spheroid paths? And I guess the difference between "mechanically eroded" and "chemically weathered" is becoming important.--Percy

Hi Faith,Before replying I'd like to make some general requests to everyone:

• When you're talking about a specific place, mention the place at least once. For instance, do not begin, "Below the unconformity...", but rather, "Below the Siccar Point unconformity..."
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• Every time you refer to an image, include the image. Images are cheap, each one is only cached once. To get an image's URL just right click on it on a Windows machine, or control click on a Mac. You'll be presented a menu that includes the item "Copy Image Address". You can now paste the image address into the [img] dBCode.
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• Quote the text you're replying to.

The cross section is the mostly vertical portion showing the relationships between all the strata: The appearance of weathering is not a measure of how long something has been weathered. Likely none of the surfaces present when Hutton viewed Siccar Point are present today because they've been eroded away. Contrast a recent image with Hutton's drawing - they're different: So you can't reason about the relative age of layers based upon degree of weathering, because that depends upon degree of exposure to the elements (in this case, wave/wind/rain exposure is most important) and the material composing the individual strata. It couldn't be any other way. Very small weathered material has been carried away by wind and water. Larger material is still there at the base of the cliff face where it accumulates as minor scree or talus but is eventually weathered itself into smaller particles and is carried away, or perhaps becomes part of the ground. Of course erosion removed the weathered material. The weathered material from the strata above the unconformity has to fall away from gravity and be washed down and/or blown away. This is a very exposed place just on the water.--Percy