Rock layers can be dated, and their depositional history can be determined. This can be done using a number of different techniques which complement one another.
Mt. St. Helens deposited a lot of material, but the material in the river beds is not rock. Rock takes a long time to form. What St. Helens deposited in the lower valleys is still more akin to mud -- sediments. It washes downhill very easily and accumulates in the valleys. This is very different from the lava flows that you see from various volcanoes.
Erosion can cut through mud very quickly, but that says nothing about the rate of erosion in rock layers. That takes a long time, depending on a wide variety of factors.
For creationists to try to equate the two, mud vs. rock (St. Helens vs. the Grand Canyon), is neither accurate nor honest.
Why can't you folks just stick to the facts?
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.
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