In this image, you can see a zone of mud that constituted the flood layer where the sea life was deposited. Once the flood waters receded, the mud dried and hardened. All of the shelled creatures would have died when the water receded and their soft tissues decayed, leaving only the remnants of the shells themselves.
Over time, the stream cut through the flood layer, loosening the rock by natural forces. In cold winters, ice crystals form in cracks and crevices and, since ice expands, it forces the rocks apart. In this image, you can see the ice crystals that had formed during a cold snap.
Here, you can see how a decaying bank frees up the rocks and gravity takes over. Tree roots are another force accelerating the breakdown.
In parts of the stream, the water hasn't cut completely through the flood layer. It creates a hardened sluice where almost nothing sticks, but gets rolled further downstream.
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