Excerpt from "The Fossil Cliffs of Joggins" by Laing Ferguson.
This sequence of diagrams shows how a lycopod tree at Joggins was gradually preserved. The bottom of each stump and any preserved roots are commonly surrounded by shale, and this is overlain by several feet of sandstone, which surrounds the lower part of the trunk. We know that shale is formed when mud becomes rock under pressure and that sandstone was originally sand.
So the sequence of sedimentary rocks we find around the base of the fossil trees at Joggins suggests that mud was slowly deposited in the flood plain, permitting trees to become established and grow to maturity. Then, when the river burst its banks after a heavy rainfall, it brought in the coarser sediment from the river channel which surrounded the tree and killed it. Later, the top of the tree was blown over by wind, leaving only a hollow stump. The inner tissues if the stump continued to rot as sediment accumulated around the trunk over the years. After the interior rotted and the upper part of the tree was removed, sediment accumulated until it reached the rim of the tree, spilled into its interior and rapidly filled it. It was at this stage, according to Dawson's theory, that the hollow stumps acted as traps for unwary reptiles and amphibians.
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