During late Westphalian and early Stephanian (late Moscovian) times there was a progressive westward withdrawal of the coal swamps from across Euramerica, eventually being restricted mainly to the basins of the Interior Province of North America. These changes can be correlated with the northwards migrations of the Variscan Front that disrupted landscapes and drainage patterns.
The resulting better-drained substrates caused localised change in the swamp vegetation from being lycopsid- to fern-dominated.