In the Upper Carboniferous, the fusion of Laurentia with Gondwanaland forced up ranges of mountains through the Appalachian–Hercynian orogeny (a mountain-building process). Of the resulting landmass, only the land that is now North America sat astride the equator, with Angaraland, containing present day Siberia/Kazakhstan a large island to the north. The vast Gondwanaland land mass extended down to the southern pole, and experienced extensive glaciation, fluctuations in which produced variations in sea-level. There were vast, dense rainforests in which lycophyte trees predominated; seed ferns, cycads and conifers were also increasingly common. The richness and denseness of plant life was responsible for generating the coal deposits that give the Carboniferous its name. The atmosphere had significantly higher oxygen levels than today, permitting gigantism in insects. Amphibians were the dominant land vertebrates, while lungfish and sharks replaced armoured fish in the oceans.