In time the clear tropical sea that dominated the early Carboniferous became increasingly choked with mud, silt and sand brought in by large rivers draining upland areas to the north. When a river empties into the sea, the sediment may build a delta. Deltas are complex environments and are shaped by the interaction of waves, tides and river activity. Over time sediment is deposited at the coastline and the delta is built up above sea level into a system of channels with intervening swampy areas, similar to the Mississippi delta, Gulf of Mexico today. During the Carboniferous, large distributary channels carrying sand and mud were interspersed with floodplains and swamps dominated by muds and silts. The soft delta sands and muds eventually hardened into sandstone and mudstone, which now contain features such as ripples, cross-bedding, feeding traces of ancient creatures and abundant plant fossil remains. The floodplain and swamps are dominated by mudstones and siltstones and coals (e.g Tanhill coal seam of the Millstone Grit Group). The name Carboniferous means ‘coal-bearing’ due to the occurrence of coal in many sections of the stratigraphy around the UK. The occurrence of coal deposits owes their existence to the abundant woody tissues and bark-bearing trees.
Plants in the Carboniferous were lush and extensive, growing to sizes that are not reached today. There were no flowering plants, but large trees and ferns were common. The descendants of some Carboniferous plants can still be seen today. The Carboniferous lycophytes of the order Lepidodendrales, which are cousins (but not ancestors) of the tiny club-moss (2-3cm high) of today, were huge trees with trunks 30m high and up to 1.5m in diameter. These included Lepidodendron (giant club moss trees). So vigorous is the growth of these ancient trees that they seemed to have sucked much of the carbon dioxide out of the atmosphere, producing a surfeit of oxygen. Oxygen levels were higher during this time than at any other time in the history of the Earth and partly accounts for the occurrence of giant dragonflies and millipedes. But as great masses of dead plants became buried in swamps and out of contact with oxygen, the level of carbon dioxide in the atmosphere actually dropped. The world during the Carboniferous gradually became cooler.
Lepidodendron is distinctive in having diamond-shaped leaf attachment scars. As the tree grew, new leaves formed at the top and the old ones dropped off, leaving diamond-shaped pits that look like the scales of a reptile and hence it is also called the ‘scale tree’.
Tree-sized horsetails (Calamites) grew in the coal swamps. They are relatives of modern horsetails but looked more like a pine tree and grew up to about 15m. Calamites had upright stems that were woody and interconnected by an underground runner, however, the central part of the stem was hollow, and fossils are commonly preserved as casts of this hollow central portion. It is a commonly encountered fossil throughout much of Swaledale, but restricted only to the sandstones and mudstones of the swampy deltaic conditions that existed. Many samples can be found in the stream sections throughout Swaledale.