Tropical Rivers and Deserts
Devonian – Early Carboniferous (415 to 345 million years ago)
By early Devonian times, Scotland was situated just to the south of the equator. The climate was hot, with wet and dry seasons. The Caledonian Mountains had been eroded and as this burden was reduced, so the land rose to expose the rocks we see today which were originally several kilometres deep in the earth. The landscape consisted of eroded mountains to the north and alluvial fans along the Highland Border. Farther south was an alluvial plain which stretched from Scandinavia to Wales where it met the sea. The rivers were large and braided with channels as wide and deep as the Mississippi. The finer silty floodplain deposits were usually eroded and carried away so that the Devonian rocks are mostly sandstone and conglomerate. Plants were only just beginning to colonise the land and were probably found in isolated damp places on the floodplains and in marshes. Large armoured fish, with life cycles similar to those of salmon, lived in the rivers and lakes. Evaporation of groundwater in drier times caused the chalky compound calcium carbonate to harden in the soils as nodules and hard ground known as calcrete.
Volcanic processes were also at work. Andesitic and basaltic lava flows were erupted from large volcanoes similar to those in the Andes today. Volcanic mudflow conglomerates and some airfall ash deposits are interbedded with the lava. These rocks now form the Ochil Hills.
Following earth movements during Middle Devonian times, when the older rocks were folded, uplifted as mountains and eroded, the landscape was low-lying near sea level, with only a few low hills in the Highlands. In the late Devonian, the area was part of a central Scottish alluvial plain in which braided rivers mainly drained eastwards to the sea. Initially, there was little vegetation, and windblown sand dunes formed when the rivers periodically dried up. As groundwaters evaporated in the dry seasons calcium carbonate again hardened the soils as calcrete.
For a while in early Carboniferous times, muddy deposits were laid down on a coastal floodplain, with storms bringing seawater well inland to mix with the groundwater. In the dry seasons, surface and groundwater evaporated, drawing up and depositing gypsum, anhydryite and sale (which later dissolved) in the sun-cracked muds, and also transforming beds of limestone to dolomite (cementstone) in lakes or lagoons.
Rivers later returned to the plains, but these flowed into the area from higher ground to the north depositing mainly sandstones. Vegetation was probably patchy with only limited stands of trees.
Volcanoes were also active at this time. The 340 million years old basaltic lava flows and volcanic ashes of the Campsie Fells and the Kilpatrick and Gargunnock hills were erupted from fissures and small volcanic cones which were often grouped along fault lines. The individual flows form bold scarp or ‘trap’ features on the northern face of the Gargunnock Hills and southern face of the Kilpatricks. The eroded roots of the volcanoes, now hard plugs of basalt, form characteristic isolated hills like Dumgoyne and Dumbarton Rock. The volcanic activity on Iceland, where new ocean floor is currently being generated causing the European and North American plates to drift farther apart, shows how this area might have been in Carboniferous volcanic times. However volcanic activity in Scotland ended before actual rifts were created.