The Carboniferous was a time of great change for the area. Shropshire would have been near the equator, and the Old Red Sandstone continent had been eroded away; in the early part of the era, the county was under a shallow sea. However, tectonic activity pushed Britain out of the sea. South of Shropshire this effect was felt greatly, though Shropshire was relatively quiet. Mountains to the north were being worn down by rivers, creating enormous deltas that were colonised by plant life. A tropical forest took hold all over Shropshire, with ancient tree ferns and horsetails. Shropshire eventually crossed the equator during this era, and became a part of Pangaea during the Permian; the area would have been very similar to the Sahara Desert, and would have been in the vicinity, around 20° to 30° north of the equator.