Life story

At school we are taught that erosion is a slow process - the grinding down of mountains by rivers, waves, rain, wind-blown sand, frost, plant roots and soil micro-organisms taking hundreds of thousands or millions of years. Glaciation speeded up the process over a lot of Britain. Glacial ice is fast-flowing, by geological standards. The ice grips protruding rocks and pulls them away from the ground. Rocks and stones at the base of a glacier work like giant sandpaper, scouring the bedrock below, deepening and widening valley into the well-known U-shaped profile, rounding off the tops of summits that do not protrude through the top of the icesheet

In this corner of Wales, the glaciers had a special party-trick in their ice-cave. As the ice age began to draw to a close, and the ice sheets started their slow retreat north, the enormous volumes of melt-water formed a glacial lake under the ice in the upper reaches of the Teifi. As the mouth of the Teifi cleared of ice, the water in the lake was released but, rather than following the slow, meandering, pre-glacial course of the river, the large volumes of water leaving the lake took short-cuts, finding the fastest route to the sea available

The powerful current carried rocks and stones of varying sizes, materiel dropped by the retreating ice. The melt-water river acted like a highly abrasive jet - similar to water-based sand blasting to clean buildings - and rapidly (in geological terms) scoured a narrow, deep channel in the rock through which it flowed. When the ice and lake had gone, and the Teifi returned to its normal flow, it found itself at the bottom of several narrow steep gorges. Even small feeder streams, like the one where this picture was taken, had their valleys deepened by many metres, to create this steep-sided, narrow, rapidly decending gorge

The retreat of the glaciers was succeeded by the development of mixed woodland, dominated by oak, across most of the landscape. When humankind returned to the land, we cleared the forests to make the land available for agriculture. Over millenia, these old-growth forests were almost entirely cleared, leaving remnants only in these steep inaccesible gorges where farming is impractical. These dark, forgotten refuges are now declared nature reserves, replete with lichens, liverworts, rare insects, birds and butterflies. Even here, some quarrying has gone on, leaving a trackway where we can get a glimpse of the ancient wood, and autumn leaves can settle in front of us, like the laying of a fresh carpet, as we walk