The Mesolithic Era follows on from the Palaeolithic and, in Britain, commences round the end of the last Ice Age, some 15,000 years ago. This era sees a fundamental change in stone technology. In the earlier periods, tools were predominantly of the 'core' type; chips were removed from a flint pebble or nodule to leave the core as, usually, a general-purpose shaped implement. In the Mesolithic peiod, the emaphasis changed from the earlier core-tool technology to a flake- and blade-based method. The process consisted of carefully shaping a flint nodule into a cylinder and then striking many long, parallel-sided blades sequentially along its length; from about 10mm to sometimes over 150mm long. This technique resulted in a much better usage of the raw material. The blades could be retouched by pressure-flaking to make many fine tools, suitable for intricate work. These include scrapers, points, burins (for engraving or cutting) and knives. Arrows and light javelins were sometimes provided with multiple heads: a single transverse point with additional barbs inserted to the side of the shaft. Several short blades could be inserted into a curved length of wood or bone to make a sickle, and so on. Some core tools were still produced but the beauty of the best Palaeolithic handaxes has gone. The crude 'Thames Picks' (so-called from the finding of many of them in and round the Thames valley) are fit for their purpose but owe little to aesthetics. Sometimes, these are better formed, as 'Tranchet' axes, from the single edge sharpening flake struck off to complete the implement. Characteristic artefacts of this era in other natural materials include superbly carved harpoon points for fishing. Most Mesolithic implements may be readily distinguished from both their Paleolithic forebears and from those of later periods. The Mesolithic was an era of hunter-gatherers. People followed a nomadic lifestyle through the seasons so that they could take best advantage of what nature provided and would often involve following herds of migrating animals.