The Rhine Rift is a striking reminder of the tectonic plates that created Europe. Rift valleys are usually formed when two plates pull apart causing land to collapse into the gap. However, the Rhine Rift is believed to have formed a different way. It is believed that the plates knitted together over 40 million years ago and this caused a long strip of rock to drop between the two continental massifs, the Vosges in France and the German Black Forest. It was initially thought that the rift was inactive however, scientists have detected signs of movement underneath the surface. Faults within stable continental interiors rarely rupture, however if they do, the earthquake could be sudden and extremely violent. Northwest Europe's worst earthquake was occured in 1356 in the Upper Rhine Valley, destroying the Swiss city of Basle and destroying buildings as far as 200km away. Some geoligists believe the earthquake was not caused by the Rhine Rift but by a fault in the Alps. However, Basle may become seriously affected by a huge earthquake released by the Rhine Rift, because an active fault has not been discovered southwest of the city. In 1992, an earthquake along the Peel Fault which is further down the Rhine, rocked the town of Roermond in the Netherlands.