Wind and rain take their toll and lead to the filling in of craters over many millions of years. Traces remain however, and the trick is to look for the deformation in the rock beneath that leads to an old wound still being recognisable. For this, we need to see if the rocks around a suspected impact site have been subjected to extreme changes in pressure and temperate. Such effects are called shock metamorphism . One result of such conditions is the formation of rare types of quartz minerals known as coesite and stishovite , also known as Shocked Quartz .
Shocked quartz contains tiny fracture planes.
No natural mechanism other than an impact event produces this effect. Another effect is the change in the structure of rocks from a crystalline to a glassy state. An impact site may also be surrounded by an ejector blanket. This is a layer of material thrown out by the formation of a crater. For example, the Ries crater in southern Germany is surrounded by a blanket of material up to 100 metres thick.