Breccia lens
A breccia lens is a lens-shaped layer of broken rock that fills the bottom of a simple impact crater.
In a simple crater, a bowl-shaped crater usually less than a few kilometres across on Earth, much of the rock blasted out during the impact falls back into the hole or slides down the steep walls. This forms a lens-shaped deposit of broken and mixed rock, called a breccia lens, filling the lower part of the crater.
The breccia lens may contain fragments of all the rock layers the crater cut through, along with impact melt and glass, and sometimes pieces of the impactor. In Meteor Crater in Arizona, drilling found a breccia lens about 200 metres thick, which reduces the crater's depth below what it was immediately after the impact.
Because the breccia lens fills the crater, the visible depth of a simple crater is typically about a third of the original depth reached by the transient crater. Its study helps geologists understand how simple craters form and how much material is excavated.
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