Glossary: Impact Melt Sheets
Impact melt that remains inside the crater often pools at the bottom, forming a melt sheet that covers the crater floor.
The amount of melt, and therefore the size and look of the melt sheet, depends on the size of the crater. For craters that are small, the melt sheet will be very thin. So, the boulders and other rubble, which get mixed in with the melt, will make it difficult to see a clear melt sheet. For larger craters, the melt sheet can be very thick. Boulders and other rubble that get mixed into the melt will be swamped. So, large craters will have very smooth-looking melt sheets lining their floors.
Examples of impact melt sheets on the planets