Part of Hall of Planet Earth.
AMNH/R.Mickens
These rocks were originally deposited in a stream as layers of rounded pebbles. They became mixed with sands and silts as the original layers were eroded and re-deposited. Much later, the rocks were transformed by high temperatures and pressures deep in the crust. The rough surface of the sample on the left was caused by weathering and erosion.
AMNH collection