Yes, "packed down" is a good way to describe the process of sediment becoming rock. It's a simplified way to explain lithification, which is the process that transforms loose sediment into solid rock.

Here's a more detailed breakdown:

1. Sediment Deposition: Loose particles like sand, silt, clay, or even the remains of organisms, are deposited in layers.

2. Compaction: Over time, the weight of overlying layers compresses the sediment, squeezing out water and air. This process is called compaction.

3. Cementation: Dissolved minerals in the water that was squeezed out begin to crystallize between the sediment particles, binding them together. This is called cementation.

So, the "packed down" process is actually a combination of compaction and cementation. These processes turn loose sediment into a solid, coherent rock.