The first picture shows a cross-section of ground with four different sediment layers. The red, purple, yellow, and black colors represent four different types of sediments. Rainwater (the blue circles) has fallen on the ground and has moved through some of the layers.
The second picture shows close-up views of four types of sediments: gravel, sand, silt, and clay.
Based on the pictures, what type of sediment is represented by the black layer?

Explain your answer.