The materials are little red to black fine-grained rock fragments. Some are pitted. Some may be a few inches across.
The rock is round and smooth and lighter in color. This pebble is embedded in the darker rocks.
This hill is an old volcano made of volcanic rock. This hill is called an ash fall tuff cone.
The rounded pebbles are formed in rivers where running water reworks until they are round and smooth.
This tells us that, on its way to the surface, hot lava feeding the volcano went through a layer of wet gravel. Being very, very hot, the hot lava caused the water to rapidly turn into steam. Because steam takes up more space than water, the pressure created a violent explosion. The explosion shattered the magma and small angular fragments of basalt rock, ash, lava, and the pebbles erupted with great force. As the material fell, it settled on the slope of the hill and the heat helped to fuse the ash around the pebbles.
Connections to Mars
Suppose the Mars rovers discovered this kind of smooth rounded rock in the plains of Mars. Would it provide evidence for water on Mars at some point in its point?