The type of sedimentary rock you're describing is called a conglomerate.

Here's a breakdown:

* Conglomerate: A clastic sedimentary rock composed of rounded gravel-sized clasts (pebbles) embedded in a matrix of sand, silt, or clay.

* Clastic: Meaning the rock is formed from fragments of other rocks.

* Rounded Pebbles: The rounded shape of the pebbles indicates they were transported a significant distance, likely by water, which smoothed their edges.

Let me know if you have any other questions about sedimentary rocks!