Your sedimentary rock is probably a conglomerate.
Conglomerates are made up of rounded pebbles or bits of pebbles stuck together
with a finer rock cement. They are formed by the flow of water; the sea
depositing pebbles on a beach, or a flash flood sweeping lots of large pebbles
together. The pebbles can range in size from tiny fragments to large stones and
indicate the speed of flow of the water which deposited them: large pebbles need
very fast water to move them!
Conglomerates which angular fragments instead of rounded pebbles are called
breccias.
