Final answer:
A mountain stream's well-rounded pebbles, cobbles, and boulders, if lithified, would turn into conglomerate, which is a type of sedimentary rock formed through compaction and cementation.
Step-by-step explanation:
If a mountain stream's fast-moving water carries away all grains except for well-rounded pebbles, cobbles, and boulders, and this gravel is then lithified, it would turn into conglomerate. This type of sedimentary rock is formed by the lithification process, where loose grains of sediment are compressed and cemented together to become solid rock.
The processes of compaction and cementation are critical in forming this sedimentary rock. During compaction, the layers of sediment are pressed down by the weight of new sediments above, reducing the spaces between particles.