The rocks that are a mix of rounded pebbles and sand are known as conglomerates. Conglomerate is a type of sedimentary rock that forms from the compaction and cementation of rounded clasts, usually pebble-sized, along with finer materials like sand, silt, or clay.
When these small, rounded fragments are deposited in a river or a beach environment, they often get mixed with finer sediments. Over time, these layers of sediment build up, and with the addition of pressure and mineral cement like silica or calcite, the sediments turn into solid rock. This process can take thousands or even millions of years.
The appearance of conglomerate rocks can vary greatly depending on the types of pebbles and stones that are present, making them quite interesting to study in geology. Often, you can identify them by their gritty texture and the visible pebbles within the rock matrix.