They have not experienced important geological modification.
Explanation:
Coastal plains are those parts of continents that border the sea and are low lying. This is due partly to the fact that coastal plains not too long ago formed part of the continental shelf and were, as such, submerged. With the subsequent retreat of sea levels, the coast plain was exposed, along with the sedimentary rock layers that formed under the ocean.
Another factor is that coastal plains lie in geologically quiet areas. This means that the formation of igneous or metamorphic rocks does not take place in that region and has not for a very long time. As such, the predominant rocks in these relatively flat areas are sedimentary.
