Here's why:

* Formation: The outer coastal plain is formed by sediment deposited by rivers and ocean currents over millions of years. This sediment is typically fine-grained, resulting in a flat surface.

* Erosion: The outer coastal plain is also subject to erosion by wind and waves, which further flattens the terrain.

* Elevation: The outer coastal plain is typically located at a relatively low elevation, often only a few feet above sea level. This low elevation contributes to its flatness.

However, there can be minor variations in elevation within the outer coastal plain, such as:

* River terraces: Some areas may exhibit slight rises in elevation due to the presence of old riverbeds.

* Coastal dunes: Coastal areas may have sand dunes, which can create small hills.

* Isolated hills: In rare cases, there might be isolated hills or outcroppings of bedrock that protrude above the flat terrain.

Overall, though, the outer coastal plain is generally characterized by flat, low-lying terrain.