Geology Reference
In-Depth Information
Rip currents are informally called “undertow,” and when rip currents are es-
pecially strong, public beaches may be closed to prevent swimmers from being
pulled out into dangerous water farther from shore.
Shaping Shorelines
Waves, currents, and tidal motion change the shape of the coastlines by creat-
ing (and destroying) geologic features. In Chapter 12, I explain the relationship
between the speed of flowing water and sediment grain size that determines how
sediments are moved by water. The same rules apply to how water carries sedi-
ments in the oceans.
Water is a powerful agent for geologic change. Along coasts where it contacts rocks and
cliffs, it erodes these materials — removing sediment particles small amounts at a time
and transporting the sediments to other locations. The processes of erosion and depos-
ition produce distinct features along the coast.
Carving cliffs and other features
Where waves meet rocky, steep shores they carve away sediments, creating a
wave-cut
cliff.
Materials are removed from the cliff by the force of water, leaving behind a flat area
below the cliff called the
wave-cut platform.
A rocky coastline being eroded will often
have cliffs that extend out into the water, called
headlands.
These headlands are made of
rock or sediment that resist erosion by the waves. Other features found along rocky
coastal cliffs are illustrated in Figure 15-7 and include:
Sea caves:
Sea caves are created as the waves remove easily eroded sediments and
rock from the cliff, carving out a cave.
Sea arch:
When two caves being carved out near each other connect (often on op-
posite sides of a headland), they form a sea arch.
Sea stack:
After enough time, the top of the sea arch will weaken and fall, leaving
tall, freestanding sea stack features.
