ROCKS THAT ARE EXPOSED to the powerful waves and harsh weather along coastlines form some of Canada's most dramatic and ever-changing geological formations. Rocky coasts are victims of constant erosion, which, within a geological time frame, cause rapid changes in rock formations.   Folded sedimentary rock lines a coastal cliff on Ile du Cap aux Meules, Iles de la Madeleine. Sea cliffs are sloping land surfaces that descend below the water. They can range in height from a few metres to hundreds of metres above sea level. Their stature is a result of wave -induced erosion near the surface of the sea and the subsequent collapse of rocks at a higher elevation. The steepness of the surface of the cliffs is directly related to the geological characteristics of its rock and to the size of the waves crashing against it. When you have rocks resistant to erosion combined with energetic waves you tend to get the steepest rock faces. Smaller, gentler sloping cliffs are usually a result of rocks that are more susceptible to erosion and lighter wave action combined with weathering from spray and wind borne gravel.   Shore platform cut by frost and waves. The slate platform readily disintegrates with freezing. In Petit Cap, Gaspesie, Que. At the base of most cliffs (at about mid-tide elevation), there are often flat, bench-like surface called wave-cut platforms (LEFT). As their name suggests, these platforms are made by wave action acting on the coastal bedrock. Small pools of standing seawater that collect during receding tides also helps to weather the bedrock and form the horizontal surfaces. Extensive wave-cut platforms of a certain height are a sign of consistent sea levels during the periods of formation. Conversely, wave-cut platforms at multiple heights signify fluctuating sea levels. Because erosion occurs at various rates depending on rock type and wave movement wave-cut platforms can also form sea stacks (BELOW) and sea arches (RIGHT). These are spectacular coastal landforms are isolated pinnacles that can reach towering heights. They are formed from coastal rock that is more resistant to some surrounding rock. Shoreline that is chiseled away by wave action can end up separated from the mainland causing sea arches, which erode to become sea stacks. Arches and stacks are impermanent formations since continued erosion will eventually wear them down to nothing.   Sandstone sea arch at Anse de l'Est, Ile aux Loups, Iles de la Madeleine.   A sandstone sea stack supports a patch of sod off the coast of Ile du Cap aux Meules, Iles de la Madeleine. The variations in the strength of the different types of rock are shown in the unusual formations. All images courtesy of the Terrain Sciences Division of the Geological Survey of Canada. Reach their Canadian Landscapes site at: sts.gsc.nrcan.gc.ca/tsdweb/landf_new.asp