Sea levels have varied considerably over time – especially during the Pleistocene (approximately 1 million years ago).
Present situation was reached around 7000 years ago.
Rise in sea level resulted in rias, drowned glaciated river valleys (fjords), and submerged forests. Most estuaries are a result of postglacial sea level rise.
Earth crust movement and/or tectonic activity can further complicate the situation.
Land surfaces are rising relative to sea level (isosatic uplift/recoil). Rate is related to the thickness of ice that existed.
Coastal landforms need to be considered with regard to both world-wide (eustatic) sea level rise and rising land.
Isostatic uplift is usually faster than eustatic sea level rise.
Negative change: A fall in sea level in relation to the land.
Positive change: Sea level rises in relation to the land (or land sinking in relation to the sea).
Local changes in sea level, due to ice weight depressing earth’s crust lying beneath it.
Large scale/worldwide changes. In times of maximum glaciations, 3 to 4 times more water was stored on land than it is today. Global change in the hydrological cycle resulted and a worldwide eustatic fall in sea level of up to 150m.
Sequence of changes
- Ice sheets and glaciers form, eustatic fall in sea level, and negative change in base level.
- Ice sheets continue to grow. Land is depressed by weight, isostatic change, positive change in base level.
- Ice sheets begin to melt, eustatic sea level rise, positive change in base level.
- Continued decline of glaciers, isostatic uplift of land, positive change in base level.
Drowned river valleys, such as Milford haven in South Wales. In the ice age when rivers still flowed they cut down to the new lower sea level. With the rise in temperatures and release of water from land, they became flooded.
Flooded U-shaped valleys. Glaciers form U-shaped valleys and during the ice age they eroded to below sea level. With ice melt the valleys were flooded, creating deep, narrow inlets, common in Norway.
Glaciated lowland areas that have since been flooded.
Landforms that occur due to isolated uplift of the land in comparison to the sea, as the weight of ice is slowly removed. Old wave cut platforms and beaches are now above the impact of waves (as shown in the diagram above).