GEOLOGY OF THE SAIMAA GEOPARK AREA

1. Ground moraine

The flowing ice eroded boulders and smithereens out of the bedrock. When the ice melted away, the rocky material was deposited as dense ground moraine which covers most of the bedrock in Finland, typically as a layer a few meters thick.

2. Ice-marginal moraine

The ice-marginal moraine was deposited when the Salpausselkä’s were formed. It typically consists of sandy and rocky till placed by the glacier at its edge.

3. Sand and gravel

The melt waters of the glacier transported and sorted material the same way as rivers do, forming gentle-sloping sandy deltas at the distal side of the Salpausselkä’s. Eskers consist of gravel and sand, deposited in meltwater tunnels inside the glacier. Today, delta and esker territories are important sources of sand and gravel.

4. Varved clay (glacial clay)

The melt waters of the ice sheet transported the finest material far out to the open waters. When it accumulated, deposits of varved clay were formed. The continuous layers (or varves) are connected to seasonal variation: dark thin layers were formed in wintertimes when the melting was slow and pale thicker layers in summertimes when the melting accelerated.

5. Post-glacial clay

Clays that were deposited after the glacier was melted are called post-glacial. They have no layered structure and they contain more organic matter than glacial clay.

6. Peat

Peat is a soil type that consists of incompletely degraded organic material which accumulates in marsh areas. The first peat lands in Finland started to form locally quite soon after the land was set free from ice and the low-lying territories were uncovered from water. 

In the areas of active ice flow, till deposited in the direction of the flow, forming longitudinal moraine ridges called drumlins and flutings. Drumlins are typically few meters tall and droplet-shaped formations that can extend up to few kilometers in length. In many cases, crescent troughs can be found in front of them. Flutings are similar formations but they are linear and smaller in height. These two kinds of moraine ridges are present as fan-like fields in many areas. Different suggestions about the exact mechanism of their formation have been proposed but it has been noticed that they often have a rocky core in their topside head. According to the generally accepted theory, the ridges have formed when the flowing glacier deposited its rocky bed load behind a strong obstacle.

Sand is a sorted soil type whose grain size variation is relatively even. Sands are deposited at beaches and river environments because of the streaming and sorting action of water. In the Ice Age, enormous quantities of sand was deposited in melt water ravines and deltas of the glacier. The melt waters on the ice surface caved tunnels through the glacier and strong rapids transported the material the glacier had captured. When the currents calmed near the edge of the glacier, the sandy material was deposited on the riverbed. When the glacier melted down and its edge receded the sand deposits were left behind and can now be seen in the landscape as longitudinal esker formations. In front of the ice sheet, at the river outlet channels, the sand formed delta areas. Sand and gravel formations are our best ground water sources.

Grain size: 0.2–2.0 mm

During the melting phase of the continental ice sheet, boulders stocked in glacial rivers which flowed inside the ice. In some places the strong currents lasted for centuries, whetting stones to rounded esker boulders. These symphatic rock spheres can be found on esker terrains but also elsewhere, where they have been transported by the ice.

Gravel, like sand, is a well sorted soil type. Esker gravel was deposited in the glacial melt water tunnels by strong currents. It is typical that the core of the eskers in composed of coarser material than the upper parts, even boulders. This kind of structure is a result from the fact that when the eskers started to form the flow velocity was often high and was able to deliver bigger rocks. The strong current eroded the stones and, because of this eroding ability of streaming water, the esker gravel clasts are rounded. While the gravel-sized particles were deposited the finer material still drifted with the current, to be deposited in some place else with calmer flowing conditions.

Grain size: 2–20 mm

The melt waters from the glacier transported fine material which did not immediately sink to the bottom but drifted along with the current to the open waters before finally descending down to the bottom of the Baltic Ice Lake. Thus, the clay terrains in Finland have been deep-water stratification environments. The varved clay or glacial clay deposits consist of continuous layers of pale and dark material. The layers resemble the growth rings of trees. The pale layers have formed in summertime when the melting of the ice accelerated, while the more fine-grained layers formed in wintertime when the melting correspondingly slowed down.

Grain size: < 0.002 mm

Till is our most common soil type. It is unsorted or poorly sorted soil type, meaning that it contains all grain sizes from clay to big boulders in variating ratios. There is not flowing water action related to the deposition of glacial till formations (or moraines) because till is material mechanically eroded by glacier and after the glacier melted away the material was deposited in situ. Because there has not been eroding action of water the clasts are typically poorly rounded. Many of our terrain shapes are consisted of till, for instance the great ice-marginal moraines of the Salpausselkä’s and the droplet-shaped drumlins following the flow direction of the ice. Most of our bedrock is covered by ground moraine, composed of the material crumbled and deposited beneath the glacier.