Salpausselkä is about 600 km long ridge from Eastern Finland
to Southern Finland which was formed during the Ice age
ca. 10 000 - 11 000 years ago when edge of the ice stopped for ca. 200 years and melting water brought tons of moraine
and banked it.
It's average height from sea level is only 20 meters but peaks are even 60 - 70 meters. Highest point is at Tiirismaa
which is 222 m.
Nowadays Salpausselkä blocks vast amounts of water to it's northern side thus creating most of the thousands of lakes of Finland including Päijänne and Saimaa.