Bavaria's big lakes are prints: each one is the hollow left by a tongue of ice

Look at a map of southern Bavaria and you see a row of big lakes set just north of the Alps: Chiemsee, Starnberger See, Ammersee, Tegernsee, Walchensee. They are not scattered at random and they are not like lakes elsewhere in Europe. They have a very particular shape: long, narrow, oriented south to north and surprisingly deep.

That shape is a signature. Ice dug them, one by one, and each lake corresponds to the tongue of a glacier that came down from the Alps and stopped exactly there.

01 A shape that gives it away

The clearest case is Lake Starnberg. It runs 19.45 kilometres north to south and 3 to 5 east to west. That is, between four and six times longer than it is wide. And in the middle it reaches 127.8 metres deep, an enormous figure for a lake sitting at 596 metres of altitude on a plain.

A lake carved by a river would have a different plan: branching, winding, shallow and with gently sloping shores. A tectonic lake would be more irregular. This geometry — a long, deep blade pointing at the mountains — is produced by one thing only: a mass of ice advancing along a valley, pushing and grinding the floor.

German has a word for it, Zungenbecken: tongue basin. And it is literal. The lake is the mould left by the tip of a glacier.

02 How you dig a lake with ice

During the glaciations, Alpine glaciers did not stay in the mountains. They came down the valleys and out onto the plain to the north, where the ice fanned out into lobes, one for each outlet valley. Each lobe was a tongue, and each tongue worked its own hollow.

The mechanism is friction and weight. A tongue of ice hundreds of metres thick drags blocks of rock along its base that act as a giant abrasive, lowering the floor far below the level of the surrounding ground. When the climate warmed and the ice withdrew, those hollows were left open and meltwater filled them.

That is why the depth is so disproportionate. The 127.8 metres of the Starnberger See are not the result of a deep valley having been there: they are the result of a tongue of ice working the same spot for millennia.

03 The rubble is the landscape

Everything a glacier excavates has to go somewhere, and there is the second half of the story. The stripped material piles up along the edges and at the front of the tongue as moraines: ridges of gravel, sand and boulders mixed without order.

Those moraines are the postcard landscape of southern Bavaria: low rounded hills, rolling meadows, hollows with peat bogs and small ponds, all of it without a scrap of bedrock in sight. What looks like gentle, ancient relief is in fact a ten-thousand-year-old glacial spoil heap, very young in geological terms.

Further north, where the meltwater torrents ran out, the material was deposited sorted by size into perfectly flat gravel plains. The state capital is built on one of them. The reason Munich is flat and sits on rounded pebbles is the same reason Starnberg is deep.

04 The lakes are filling in

One figure is startling, and it is visible from any plane taking off from Munich. These lakes are not stable: they are filling in. The Chiemsee originally covered almost 240 square kilometres when the ice withdrew, some 10,000 years ago. Today it is around 80.

That is, it has lost two thirds of its area in ten millennia. The cause is simple: the Alpine rivers feeding it arrive loaded with fine sediment and deposit it on the bottom and along the shores. Every melt season adds another layer.

That process explains the great gravel deltas and reed beds occupying the southern end of nearly all these lakes. They are not permanent marsh: they are the front along which the lake is retreating. On a geological scale, what is a lake today will be a marshy plain in a few thousand years and then a meadow.

05 Four storeys from north to south

With that in mind, the state's map sorts itself into bands. In the south, a narrow strip of limestone Alps topping out at the Zugspitze's 2,962 metres. Immediately north, the Alpine Foreland: the country of moraines, lakes and gravel plains we have been discussing, which is where almost everyone lives.

Crossing the state from west to east runs the Danube, acting as a hinge and collecting everything coming down from the Alps. And north of the river a completely different Bavaria begins: limestone plateaus, old granite massifs and extensive forest, with relief the Alpine ice never reached.

06 What this changes for the traveller

The first consequence is practical: the lakes are cold and deep. With over a hundred metres of depth and Alpine feed, the water takes a very long time to warm and does not become comfortable until well into August. Swimming in one of these lakes in June is a decision, not a plan.

The second is that the south of the state travels better horizontally than vertically. The lakes, moraines and villages line up in a band parallel to the Alps, so moving east to west is easy and flat. Heading south means entering the mountains all at once, with no transition.

The third is about how you look. That rolling landscape of meadows and copses everyone photographs is not old countryside: it is demolition material dumped ten thousand years ago and still on the move. Knowing that, a reed bed at the far end of a lake stops being a pretty corner and becomes the proof that the landscape is still changing shape.