Saturday, May 25, 2013

Some Gneiss Granite You Got There

I have an unfortunate weakness for really lame inside-family jokes, something my wife and kids will certainly vouch for.

There is one such joke that I unfailingly trot out whenever we travel somewhere where the landscape is especially rocky, for example, the Canary Islands or Utah. I look to my wife and say, “There’s a lot of geology around here.” She cringes.

This is, of course, not a “joke” in any sense other than it exasperates my wife to hear this again, as she has pointed out long ago and repeatedly that there is the same amount of geology everywhere, in the same way there is the same amount of weather everywhere.

That is true, of course. Geology is everywhere; it’s just that it's more noticeable and interesting in some places than others, Finland being one of those.

Devil's Churn at Pihlajamäki.
Here in Helsinki, we live atop the Baltic Shield, a remnant of the Earth’s crust that has been around some for 500 million years since the Precambrian period (that is, before life really got going).

Coincidentally, I stood last summer on the edge of the Canadian Shield, another giant layer of Precambrian rock that, in the vicinity of Ottawa, ends in an impressive 270-meter (almost 900-foot) high escarpment overlooking a flat plain along the St. Lawrence River. See, geology is everywhere.

And in Finland, the Baltic Shield is everywhere, not that most Finns would think of it that way. For most people here, the visible parts of the shield is just the ordinary bedrock (kallioperä) that you find practically everywhere, if not fully exposed as naked stone, then just barely out of sight beneath the ground. (When we built an extension to our house, we were dismayed to find just how near the surface the granite bedrock in our backyard was. It eventually took three attempts to blast out a pit big enough for a small basement.)

The kallioperä is often covered only by a thin layer of soil, which has consequences beyond basement excavation. Trees here can be toppled in winds far below hurricane strength because their grip on the Earth is especially tenuous when their roots can’t penetrate much below the surface.

Tree toppled by winds last autumn.
Often the bedrock is the surface, with exposed granite taking the form of dramatic shorelines, barren islets and the countless rocky outcrops (kalliot in Finnish) that punctuate the generally flat landscape all over Southern Finland. These hills of solid rock often have to be cut in half to make way for highways, leaving some impressive road cuts that in winter are plastered with impressive columns of ice. It also means that Finns have a special expertise in carving out portions of the Baltic Shield to suit their needs.

In the lush, forested landscapes of my native Georgia, exposed rocky surfaces are rare, except for most notably places like Stone Mountain, a giant dome of granite near Atlanta – and a place I have still never visited. Typical, isn’t it?

Any lushness in the forests of Finland has been hard won in the last 10,000 years or so since living things started to reclaim this part of Earth bulldozed, sandpapered, scraped to the bare stone by a layer of ice three-and-half kilometers (over two-miles) thick. Signs of the so-called Weichsel glaciation, which covered most of Northern Europe, can be seen everywhere here.

One such sign was uncovered, and almost destroyed, a few kilometers from my home, when some roadwork was undertaken twenty years ago on the side of a kallio in Pihlajamäki. A portion of bedrock was being prepared to be blasted when a passerby noticed the outlines of two large “giant’s kettles” (called hiidenkirnut in Finnish, or “Devil’s Churns”).

Baltic Shield bedrock is never far from the surface.
These “kettles” are potholes formed when fast-flowing water carves out a round cavity in bedrock through the twirling, grinding action of stones that become trapped in the hole. I recall seeing miniature versions of these in the mountain creeks I fished with my father back in Georgia.

The giant’s kettles in Pihlajamäki are much bigger than those, the larger one being almost 7 meters (23 feet) across and eight and half deep (28 feet). They were formed over 50,000 years ago as the retreating Weichsel icecap unleashed torrents of meltwater lasting hundreds of years, eventually digging a hole spacious enough to dump ten VW Beetles.

This massive pit wasn’t the only geological fingerprint left by the retreating ice sheet, not by a long shot. Besides the gouge marks left on exposed rock by glacial scraping and scouring, evidence of the Ice Age is easy to spot in the innumerable glacial erratics (siirtolohkareet), eskers (harjut), and kettle holes (supat).

Erratic and uprooted trees in Helsinki's Central Park near Paloheinä.
 Erratics are isolated boulders, large and small, torn from their original location and carried off by glaciers before being dropped in a new neighborhood as the ice sheet vanished. Eskers are long narrow ridges of sand and looser glacial debris that can wind for miles, sometimes with lakes on either side, as with the scenic Punkaharju in eastern Finland. They are sometimes mined for building material, creating large sand pits that have also been used (at least once) as locations for Finland’s own low-low-budget versions of spaghetti Westerns.

Kettle holes (not to be confused with giant’s kettles) are large cauldron-like dents in the ground where giant blocks of long-gone ice left deep impressions in the sandy deposits around them.

Still, the most impressive holdover from the icy Weichsel episode is the one that is the most subtle, in fact, practically impossible for mere mortals to see except near the coast.

With the Baltic Sea on two sides, Finland has a long coastline that is rustic, rocky and, well...rising. In many spots you can find the rocky beds of ancient beaches well inland from the modern shoreline, signs that the waterline was once much higher than now.

Erratic near the seashore at Sipoo.
The reason is that frozen water doesn’t weigh nothing, and terra firma isn’t always as firma as it seems. An ice sheet two-miles thick exerts punishing pressure on the earth’s crust, which it turns out is a bit spongier than you might think. During the last Ice Age, parts of the Baltic Shield were pushed down by this tremendous weight. After the icecap disappeared 100 centuries ago, the Earth's crust started to bounce back – slowly.

It’s still rebounding today, at a rate in some places of a meter (three feet) every hundred years, most notably at the, ah, poetically named Merenkurkku (Throat of the Sea), the 25-kilometer-wide chokepoint in the Gulf of Bothnia between Finland and Sweden. (By the way, Merenkurkku should not be confused with merikurkku, “sea cucumber”, a distinct possibility thanks to Finnish’s odd use of kurkku to mean both “throat” and “cucumber”. Why not, it’s their language?)

Most likely, even in my lifetime some rocks that were once merely hidden boating hazards in the waters of Merenkurkku have now emerged to be visible boating hazards. Because Merenkurkku is only 25-meters (75-feet) deep, it’s possible that, if the rate of rebounding remains the same, you (okay, let’s be honest, someone who is not you) will be able to drive across dry land straight from Vaasa to Umeå, by the year 4513. Give or take a lifetime or two.

Typical rocky Finnish seascape.

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