I noticed a plaque recently near the bandshell at Lake Harriet titled “Geology of Minnesota, Lake Harriet Region.” The date on it is 1955, but I had never noticed it before. It is a big metal slab with lettering on it, and it is installed on a concrete base. I often walk and bike near it. How much do we miss?
Below, braided together, are sentences from the plaque as well as from two other sources. Those from the plaque are in regular type. The definitions of terms (glacier, glacial valley, and glacial drift), from “Home Ground: Language for an American Landscape,” edited by Barry Lopez, are in italics. Those on emotion, from Pema Chodron’s book, “When Things Fall Apart: Heart Advice for Difficult Times,” are in boldface.
The continental glaciers spreading over Minnesota during the Great Ice Ages brought vast quantities of rock material from the North to be dumped indiscriminately during the recession of the ice.
A tremendous accumulation of snow, compacted by weight and turned to ice, often carrying rock and sediment debris with sometimes a meltwater river below, a glacier moves with gravity due to its own mass.
We use our emotions.
Old river valleys were filled and belts of hills were formed as conditions changed. A glacial valley is distinguished by it distinctive U-shape: a broad flat floor flanked by high walls of rock. Such a valley has been bulldozed wide by a glacier scraping its way down an existing V-shaped valley, created by a preglacial river.
We use them.
The Lake Harriet landscape has such an origin.
In their essence, they are simply part of the goodness of being alive, but instead of letting them be, we take them and use them to regain our ground.
Leaving the present channel of the Mississippi River at the Plymouth Avenue Bridge, a pre-glacial valley runs almost directly south beneath Lake of the Isles, Lake Calhoun, and Lake Harriet to the Minnesota River at Bloomington.
We use them to try to deny that in fact no one has ever known or will ever know what’s happening.
This valley was mostly filled but not completely obliterated by glacial deposits. The random spoils of a glacial incursion over land, whereby detritus is picked up in one place and set down in another, is called glacial drift. Occupying the time stratum between the drifting of snow and the drifting of continents, glacial drift represents the products of abrasion between resident earth and moving glacier.
We use them to try to make everything secure and predictable and real again, to fool ourselves about what’s really true.
The unfilled portions of the valley are now basins, which are filled by lakes perched high on glacial debris.
We could just sit with the emotional energy and let it pass.
Located last in a chain of five lakes located in Uptown Minneapolis, Lake Harriet lies directly over this ancient valley, formed as an ice block depression during the most recent glaciation. Its surface is in a setting of hills piled up while the ice front paused here in its final retreat about 10,000 years ago.
Drift materials may consist of erratic stones, landforms such as massed terminal and lateral moraines, drumlins dozed up by moving ice, or broad sheet deposits such as till or outwash, which are shaped by ice and left when glaciers retreat.
There’s no particular need to spread blame and self-justification.
The weight of the great continental glaciers not only shaped in passage but also physically depressed whole landscapes so severely that the land continues to rise (“isostatic rebound”) thousands of years after the melting of the glacial ice.
Lake Harriet has a surface area of 335 acres and is the second largest lake in Minneapolis. Its average depth of twenty-nine feet (eighty-two feet maximum).
Instead, we throw kerosene on the emotion so it will feel more real.
Now located in an urban residential area, the lake is fed largely by storm drainage, runoff, direct precipitation, and groundwater seepage. Throughout its history, Lake Harriet has undergone many changes, driven by both natural and human influences.
We, each of us, have undergone many changes, driven by both natural and human influences. We are mysteries made up of layers. We are great sheets of ice. We move with gravity, due to our own mass.
Too, we are old valleys, with broader new valleys carved out of them, on top of them. We are ghost meltwater rivers down low, and frenzied warm rivers above. We are hills; we can see a long way. We are tamed residential lakes. We are piles of detritus. We are both the land weighted down, severely compressed by the past, and the land rebounding. We are the seen and the unseen.
Mary Jean Port writes at home, near Minnehaha Creek and Lake Harriet, and teaches at the Loft Literary Center.