Integrative Natural History of Minnesota's North Shore, Part 12: A Groovy Close-up of Keys, Cracks, and Crescents

Integrative Natural History of Minnesota's North Shore, Part 12: A Groovy Close-up of Keys, Cracks, and Crescents | Duluth

Facing southwestward on the same glacially abraded outcrop of North Shore Volcanic Group basalt shown in Part 9, Part 10, and Part 11 of this series. In other words, we're on the Lake Superior coast at Kitchi Gammi Park. The car keys provide scale.

In geology, as in many other aspects of life, what may at first glance seem to be an uninteresting jumble of trivia can reveal itself to be, on closer and thoughtful inspection, a set of relata—interconnected things that when woven together have a wonderful story to tell.

Just of the left of the keys, for example, there's a groove that runs straight as an arrow and more or less vertically. This is one of many glacial striations to be found on this bedrock surface. It was etched into the hard igneous rock at some point 15-10 ky ago, when a stone embedded in the bottom of the the Superior Lobe of the Wisconsin-episode ice sheet moved over what is now the Duluth area.

And if you look closely to the left of the striation, you should be able to discern quite a collection of subtle indentations bordered at the top with raised and gently curved rims. These features are crescentic gouges, another type of glacial-abrasion effect, where downward-pushing rocks in the ice produced shallow divots.

The midpoints of the crescents indicate the way the glacier was moving, while the 'horns" or tips face upstream. And, not surprisingly, the direction indicated by the gouges is parallel to the striations here.

What aren't oriented well with the other two, though, are the deeper cracks in the basalt. These are joints, fissures in the rock that formed once this portion of the Earth's crust, previously buried for over a billion years by overlying rock, was finally exposed at the surface.

Relieved of its confining pressure, and also probably cooler than it was when underground, the basalt expanded vertically, shrank horizontally, and cracked. This process of stretching in one direction and contracting at right angles to that direction is know as Poisson's Effect.

To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.

Taken in: United States / Minnesota / Duluth / Lester Park (show map)

Taken on: May 21, 2004

Tags:

geology

nature

rock

rocks

stone

Duluth

Minnesota

Kitchi Gammi Park

North Shore Volcanic Group

basalt

glacial abrasion

glacial striation

crescentic gouge

joints

Midcontinent Rift « less

Sets appears in: • Integrative Natural History of Minnesota's North Shore • MCR: Homage to the Midcontinent Rift more » • Natural History: Minnesota • Rock Forms, Rock Formations « less

Groups appears in: • Rocks and Geology • Geology Rockers more » • Midwest • GeoTagging Flickr • Geotagged • Rocks • Geology • Rocks and Stones • Minnesota Parks • Geologists • USA - UNITED STATES OF AMERICA • nature and landscape photography • North American Nature & Wildlife Photography • Geology for all • Geology/EarthScience • Celebrating Minnesota • Minnesota North Photography • American Photography • Rocks-Fossils-Minerals • Nature Photography // • stones n rocks • I Love Rocks! • Midwest Nature • Natural Midwest *Please invite friends* • Geotagged: Minnesota • Geology Flickrs • Nature Album • Photography with a Story • Geology / Geologie • Nature and all that is beautiful • The Very Great Outdoors • We love Nature and Geology • AMERICAN PHOTOGRAPHER « less

Camera: Sony CD MAVICA, ISO 200 (more info) (hide info)

Camera:	Sony CD MAVICA
ISO:	200
Flash:	On, Red-eye reduction, Return detected
Exposure Program:	Program AE
Metering Mode:	Spot
White Balance:	Auto
Color Space:	Uncalibrated
Show full info

Stats: 279 views