Showing posts with label Columbus Limestone. Show all posts
Showing posts with label Columbus Limestone. Show all posts

Monday, March 2, 2009

A piece of the Ice Age puzzle, glacial striations...

We find a rare faceted and striated boulder...
Location: 40.24N 83.07W
(construction site currently off-limits)

See it at the new Nature Lodge opening during the summer of 2009 at Deer Haven Preserve, Preservation Parks of Delaware County, Ohio.

Ice sheets scoured the high spots and filled-in the low spots throughout the northwestern two-thirds of Ohio and much of the Midwest, North America's glaciated flat-lands. Glacial geology lessons abound throughout the landscapes of Ohio's glaciated counties, and beyond; but we rarely find well preserved examples of eroded bedrock like this striated boulder recently unearthed during back-fill operations for the new Nature Lodge under construction.

This rough boulder is a gem of a find, a 'diamond-in-the-rough' reflecting a sub-glacial erosional process that shaped Ohio and the Midwest.

A limestone boulder, plucked by glacier ice from the glacially scoured surface of the Middle Devonian age Columbus Limestone formation, shows angular fractures surrounding a striated facet. Much of the surface of Ohio's buried limestone bedrock is found flattened, striated, grooved, and chatter-marked when glacial deposits are removed, exposing the buried bedrock surface. This boulder offers a rare glimpse of the buried bedrock surface.

The flattish facet surfacing the boulder pictured above was chiseled by debris 'tools' once frozen in a basal ice-debris matrix, the 'dirty' ice at the base of the moving Wisconsin-age Laurentide Ice Sheet.

Resistant bedrock surfaces were scoured and gouged by debris held in glacier ice for tens of thousands of years underneath the Laurentide Ice Sheet that spread across northern North America. In central Ohio, the erosion process continued until at least twenty-thousand years ago.

The sharp fractures surrounding the boulder's facet suggest the boulder did not travel far after it was faceted, and then plucked from the bedrock surface. The striations would have been smoothed, and the sharp edges of the fractures would have been rounded by long dynamic transport within the ice or by transport in flowing melt water.

Detail of striations. Parallel striations were cut when the boulder was part of the bedrock surface the glacier slid over. Hard rocks, frozen in the basal ice-debris matrix, scraped the soft limestone bedrock like coarse sandpaper on wood. A second group of deep gouges, crossing the first, may have resulted when the bedrock block was rotated and plucked from its original position at the ice-rock interface (the bedrock surface) to begin its transport within the ice from its nearby origin to the site of the new nature lodge at Deer Haven.

The boulder was unearthed from clayey till, a ground moraine left in central Delaware County, Ohio by the most recent retreating ice sheet. Till is a mix of fine and coarse grained sediments kneaded together by pushing-grinding ice flow, and released from melting ice without the sorting action of flowing water.

During the 1.8 million years of the Pleistocene Epoch (beginning 1.806 million years ago*), much of the Midwest was buried by ice sheets for most of the time. We live in the Ice Age still, though we formally end the Pleistocene Epoch at 11,700 years ago.

*International Commission on Stratigraphy.