Monday, September 12, 2011

towering chimney rock

towering chimney rock,
weathered fossil palisades
line the river gorge

“That’s got to be Chimney Rock,” I say, as a towering stone pillar comes into view around a bend in the Oneota River. We’ve been paddling past palisades of white bluffs, fractured and weathered, but still forming a contiguous wall of stone. How did this lone, freestanding pillar get that way? The answer to that question leads me deep into Iowa’s geologic history, which mostly lies buried beneath the ground.

We are drawn to this area of northeast Iowa by white bluffs along a clear river, lots of wildlife and pleasant paddling. The trip turns out to be a fascinating excursion through time and space. In one afternoon we travel in geologic time from 350 to 600 million years before the present. In space we traverse from the Southern Iowa Drift Plain through the Iowan Surface to the Paleozoic Plateau.

The landscape where we live in the southeast corner of Iowa consists of rolling hills, the eroded remains of glacial deposits on top of bedrock. The sedimentary bedrock consists of sandstone, limestone, dolomite and shale, over 3,000 feet thick in places, laid down by shallow seas that covered the area between 530 and 74 million years ago. The fertile soil on the surface presently supports fields of corn and soybeans. Exposed rock is rarely seen, except at road cuts or river valleys.

As we travel north, the terrain gradually becomes more gently rolling. The older glacial deposits in the Iowan Surface region were leveled by intense erosion during a glacial cold spell between 16,000 and 21,000 years ago. Near rivers, the hills become steeper, the valleys deeper, the picturesque terrain depicted in many of the landscapes of Grant Wood.

Finally, we enter the Paleozoic Plateau, a region of deep, narrow valleys and exposed bedrock in the form of bluffs and outcrops. This is the only region of Iowa where bedrock dominates the landscape. The sandstone and carbonate rocks that are more resistant to erosion are the ones that form cliffs and escarpments. The answer to my question about the unique form of Chimney Rock is unequal weathering. Why this lone pillar? That's an even deeper question.

The rock outcrops, studded with fossils, lie in layers, reflecting their sedimentary origin. Vertical fractures in the brittle rock, caused by earth stresses, give the outcrops a blocky appearance. These breaks become flow paths for groundwater. In addition to the distinctive bluffs, this is also a karst area of springs, sinkholes and caves. As we continue maneuvering our canoes through the shallow rapids of the narrow gorge, we encounter many springs and small waterfalls pouring their cold, clear water into the river. I dip my hand into one of the outflows as we drift past. It feels glacial.

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