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Time, on a geologic scale, sets things in perspective

June 5, 2010
By Garry Brandenburg

Here is a question for all of you readers: Has Iowa passed more time (geologically speaking) as dry land or under the ocean? The answer is ... without any doubt ... under the ocean. And the proof is written in the rocks. The nice thing about rocks is this. They do not lie. They always tell the truth. It may take a geologist and other physical scientists to decipher the clues within the rocks. But once the story of rocks is told, it helps paint a picture for us mere mortal humans so we can try to comprehend the forces of nature that continue to run the show on earth.

Remember this also: the earth is not static. Geological forces are still with us, always have been and always will be. It is also true that human lifetimes are way too short to see the big picture of the natural cycles that the earth experiences. Those cycles are unevenly distributed on the land masses but in general, reflect times that fit into these categories: warm and dry, warm and wet, cold and dry, cold and wet, or a mix of any one transitioning into the other and back again, and again and again. And these natural rhythms take place over huge time scales of geologic time.

Samuel Calvin, in 1906, wrote in his "Report on Geology of Iowa's Counties" this statement: "The rocks in question, therefore, so far as relates to Iowa, are nothing more than the consolidated sands and muds of old sea bottoms preserving for our inspection samples of life that occupied the seas at the time each successive bed was in the process of accumulation. Iowa has passed more time under the ocean than as dry land."

Article Photos

This view of the Gifford Quarry in Hardin County, not too far north of Union, reveals layered limestone strata, proof of many ancient shallow oceans of Iowa’s geologic past. To keep the quarry operating, groundwater infiltration must be diverted to a pool and then pumped up and over a levee. Limestone can be used in many ways as a construction material. However, also note that the relative depth of soils over the top limestone layer is comparatively shallow. In effect, the glaciated mix of fine grained materials we call soil, the stuff that grows things like trees, grasses and our crops, is the product of the most recent geologic episodes of earth history.

Over most of Iowa, the rock record is covered and obscured by unconsolidated sediments that were laid down by wind, streams or glaciers. The geologic time frame for unconsolidated materials is within the Pleistocene epoch ( 0 to 1.65 million years ago ).These records are the most recent and give geologists a very complete record of Iowa history.

If one can visualize Iowa as a big thick sheet cake, and you took a knife and sliced it from northeast Iowa, McGregor, to southwest Iowa at Council Bluffs, and then looked at the exposed layers of the cake, the bedrock at the surface at the Mississippi side of the cake is going to be deep at the Council Bluffs side. In fact, one has to go about 3,000 feet down at Council Bluffs to find the same rock types you can touch at a State Park overlooking the Mississippi River. This hypothetical 'cake' is really quite deep, and under every layer of rock is another layer, and another and another, each with their own histories.

Rocks exist deep below us from Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurrasic and Cretaceous periods.You and I can see Devonian age rocks ( 409 to 363 million years old ) at the Ferguson Quarry, the whitish limestone products mined there. Mississippian rock layers ( 363 to 323 million years old ) are exposed at Three Bridges County Park along the base of the cliff faces. Fossil animals called crinoids are on exhibit at the Marshall County Historical Society and many geology departments of major Universities which came from the quarry north of LeGrand. They are from rocks now 330 million years old but at the time of their growth, were in warm shallow oceans.

To better understand geologic time, I'll compare it to a 365 day calendar year. Precambrian time would go from January first through mid-November. All the rock formations and depositions of the Cambrian through Permian periods would take from mid-November to about Dec. 13. The Mesozoic era is a combination of the Triassic, Jurassic and Cretaceous periods and would be represented on the calendar from Dec.13 to Dec. 27. Cenozoic era of the last 1.65 million years is represented by the last five days. The most recent geologic time, the Pleistocene and our known ice age history would be represented by the last three hours. The last 10,500 years is represented by only 1.1 minutes!

Here is another way to look at geologic time. Iowa is about 300 miles wide from Davenport to Council Bluffs. Precambrian time is the first 264 miles. Paleozoic time, also called the age of invertebrates, is in the next 20 miles. The age of reptiles, the Mesozoic Era is the next 12 miles. The age of mammals, Cenozoic, is the last 4 miles of the journey. Pleistocene and Holocene epochs can be depicted by the final 565 feet of the trip as you cross a bridge over the Missouri River from Council Bluffs to Omaha, Neb.

I hope you have enjoyed this very brief description of geologic time. The story of earth's history is written in the rocks.And the rocks tell the truth. How mankind adapts to the present and the near future will be written on paper in history books. I can only hope the story on paper is truthful.


Last weekend was not so nice for water based recreationists. Errors were made in timing and judgment and accidents happened. Boaters had fun for the most part ... until. A 10 year old boy on Storm Lake is dead when the motor on his boat was flipped up and became dislodged. A 12 year old Fairfield girl was hit and killed by another boater after she had fallen off her tube. Two teenagers on the Mississippi River were injured when their personal watercraft exploded. A family in a stalled boat on the Mississippi river escaped before their boat was hit by a barge. The boat was destroyed but the people survived when picked up by other boaters. Safety on the water is a constant necessity.


Registration is now open for the "Bird Bath Bonanza" Brown Bag bunch program. You can create a sand-cast bird bath from a large hosta or rhubarb leaf in this two-part workshop. The workshop will be held at the Conservation Center from 11:30 a.m. 1 p.m. on June 14 and June 22. Cost is $5. Call 752-5490 to register and pay by June 9. Space is limited.


The Amateur Astronomers of Central Iowa invite the public for a solar viewing and potluck on June 12 at 4 p.m. at the Green Castle Recreation Area. For more information contact Jim Bonser at 641/751-8744.


"Truth is such a rare thing, it is delightful to tell it." -Emily Dickinson, American Poet


Garry Brandenburg is a graduate of Iowa State University with BS degree in Fish & Wildlife Biology. He is the retired director of the Marshall County Conservation Board. Contact him at PO Box 96, Albion, IA 50005.



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