Erratic on the Southern Iowa Drift Plain EarthCache

This EarthCache is located on Warren County Conservation Board property. The park sits on 160 acres, includes a six-acre pond and ten-acre wetland, and lies adjacent to Lake Ahquabi State Park. In addition to this Earthcache, the park also contains miles of trails, a 30" observation tower, osprey reintroduction site, butterfly gardens, fishing, a bird blind and more. Take your time and enjoy all this park has to offer! Please take time to visit the Nature Center while you are here. There are 2 levels of exhibits to explore inside the Nature Center as well restrooms for those in need. And don't miss the giant Iowa Bison statue outside too!

 

Park hours are sunrise to sunset.  Please do not hunt for this at night.

Hours for the Nature Center are:

Monday thru Friday 8am - 4:30pm

Saturday and Sunday - CLOSED

The 3rd Thursday of every month the Nature Center is open til 6pm.

 

Parking is available at the Annett Nature Center. See the Parking Waypoint below for coords.

The boulder at the listed coordinates is a glacial erratic, a rock not native to the area in which it has come to rest, hence the name “erratic”. These rocks were carried often hundreds of miles to their current locations by glacial ice. This area of Iowa is known as the Southern Iowa Drift Plain and was most recently covered by ice over 500,000 years ago. Extremely cold climatic conditions led to erosional beveling in this area and removal of much of the finer-grained glacial materials, thus concentrating the larger pebbles and boulder at the land surface. In the recent past this area was used for farming. When the conservation board purchased the land, they brought it back to a more natural state of prairie.

 

This boulder and the boulder in the butterfly garden were unearthed when building 118th street and were moved to the Nature Center for landscaping and display.

 

 

What is it, were did come from, how did it get there? The cache is a traveler from the ancient past.

 

A glacial erratic is a piece of rock that deviates from the size and type of rock native to the area in which it rests; the name "erratic" is based on the errant location of these boulders. These rocks were carried to their current locations by glacial ice, often over hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta.
 

“Big Rock” in Alberta, Canada

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics were once considered evidence of a massive flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient civilizations throughout the world. Ancient legends of an epic flood come from many cultures including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old Testament) and Indian culture. In the 19th century, many scientists came to favor erratics as evidence for the end of the last glacial maximum (ice age) 10,000 years ago, rather than a flood. Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with them. When the ice melted, the erratics were left in their present locations.

Shown here is a field strewn with glacial erratics. This is typical of many pastures in Iowa.

 

 

"Peculiar," "irregular," and "uncommon," are words used to describe one class of Iowa rocks -- glacial boulders or "erratics." Geologists define erratics as stones or boulders that have been carried from their place of origin by a glacier and then left stranded by melting ice on bedrock of a different composition. In Iowa, glacial erratics are commonly observed where glacial deposits occur at the land surface, primarily in the north-central and northeastern parts of the state. In western and southern Iowa, erratics generally lie buried beneath wind-deposited silts (loess) that cover the glacial materials. In these areas, erratics generally are restricted to valleys, where streams have eroded through the loess and into the underlying glacial deposits.

 

The erratics seen in central and north-central Iowa are the most recent to arrive in the state. The ice sheet responsible for them entered Iowa from Minnesota and moved southward between what is now Mason City and Spencer, advancing as far as the capital city of Des Moines and forming the Des Moines Lobe just north of here.  This ice melted away around 12,500 years ago. The same process formed the Southern Iowa Drift Plain, but around 500,00 years ago instead. This is why landforms in the counties in the Southern Iowa Drift Plain are so much more distinct than its neighboring counties to the north.  Features typical of a freshly glaciated landscape have been obliterated by time. The only remaining evidence to verify passage of these earlier ice sheets is the ten to hundreds of feet of glacial drift covering the bedrock surface. Gone are the moraines, kames, kettles, bogs and lakes – all the distinct visual clues to recent contact (geologically speaking) with glacial ice.

 

The Drift Plain.....

 

Instead of poorly drained, low-relief landscapes, streams have had time to establish drainage systems and carve the land surface deeply. Hillslopes often display a texture of finely etched rills that give a distinct ribbed or furrowed appearance to the terrain. These rills give way to ravines, then to creeks that flow part of the year, and eventually to perennial streams and rivers in major valleys. Patterned like the branching veins in a leaf, this network has drained the postglacial wetlands, erased the ice-contact landforms, and over time turned the glacial plains into deeply creased landscapes so familiar to this region today, and makes the erratics in the Southern Iowa Drift Plain so much more distinctive and noticeable on the lands surface.

With most of the land surface sloping toward some drainageway, the terrain of this region projects a feeling of enclosure when we travel among its hills. Views extend only as far as the next rise or the next bend in the road. There are no long-distance vistas except those seen from hillcrests that return again and again to the same elevation, each time providing a glimpse over the billowy landscape beyond.  The observation that Southern Iowa Drift Plain summits always seem to return to a uniform elevation is a clue to their geologic origins. These even-topped uplands disclose the approximate level of the original, once-continuous land surface constructed by the last ice sheet to pass this way over 5000,000 years ago. Every hillslope and valley floor mark the extent of erosion into the old glacial plain. The space between hills emphasizes the great amount of material that has been removed by flowing water, wind and rain, and the hundreds of thousands of years this process has taken.

The erosional processes that have carved these hills from the earlier glacial plain have not been at work continuously or uniformly through time. Instead, the past has been punctuated with episodes of rapid erosion accompanied by valley deepening and lengthening. The episodes of downcutting alternated with periods of greater landscape stability when soil profiles could weather deeply into the exposed glacial deposits. This variable intensity in the erosional shaping of the landscape actually left broad steps notched into the hillslopes of the region’s drainage basins. These stepped erosion surfaces occur throughout the portions of Iowa where Illinoian and Pre-Illinoian glacial deposits are the dominant landscape materials. Thus, hillsides are a key element of Southern Iowa Drift Plain landscapes. They may appear at first glance to be smoothly flowing slopes, but subtle changes of contour from more steep to less steep reveal past irregularities during their long erosional history.

 

How the distinctive terrain of the Southern Iowa Drift Plain looks below the surface

 

More on erratics......

 

When these areas of the state were settled, farmers were forced to clear fields of the rock obstacles in order to plow and cultivate. Many of the erratics were used to build fences and foundations, while others were just piled along fence rows or into unused field corners where they are seen today. Clearing farm fields of glacial erratics is a necessary and frequent chore wherever glacial deposits are cultivated. Over time, seasonal freezes and thaws work these rocks upward from below the plow zone to the land surface. Smaller glacial erratics can be hauled out of the fields; larger ones are frequently blasted apart by dynamite and the pieces hauled away; while some of the largest are just left in place and avoided. At the municipal park in Nora Springs (Floyd County), an adjoining city street actually narrows to accommodate an erratic protruding into the right-of-way.

 

Erratic left in roadway in Nora Springs, Iowa

 

Glacial erratics in Iowa are not difficult to identify. The vast majority are igneous or metamorphic rocks, rather than the usual sedimentary rocks of sandstone, limestone, dolomite, and shale that constitute the bedrock under most of Iowa. If you pick up a granite rock, composed of interlocking crystals of pink feldspar and glassy quartz, you can be sure it is not native and that it came from outside the state, most likely carried by glacial ice.

 

Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. Glacial transport also caused some boulders to fracture, producing fresh angular edges. Rocks carried by rivers also undergo abrasion and become rounded in the process. In fact, most of the igneous and metamorphic rocks in Iowa's river valleys were originally transported into the general area by glaciers, then eroded from the glacial deposits and moved some additional distance by a river.

Transportation by glacial ice, however, produces some other features unique to this mode of travel. The most easily observed of these tell-tale signs are glacial striations, a series of parallel lines or fine grooves gouged across the beveled faces of erratics or inscribed on the underlying bedrock surface. .

Information for this Earthcache obtained from the information in the Annett Nature Center, various internet sources, and the Iowa Geological Survey from the University of Iowa College of Engineering.

 

 

1) What type of rock do you think this errratic is made of? If you don't know, then your best guess. 

2) Although this was transported here for display from nearby construction, how do you think it originally made it's way here?

3) Is the surface rough or smooth? What is the texture like? Why do you think it is that way? Are there any striations along the surface of this erratic? If so how did they get there?

4) Compare this erratic to the smaller one located in the nearby butterfly garden. How are they similar? How are they different other then just size? 

5) Looking out over the prairie away from the erratic, what evidence do you see, of the millenia of erosion thats taken place here?

6) Why are the ravines and streams and hills so much more exaggerated here when compared to the counties to the north of here in the Des Moines Lobe formation?
7) Do you see any evidence of the rilling feature mentioned in the hillsides and landscape here and if so, describe it. 

8) The water drainage system for this area has evolved and deepened over the last 500,000 years. What is the name of the large body of water that the local watershed here drains into?