GENERAL GEOLOGIC SECTION AT PIKES PEAK STATE PARK Brian J. Witzke Iowa Department of Natural Resources Geological Survey Bureau Iowa City Iowa 52242-1315

 

 

Lower Dolomite Ledges
the Ordovician Prairie du Chien Group

Lower Ordovician dolomite strata of the Prairie du Chien Group overlie the Jordan Sandstone in the park.  Although the Jordan-Prairie du Chien contact does not display significant erosional relief, an episode of missing time is marked at that position in the stratigraphic succession (a disconformity).  The Prairie du Chien Group is characterized by ledges of dolomite strata, some containing nodules of chert.  Minor sandstone and shale is also observed.  The Prairie du Chien dolomites were formed by chemical replacement of original lime sediments which accumulated in shallow tropical seas that covered much of the interior of North America between about 490 and 505 million years ago.  This chemical replacement of original lime sediments composed of calcium carbonate by calcium-magnesium carbonate (the mineral called dolomite) was a later-stage process.  Because dolomites differ chemically from limestones, they have sometimes been termed “magnesian limestone” in the older geologic literature.

The Prairie du Chien Group derives its name from the nearby city of Prairie du Chien, Wisconsin, only a few miles north of Pikes Peak.  The bluffs behind Prairie du Chien display these strata in bold cliffs and rocky slopes.  Prairie du Chien strata are prominently exposed in bluff slopes and cliff faces throughout much of the Upper Mississippi Valley (from Prairie du Chien to the Twin Cities), commonly reaching thicknesses to about 250 feet.  The interval contains rocks that are more erosionally resistant than the underlying succession of less resistant Cambrian sandstones.  The Prairie du Chien Group has been divided into two formations, the Oneota below and the Shakopee above.  Strata of the Oneota Formation are exposed in the lower bluff faces and lower ravine drainages of Pikes Peak State Park, and old quarries can be seen adjoining the railroad tracks south of McGregor.  The Oneota Formation is subdivided into two members.  The lower interval, the Coon Valley Member, has historically been considered under several different names, and it was variously included within the Jordan Sandstone or Prairie du Chien Group.  Because it is an interval dominated by dolomite and contains fossils of Ordovician age, it seems reasonable to include it within the Prairie du Chien Group (as now classified by the Minnesota Geological Survey), as opposed to the underlying Jordan sandstones of Cambrian age.  The Coon Valley interval, however, does contain some quartz sand, primarily as isolated sand grains in a dolomite matrix.  By contrast, overlying strata of the middle and upper Oneota Formation (termed the Hager City Member) are generally devoid of quartz sand, and it is characterized by remarkably pure dolomite beds, in part cherty to very cherty.

Upper Prairie du Chien strata are included in the Shakopee Formation, which differs from subjacent Oneota strata in containing some sandstone, shale, and sandy dolomite.  The lower Shakopee interval contains sandstone and sandy dolomite (the New Richmond Member), and the upper interval is less sandy and variably cherty (Willow River Member).  The Shakopee Formation is not well represented within Pikes Peak State Park, primarily because it has been erosionally removed from much of the immediate area during a prolonged period of erosion that followed the withdrawal of the Prairie du Chien seas from the area and preceded the deposition of the St. Peter Sandstone.  However, Shakopee strata are represented in the deep water well drilled in the uplands near the Pikes Peak State Park entrance, so the Shakopee Formation was not completely eroded across the entire park area.  Shakopee strata are well exposed elsewhere in northeastern Iowa (portions of Clayton and Allamakee counties) and the Upper Mississippi Valley.

 

Thick Massive Sandstone
the Middle Ordovician St. Peter Sandstone

The shallow tropical seas in which Prairie du Chien lime sediments were deposited eventually withdrew from the interior of North America during later stages of the Early Ordovician, and a long period of erosion began in the area.  This ensuing erosional episode spanned a remarkably long period of time, including portions of the Early Ordovician and much of the Middle Ordovician (approximately 25 million years duration).  This period of erosion was characterized by deep weathering across the exposed surface of Prairie du Chien dolomite strata., resulting in the development of an irregular and incised topography.  The region that now includes eastern Iowa occupied a geographic position in tropical latitudes (within about 10º of the equator) at that time.  Tropical weathering produced a network of caves and sinkholes created by karstic solution of the Prairie du Chien carbonate rocks, and these solutional openings are known to penetrate up to 350 feet though the succession of Prairie du Chien strata in areas of eastern Iowa.  Deep valleys were also eroded into this landscape, which in places in the Upper Mississippi Valley area cut through the entire Prairie du Chien succession and into underlying Cambrian strata.  In the Pikes Peak area a deep valley was incised through the entire Shakopee Formation and into strata of the Oneota Formation.

The erosional landscape of valleys and sinkholes developed on Prairie du Chien strata was subsequently infilled by sediments of the St. Peter Sandstone during the latter part of the Middle Ordovician.  This infilling was apparently initiated as shallow seas encroached once again into the continental interior of North America.  Initially, rivers within the valley systems began to aggrade their sediment load as stream gradients changed in response to rising sea level.  As the sea continued to expand into the region, the valleys likely became estuaries along the encroaching coastline.  Ultimately the valleys, like the one developed across Prairie du Chien strata in Pikes Peak State Park, entirely filled up with sediment, primarily quartz sand.  Once the valleys and karstic openings became filled, the seaway continued to expand over the region depositing a widespread body of sand across the shallow shelf.  This valley-filling and shallow-marine sandstone body is known today as the St. Peter Sandstone.  The St. Peter Sandstone comprises the thickest formation exposed today in Pikes Peak State Park.

The St. Peter Sandstone derives its name from exposures below Fort Snelling at the mouth of Minnesota River in St. Paul, Minnesota – the Minnesota River was formerly known as the St. Peter River.  The St. Peter Sandstone is a remarkably widespread sandstone formation that has been recognized as far east as Michigan and Ohio, as far south as Arkansas, and as far west as Kansas and Nebraska.  Of all the known exposures of St. Peter Sandstone across this vast area, the thickest known succession is seen within Pikes Peak State Park, where the full thickness of the formation is known to vary between 90 and 223 feet.  By contrast, in nearby areas of Clayton and Allamakee counties, the St. Peter Sandstone is more typically 40 to 55 feet in thickness.

The St. Peter Sandstone is a remarkably monotonous and homogeneous succession of quartz sand (the rock is termed a “quartzarenite”).  It appears within Pikes Peak State in thick massive beds best seen in bluff slopes and steep-walled ravines (as along the Sand Cave trail).  The St. Peter is overwhelmingly dominated by very fine to medium grains of quartz sand, with little other material present.  The quartz grains are commonly well rounded, and the sedimentologically-mature aspect of the St. Peter quartzarenites suggests that much of the sand was derived by the reworking of older sandstones (like those seen in Prairie du Chien and Cambrian strata).  Because of its homogeneity it is difficult to distinguish sedimentary features within the succession, although some low-angle crossbeds and the burrow traces of marine animals can be seen in places.  Argillaceous (clay) material is incorporated with the sand in the upper part of the sandstone succession, and clay-rich intervals with coarser reworked fragments of Prairie du Chien chert are locally present at the base of the St. Peter.  At Pikes Peak, the lower portion of the St. Peter Sandstone where it is thickest (as in the Sand Cave area) displays dramtic swirls and bands of red-colored iron-oxide cements (which geologists sometimes term “Leisegang bands”).

The St. Peter Sandstone in the region is capped by a relatively thin green-gray shale unit known as the Glenwood Shale.  This shale is only about 4 to 5 feet in thickness in the park, and because of its soft and easily erodable character, it is typically not well exposed in the wooded ravines.  The Glenwood Shale has been grouped together with the St. Peter Sandstone into a stratigraphic interval geologists have named the Ancell Group.  The Glenwood Shale is phosphatic in part, incorporating vast numbers of tiny tooth-like phosphatic microfossils known as conodonts.  Chitinous jaws of annelid worms (scolecodonts) are also abundant.  The clay sediments of the Glenwood Shale in this area were deposited very slowly within the shallow seaway (geologists refer to such a slowly-deposited unit as a “condensed section”), likely in far offshore areas.  The slow rates of sediment accumulation are reflected by the thinness of the Glenwood Shale across the Iowa area (generally less than 5 feet), but elsewhere in southeastern Iowa and Illinois equivalent Glenwood strata reach thicknesses of 75 to 150 feet.  The Glenwood Shale is interpreted to have been deposited as the seaway continued to deepen (transgress) across the region.

 

Dolomite and Limestone
the Ordovician Platteville Formation

The Platteville Formation is exposed in the upper bluff slopes within Pikes Peak State Park, where it comprises a succession of carbonate rock strata (limestone and dolomite) about 45 feet thick.  These strata form the lip of Bridal Veil Falls in the park.  The Platteville Formation derives its name from characteristic exposures at Platteville, Wisconsin, about 40 miles southeast of the park.  The lower part of the formation is characterized by ledges of fossiliferous dolomite (the Pecatonica Member), and upper Platteville strata (the McGregor Member) which form an interval of mostly wavy-bedded limestones, many containing a beautiful assemblage of well-preserved fossils.  The member names derive from the Pecatonica River of southwest Wisconsin, and, of course, the nearby city of McGregor, Iowa.  Platteville strata have previously been assigned a Middle Ordovician age based on the standard usage of geologic series in North America, but the recent definitions promoted by the International Subcommission on Ordovician Stratigraphy would now place the Platteville Formation and all overlying strata of the Galena Group within the Upper Ordovician Series.

The Platteville Formation represents the lithified sediments that were deposited within a broad tropical sea which supported a diversity of shelled bottom-dwelling animals.  The influx of quartz sand, which marked earlier St. Peter deposition, waned and ceased altogether as depositon of the Pecatonica Member proceeded and as shorelines advanced deep into the continental interior.  The deposition of carbonate mud along with the calcite shells and skeletons of invertebrate animals now prevailed in the region, and only a minor input of land-derived detrital siliciclastic sediment (largely clays) originated from distant riverine input to the sea.  These carbonate sediments were largely precititated directly from seawater, a chemical precipitation typically mediated by biological processes.  As sea levels fluctuated within the seaway, sediment accumulation varied in response to deepening and shallowing trends.  The close of Pecatonica deposition was marked by widespread development of a so-called “hardground” surface, a surface formed across the seafloor at a time when sediment accumulation ceased altogether.

The overlying succession of wavy-bedded to nodular limestone strata comprise the McGregor Member.  Fossiliferous stringers provide evidence of episodic transport and concentration of shell material during deposition, likely produced by storm-generated currents associated with hurricanes and other tropical storms.  Episodes of sediment starvation are marked by hardground surfaces.  The fossils are wonderfully preserved in these limestone strata.  A number of creatures inhabited the sea bottom, including brachiopods, crinoids, trilobites, ostracodes, bryozoans, solitary corals, snails, and others.  Large nautiloid cephalopods plied the waters in search of prey or scavenge, and very large molds of their chambered shells are found in the Platteville Formation of the area.  The McGregor Member is a stratigraphic term used across parts of Iowa and Minnesota, but these strata have also been included within the Mifflin and “Grand Detour” formations (of the “Platteville Group”) using a classification scheme proposed by the Illinois Geological Survey (Templeton and Willman, 1963).  Mifflin strata comprise the typical wavy-bedded fossiliferous limestone succession, and the horizontally-bedded dolomitic limestone interval at the top of the Platteville Formation has been labeled the “Grand Detour”; this classification is discussed further for the McGregor Quarry field trip stop.

The flat surface that separates the Platteville Formation from the overlying Decorah Formation is in many respects geologically remarkable, and it is characterized by extreme sediment starvation or condensation across a vast area of the central and eastern United States (Kolata et al., 1998; Ludvigson et al., 1996).  The amount of missing geologic time along this surface (a submarine disconformity) generally increases in an offshore direction (southeastward).  A thin interval of limestone and shale in the Pikes Peak area (8 inches thick), which contains a prominent volcanic ash (Deicke bentonite), is all that remains of a unit termed the Carimona Member of the upper Platteville Formation in Minnesota (where it reaches thicknesses to 6 feet).  The Carimona Member, which is included within the basal Decorah Formation (in Iowa and Wisconsin), completely disappears a short distance southward in Clayton County.

 

Ordovician Shale and Limestone
the lower Galena Group, Decorah Formation

The Decorah Formation is a succession of shale and limestone occurring high in bluff slope drainages of Pikes Peak State Park (for example, above Bridal Veil Falls).  The Decorah Formation, which derives its name from Decorah, Iowa (38 miles to the northwest of Pikes Peak), comprises the lower shaley interval of the Galena Group.  The Decorah differs from overlying strata of the Galena Group in containing an appreciable content of clay shale.  These shales are typically greenish-gray in color and contain fossiliferous lenses and thin beds (commonly brachiopod shell hashes or coquinas) of limestone.  The shale content of the Decorah Formation increases to the northwest, and these clays were sourced by erosion across an exposed low landscape that stretched across Minnesota.  Because shales are soft and easily weathered, the shaley portions of the Decorah Formation are typically not well exposed in Pikes Peak State Park.

The Decorah Formation in the park includes a lower shale member (the Spechts Ferry Shale), a middle limestone unit (the Guttenberg Member), and an upper shaley interval (the Ion Member).  The Spechts Ferry Shale, named after a location in northern Dubuque County, contains two widespread volcanic ashes (known as bentonites) visible as soft pale-colored (whitish to yellow-orange) layers 1 to 2 inches thick within the darker green-gray shale interval.  These volcanic ashes have been chemically altered, and geologists label them as “K-bentonites” (“K” for potassium alteration, sometimes lithified by potassium feldspar).  These bentonites represent volcanic ash falls blown over the interior seaway from distant volcanoes (the volcanic arc lay eastward from present-day Virginia), and the ash settled to the sea bottom where it remained largely undisturbed by bottom currents or burrowing organisms.  Some of these ash falls were among the largest known in earth history (giant Plinian volcanic eruptions), with individual bentonite beds recognized across vast areas of eastern and central North America and northern Europe.  The lowest bentonite at Pikes Peak immediately overlies the Platteville surface, and this bentonite bed is known as the Deicke K-bentonite in North America (dated at 454 million years old).  A second bentonite is present about 1 ½ feet higher, this one termed the Millbrig K-bentonite (dated at 453.7 million years old).

Limestones of the Guttenberg Member overlie the Spechts Ferry Shale.  This limestone interval, whose name derives from the town of Guttenberg about 15 miles south of Pikes Peak, resembles the McGregor Member of the Platteville Formation in possessing wavy to nodular bedded fossiliferous limestone strata.  Although not clearly visible in exposure (due to oxidation), the Guttenberg Member contains thin organic-rich brown shales between the wavy limestone beds.  Occasional whole articulated crinoid and trilobite fossils are found in the lower beds, which are interpreted to have been deposited in the deepest of the Guttenberg environments.  Thin stringers of brachiopod shells were probably concentrated by storm activity, and the general upward increase of abraded and broken skeletal grains likely relates to overall shallowing conditions during deposition.  Although the Guttenberg Member is dominated by limestone, the member incorporates progressively more shale in a northward direction.  At Decorah, Iowa, and northward in Minnesota, the Guttenberg Member is not clearly recognized and the entire Decorah Formation becomes shale dominated.  An additional two bentonite horizons occur within the Guttenberg Member.  The Elkport K-bentonite is locally as a thin streak near the base of the member in the Pikes Peak area.  The Dickeyville K-bentonite locallyoccurs in the upper Guttenberg, and it is tentatively recognized at the McGregor Quarry (field trip stop).

Upper strata of the Decorah Formation at Pikes Peak are included within the Ion Member, a named derived from a bridge crossing on the Yellow River a short distance north of Pikes Peak.  The Ion Member includes interbedded green-gray shales and fossiliferous limestones, and it is notably more shaley than the underlying Guttenberg Member.  An interval containing large trepostome bryozoans is widespread in the upper part, including distinctive hemispherical forms (the “Prasopora zone”).  The Ion shaley interval is replaced southward in the Dubuque area by non-shaley dolomite strata included within the Dunleith Formation (Buckhorn and St. James members), and northward into the Decorah area and southern Minnesota Ion equivalents are part of the thicker undifferentiated Decorah Shale.  In the Pikes Peak area, the last major influx of clay sediments occurred during Ion deposition, but Decorah shales occur at higher stratigraphic positions northwestward.  The Ion is interpreted to have been deposited in shallower environments than the underlying Guttenberg, as evidenced by the progradation of shale from shoreward areas and the higher proportion of abraded and broken shell material.

 

Uppermost Dolomite Ledges
the Ordovician Dunleith Formation, Galena Group

The highest Paleozoic bedrock strata found in Pikes Peak State Park belong to the Dunleith Formation, which along with the underlying Decorah Formation comprises the lower half of the Galena Group.  These strata form the highest cliffs and ledges above elevations of 1000 feet in the park, and these resistant dolomite and limestone beds are well displayed below the main overlook structure.  In contrast with underlying Decorah strata, the Dunleith Formation contains almost no shale and the carbonate beds are dominated by recrystallized dolomite and dolomitic limestone (unlike the fossiliferous limestones of the Decorah).  In addition, the Dunleith contains a considerable quantity of nodular chert (“flint”), whereas underlying Decorah and Platteville carbonates completely lack chert.  The Dunleith Formation reaches thicknesses to 80 feet in the park (generally less than 50 feet is represented in the bluff faces).  However, the upper part of the formation has been erosionally removed from the park, and the full thickness of the Dunleith in the area is actually about 110 feet.  The Dunleith Formation is found in cliffs and bluff faces along the Mississippi River Valley extending southward from Pikes Peak to the Dubuque area.

The Dunleith Formation, whose name derives from Dunleith Township, East Dubuque, Illinois, has been subdivided into a series of members based on variations in chert and argillaceous content (Templeton and Willman, 1963; Levorson and Gerk, 1972), and these individual members have remarkable continuity over a vast area (across Iowa, southern Minnesota, northern Illinois, Wisconsin, northern Missouri, eastern Nebraska).  This continuity indicates that widespread and uniform conditions were established on the sea bottom during Dunleith deposition.  However, a general northwestward increase in clay content characterizes the lower portion of this interval, and lower Dunleith strata at Pikes Peak correlate with shales included in the upper Decorah Formation at St. Paul. Minnesota.  At any given locality, the shift from Decorah to Dunleith deposition is marked by a significant decrease in clay content.  This change in clay content reflects the increasing distance from the eroding source area for these clays as shorelines advanced deep into the continental interior (clay sources largely vanished when much of present-day Minnesota and the Canadian Shield were submerged beneath the advancing seaway).

The Dunleith Formation is characterized by recrystallized dolomite and dolomitic limestone strata at Pikes Peak, and argillaceous (clay) content is very low to largely absent through the succession.  However, lower Dunleith strata (Beecher Member) are slightly argillaceous, with some clay streaks and shaley partings noted, but clay is nowhere near as abundant as in underlying Decorah strata.  Like the older Prairie du Chien Group, Dunleith strata were originally deposited as calcium carbonate sediments which were later replaced, partially or wholly, by dolomite.  Interestingly, the Dunleith Formation to the north (as seen at Decorah, Iowa, and in southeastern Minnesota) lacks dolomite (and the succession is entirely comprised of limestone and cherty limestone), whereas, southward into Dubuque County, the Dunleith is entirely dolomitized (completely lacks limestone).  Pikes Peak lies at an intermediate position between these extremes, where the Dunleith succession is characterized by an interfingering of dolomite and dolomitic limestone rock types (dolomitic limestones are limestones that are only partially dolomitized).

The Dunleith carbonate strata at Pikes Peak have been recrystallized during dolomitization, and original sedimentary features and fossils are thereby more difficult to distinguish than in the beautiful fossiliferous limestones of the underlying Platteville and Decorah formations.  The secondary development of vugs and pores (open spaces and holes) in the dolomite strata has further obscured the fossils and fabrics.  Nevertheless, brachiopods, crinoid material, snails, and solitary corals are seen in the park, indicating that the bottom environments in the seaway provided suitable habitat for a diversity of organisms.  Relatively large but enigmatic fossils known as receptaculitids (belonging to the genus Fisherites) occur within these strata in the park, and the stratigraphic units which contain these fossils are correlatable across a broad region (three receptaculitid “zones” are recognized in the Galena Group).  Receptacultids are sometimes called “sunflower corals” because the radiating geometric pattern formed by its skeleton resembles the seed-heads of sunflowers.  However, the biologic relationships of receptaculitids are not known with certainty.  Various proposals have allied them with corals, sponges, or algae.  Receptaculitids share many features in comon with calcareous-plated green algae, but certain differences indicate that receptaculitids are a unique extinct group of organisms (possibly allied with green algae).

The Dunleith Formation in the Mississippi Valley area is known for its profusion of widespread “hardground” surfaces (see previous discussion for Platteville), and the accumulation of carbonate sediment on the seafloor during Dunleith deposition was probably very slow (and entirely absent at times when hardgrounds formed).  Some hardgrounds are seen in lower Dunleith strata at Pikes Peak, but dolomitization and weathering have made the recognition of other widespread hardgrounds difficult to recognize.  As seen in better-preserved limestone successions of Dunleith strata to the north, episodic tropical storm activity fragmented and transported fossils grains on the seafloor.  Grains of calcareous green algae occur in some but not all of the beds, indicating that Dunleith deposition occurred, at least in part, within the zone of light penetration (photic zone).  Burrowing organisms left complex burrow networks within these strata, sometimes accentuated by preferential dolomitization.

Although higher Upper Ordovician strata of the Galena Group (Wise Lake and Dubuque formations) and Maquoketa Shale can be seen in nearby areas of Clayton County, these stratigraphic units have been removed from Pikes Peak State Park by later erosion.  Northeastern Iowa was subjected to many long-lived episodes of deep erosion marked by the erosional incision and truncation of various bedrock units.  Recurring erosional episodes separated periods of shallow marine deposition in the region during much of the Paleozoic Era, but a dominantly erosional landscape was developed across the region later in the Paleozoic (beginning about 300 million years ago).  Except for some minor Cretaceous-aged sediments in the area (e.g. at Waukon, Allamakee County), there is no evidence of any deposition in northeast Iowa for a period of time spanning most of that 300 million year interval.  Pikes Peak was certainly subjected to the erosional downcutting of bedrock strata seen across the region during that prolonged period of erosion.  Nevertheless, the steep erosion of bluff slopes and ravine drainages which has produced the dramatic and picturesque landscape of Pikes Peak State Park is primarily a reflection of erosional processes relating to the more geologically-recent incision of the Mississippi River Valley and its tributaries during the Quaternary Period, which includes the Pleistocene (“Ice Age”) and Holocene (Recent) epochs.

 

Valleys, Alluvium, Colluvium, and Loess
Quaternary Erosion and Deposition at Pikes Peak

The modern landscape of Pikes Peak State Park is dominated by steep bluff faces that border the Mississippi River Valley.  This landscape has been dramatically sculpted by erosion of the bedrock strata, and this erosion continues with each passing year as cycles of rain and snowmelt, freeze and thaw, slowly but surely degrade the slopes and transport material down the valley walls.  The origin of the deep incision of the Mississippi River Valley and adjacent tributary valleys (including the Wisconsin River) is not well constrained geologically.  As discussed earlier, the base of the Mississippi bedrock channel lies some 300 feet below the modern river level.  This deeper level of incision was likely related to episodes of continental glaciation, when vast ice sheets spread across large areas of the northern hemisphere several times during the last 2 ½ million years.  As the ice sheets grew, global sea levels progressively dropped, reaching levels some 400 feet lower than today at the height of the northern glaciations.  This lowering of sea level changed the gradient of the Mississippi River, leading to deeper erosion of its valley.  In addition, as the ice sheets began to melt, huge volumes of meltwater and sediment moved down the Mississippi Valley and other drainageways (like the Wisconsin River), initially scouring the valley.  As sea levels progressively rose once again, the sediment-laden meltwaters began to deposit large volumes of sediment within the valley.

The deepest portion of the bedrock incision in the Mississippi Valley is now buried beneath about 300 feet of alluvial sediments.  These sediments accumulated during waning floods along the river, aggrading the river channels and floodplains with deposits of sand, gravel, silt, and mud.  A complex history of sediment accumulation and erosion was likely produced in the valley by the waxing and waning of glaciers in the region, producing major variations in the river’s flow and the intensity of its floods.  Only the most recent portion of that history is known with any certainty, which largely encompasses the past 12,000 to 14,000 years.  Meltwater surges resulting from the degradation of the most recent glaciers in the region (late Wisconsinan glacial stage) are documented in the valley.  Climatic variations during the Holocene (the past 12,000 years) also influenced deposition and erosion within the river valley, producing variations in the sediment load, flow, and flood intensity of the river.  The most recent sedimentation in the valley occurs on the active floodplains, and additional siltation is occurring in the pools above the Lock and Dams.

Some sediment has locally accumulated along the slopes and tributary drainages within Pikes Peak State Park, and much of this material has been derived by the erosional breakdown of bedrock materials.  As the Cambrian and Ordovician rocks weather into fragments of varying size, these fragments roll, slide, creep, or fall down the hillslopes, aided in places by running water.  This material is termed “colluvium.”  Bedrock strata of the Prairie du Chien, Platteville, and Galena groups commonly produce coarse colluvium comprised of broken and weathered blocks of limestone and dolomite of varying size.  Other bedrock intervals of sandstone and shale typically weather in smaller particles, which may become mixed with coarser materials along the slopes.  Coarse blocky colluvium can be seen along many slopes within the park, and some of this material is being actively transported by modern geologic processes.  In other places, the colluvium has been partially stabilized beneath a vegetated cover.  Ultimately, however, all colluvium will proceed downslope to the Mississippi River as the inevitable forces of erosion continue to sculp the landscape..  In addition to the weathered bedrock units, the colluvium commonly incorporates weathered material derived from upland loess deposits as well as soil units that were developed on the loess and bedrock.

The upland bluff tops in Pikes Peak and elsewhere in the Upper Mississippi Valley have a cap of silty material known as “loess.”  The widespread blanket of loess, which covers vasts areas of east-central United States and Mississippi Valley, was deposited as wind-blown silts accumulated on the landscape during the later phases of the Wisconsinan glaciation.  The Midwestern United States was a very different place at that time.  Glaciers expanded southward to DesMoines, Iowa, at the maximum glacial advance, and much of eastern Iowa was a cold and stark landscape under periglacial conditions and permafrost.  Fierce cold winds picked up silts from the landscape, especially the glacial outwash plains and river valleys, transporting and depositing silt across the region.  These wind-blown loess deposits comprise the base materials for much of Iowa’s rich agricultural soils.

Northeast Iowa was not glaciated during either of the last two major glaciations of the Late Pleistocene (“Ice Age”), the Wisconsinan and Illinoian, but there is clear evidence of older glaciations (“pre-Illinoian”) in the region.  Although Pikes Peak lies near the western margin of the so-called “Driftless Area” (“Drift” is an older term referring to glacial deposits), eroded remnants of glacial till are known in places within the Driftless Area of northeast Iowa.  We have been unable to locate any evidence of glacial till deposits within Pikes Peak State Park, although some remnants may be locally present.  Christiansen et al. (1980) noted the possible occurrence of glacial till remnants in the park.

 

References

Christiansen, P., Hadow, H., and Hinman, E., 1980, Natural resources inventory of Pikes Peak/ Point Ann State Park, Clayton County, Iowa:  report submitted to Iowa Conservation Commission.

Kolata, D.R., Huff, W.D., and Bergström, S.M., 1998, Nature and regional significance of unconformities associated with the Middle Ordovician Hagan K-bentonite complex in the North American Midcontinent:  Geological Society of America Bulletin, v. 110, p. 723-739.

Levorson, C.O., and Gerk, A.J., 1972, A preliminary stratigraphic study of the Galena Group in Winneshiek County, Iowa:  Iowa Academy of Science Proceedings, v. 79, p. 111-122.

Ludvigson, G.A., Jacobson, S.R., Witzke, B.J., and González, L.A., 1996, Carbonate component chemostratigraphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley, in Witzke, B.J., Ludvigson, G.A., and Day, J., eds., Paleozoic Sequence Stratigraphy: Views from the North American Craton:  Geological Society of America, Special Paper 306, p. 67-86.

Templeton, J.S., and Willman, H.B., 1963, Champlainian Series (Middle Ordovician) in Illinois:  Illinois State Geological Survey Bulletin 89, 260 p.

 

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from 

Anderson, R.R. (ed.), 2000, The Natural History of Pikes Peak State Park, Clayton County, Iowa: Geological Society of Iowa Guidebook 70, p. 3-11.