The Cambrian Jordan Sandstone at Point Ann, Pikes Peak State Park, and in McGregor

Robert M. McKay
Iowa Department of Natural Resources
Geological Survey Bureau
Iowa City Iowa 52242-1319

 

A Bit of Stratigraphic History

During the early phases of Upper Mississippi Valley geologic investigations the strata currently differentiated as the Jordan Sandstone were lumped within the Potsdam Sandstone, a unit defined in upper New York State, and traced into Wisconsin, Minnesota and Iowa by Owen (1852) and Hall (1858).  As originally used the Potsdam encompassed all strata between the crystalline basement and the first substantial dolomites of the region, today’s Oneota Formation.  The Jordan remained undifferentiated in what N.H. Winchell (1873) called the Saint Croix sandstone; a name derived from extensive sandstone outcrops along the St. Croix River on the border between Minnesota and Wisconsin.  The Saint Croix, as used by Winchell, was simply a Midwest term for the same strata as that previously referred to the Potsdam by earlier geologists.  The following year Winchell (1874) recognized Saint Croix strata along the Minnesota River, and introduced the name Jordan Sandstone for a distinctive coarse-grained sandstone exposed near the village of Jordan, Minnesota.  At about the same time, Irving (1875), working in the Cambrian of Wisconsin, applied the name Madison Sandstone to equivalent sandstone exposures in the city of Madison.

The term, Saint Croix sandstone was officially introduced into Iowa literature by Keyes (1893) in the newly established Geological Survey’s first Annual Report.  Norton (1895), in his investigation of deep wells in northeastern Iowa, continued use of the term Saint Croix, and established a dual division based on the presence of a upper sandstone member (his upper Saint Croix), and a lower dolomite and shale member (his lower Saint Croix).  Norton’s upper Saint Croix sandstone was equivalent to today’s Jordan Sandstone, and his lower member referred to today’s St. Lawrence and Franconia or Lone Rock formations.

Calvin (1895), in his report on the geology of Allamakee County, maintained use of term Saint Croix sandstone and recognized that the section at Lansing, Iowa contained strata equivalent to the Jordan of Winchell and the Madison of Irving, but did not employ those terms as formalized stratigraphic names.  That distinction was left to Norton (1897) who specifically included both the Jordan and the St. Lawrence as formations on his deep well geological section of Iowa.  In 1906, in his report on the geology of Clayton County, Leonard (1906) maintained Norton’s use of the terms, as did other latter annual reports.

From 1906 to the late 1930’s essentially no published geologic investigations of the Iowa Cambrian outcrop belt were conducted.  The lack of work in Iowa, coupled with considerable new work in Minnesota and Wisconsin, inspired Walter Schuldt, a University of Iowa graduate student, to embark, in 1938, on a comprehensive study of the Iowa Cambrian outcrop with particular emphasis placed on the best exposed unit, the Jordan Sandstone.  In a summary of his thesis work Schuldt (1943) adopted the Wisconsin classification of Twenhofel et al. (1935) whereby the Jordan was included within a four-member Trempealeau Formation (Table 1).  Schuldt restricted the term Madison to a fine to medium-grained sandstone facies that was distinguished by the common presence of thin bedding, intraclast conglomerates, variable dolomite cement, and occasional green shaly horizons, and he opined that the Cambrian-Ordovician boundary lay at the Madison-Oneota boundary.

Studies of this interval continued after World War II, but little attention was devoted to the outcrops in Iowa; most new work was done in the larger outcrop belts of Wisconsin and Minnesota.  Raasch (1951) recommended dropping the term Madison because of potential confusion with the Mississippian limestone unit of the same name in the western U.S.  In place of Madison he substituted the term Sunset Point, a bluff locality in the City of Madison, Wisconsin, and promoted the Sunset Point to formational rank.  One year later Raasch (1952) extended use of the term Sunset Point to the Mississippi Valley of western Wisconsin where he recognized numerous Sunset Point Formation sections in the Stoddard area.  Nelson (1956) retained the Sunset Point Formation of Raasch and promoted the Jordan back to formational rank as it had been in publications prior to Twenhofel et al. (1935).

 Several geologists continued work on this interval in the sixties and seventies.  Ostrom (1964, 1965, and 1967) retained Raasch’s Sunset Point but reclassified it as the upper member of the Jordan.  Davis (1970), in his study of the overlying Prairie du Chien Group, maintained that the Sunset Point was extremely difficult to identify and could not be consistently recognized in the field or subsurface.  He rejected the terms Sunset Point and Madison and recommended that all dolomitic sandstone and sandy dolomite between the friable Jordan sandstone and the pure dolomites of the Oneota be included in a new Stockton Hill Member of the Oneota Formation, named after a locality west of Winona, Minnesota.  His definition emphazised the heterogenous and transitional lithic nature of the Stockton Hill and he maintained that its lower contact with the Jordan was abrupt and readily recognizable; he applied this terminology to one section in northwest Allamakee County along state highway 76.  Odom and Ostrom (1978) presented conclusions from several investigations of the Cambrian-Ordovician sequence in southwest Wisconsin.  Their studies extended into northeast Iowa where they introduced two new member names into the Jordan, the Waukon, and the Coon Valley.  After introduction, these names were adopted by the Geological Survey in Iowa, deferring to Odom and Ostrom’s detailed studies done in the adjacent and much larger outcrop belt of Wisconsin.  The Waukon Member was defined as a fine-grained lithofacies similar in lithic character to the lower Jordan (Norwalk Member), but recurring as a lenticular deposit within the coarse-grained Van Oser Member of the upper Jordan in northern Allamakee County.  The Coon Valley name was proposed to replace Davis’ Stockton Hill Member due to prior usage of the term Stockton Hill as an informal member name of the St. Lawrence Formation.  They also reassigned the Coon Valley strata to the Jordan because they felt the intervals’ high sand content implied greater lithic affinity to the Jordan than the Oneota.

 

Table 1.  Stratigraphy of the Jordan and adjacent
                units in Northeast Iowa according to
                 (Schuldt, 1943)

 

In more recent publications Smith et al. (1993), Runkel (1994), and Byers and Dott (1995) not only simplified stratigraphic nomenclature, but specifically addressed the sedimentology and depositional setting of the Jordan and adjacent units.  These investigations recognize three lithofacies in the Jordan: a lower fine-grained hummocky cross-stratified sandstone facies often called the Norwalk Member, a middle fine- to medium-grained trough cross-stratified sandstone facies called the Van Oser Member; and an upper coarse-grained large-scale cross-stratified facies also assigned to the Van Oser Member.  Runkel (1994) recognized a fourth, uppermost facies at one locality in Minnesota that consisted of very fine to coarse-grained sandstone thinly interbedded with siltstone, and shale.  Runkel recommended retaining the term Coon Valley but preferred to include the strata as the basal member of the Oneota, much as Davis had done with the Stockton Hill.  Byers and Dott (1995), following the conclusions of Smith et al. (1993), favored retaining the name Stockton Hill as the basal member of the Oneota.

Discussion of the conformable versus unconformable nature of the Jordan-Oneota contact, the exact placement of that contact, and the placement of the Cambrian-Ordovician boundary has been considered by numerous authors during the 20th century.  Ulrich (1924) and Twenhofel et al. (1935) considered the contact unconformable and coincident with the Cambrian-Ordovician boundary.  Raasch (1952) and Raasch and Unfer’s (1964) work, in the Stoddard area, located the system boundary at an unconformable Sunset Point-Oneota contact, but also concluded that the Sunset Point lay unconformably on highly cross-stratified Van Oser facies of the Jordan.  Other authors, including Schuldt (1943), Ostrom (1964) and Odom and Ostrom (1978) maintained that the Cambrian-Ordovician boundary exists within a conformable upper Jordan through lower Oneota transition zone.  In more recent works, Smith et al. (1993), Runkel (1994) and Byers and Dott (1995) concluded that lithostratigraphic and sedimentologic evidence supported the long-contested notion of a Cambrian-Ordovician unconformity between the sandstone dominated Jordan and the carbonate dominated Oneota.  It should be noted however that those authors do not universally agree on contact or unconformity placement.  Biostratigraphic studies across these unit transitions have been limited due to their sparsely fossiliferous nature, but recent investigations of conodonts by Miller and Runkel (1998), Runkel and others (1999), and Runkel (2000) support the contention that a substantial hiatus exists between the Jordan and the Oneota formations.  These ongoing studies suggest that four conodont zones are missing between the top of the Jordan and the base of the Oneota (Coon Valley Member) at several outcrops in the southwest Wisconsin and southeast Minnesota.  Missing conodont zones have also been recorded at the Jordan-Oneota contact from core samples in the subsurface of southern Minnesota and central and western Iowa.

In the absence of definitive conodont-based biostratigraphic data, the precise position of the system unconformity at any individual outcrop is still subject to debate.  Runkel (1994, 2000) favors locating the unconformity at a lithofacies change coincident with a poorly sorted, pebbly sandstone deposit.  He interprets the pebbly sandstone as a sedimentary lag deposit, or pebble concentration, that formed at the top of the Jordan regressive sequence by widespread erosion across a subaerially exposed, prograding sandy shoreline.  Modification of the pebble lag occurred by wave and tidal current reworking during the subsequent Early Ordovician transgression.  Byers and Dott (1995), in agreement with the recommendations of Smith et al.’s (1993) study of the Prairie du Chien Group, prefer to locate the Jordan’s upper boundary at the point where well-sorted siliciclastic sands are sharply overlain by carbonates with variable sand content.  They maintain that this sharp upper contact is coincident with a sequence scale boundary unconformity that separates Late Cambrian from Early Ordovician age strata.  At some outcrops in Minnesota where it appears that both conditions exist - the pebble lag and the sharp dolomite contact – placement of the boundary might be accomplished with little disagreement, but conodont data, although very difficult to obtain, probably remains the most definitive measure.

 

The Point Ann Sections

The last stops of the day will focus on looking at the upper half of the Jordan Formation in the downtown area of McGregor.  The outcrops in town are well exposed and are unique in that they are the southern most exposures of the Jordan in Iowa before the formation dips into the subsurface.  For the time being the Geological Survey is including the carbonate dominated Coon Valley transitional strata in the Oneota Formation.  In doing so the thickness of the Jordan in the McGregor area becomes approximately 100 feet, the upper 40 feet of which is exposed in town.  Several wells in town, in particular city well #6 (W5311) at the north end of Main Street, indicate that there is about 50 to 60 feet of friable sandstone below the outcrops before transitioning into dolomitic siltstones and glauconitic dolomites of the underlying St. Lawrence.

 

Figure 2.  Grain size analysis of Jordan Sandstone from the Point Ann North outcrop.  From Schuldt (1943).

 

The first stop will be at the north end of Point Ann, a promontory above the grain terminal.  There are actually two sections along this bluff, Point Ann South and Point Ann North (Fig. 1).  The north  section  is  the  same  location  as  exposure number 144 from Walter Schuldt’s thesis, and his graphical display of grain size analysis from this section is illustrated in figure 2.  The more recent measured sections from these exposures are composited in figure 3.  We will not visit the section at the south end of Point Ann but it is that section that provides the better accessibility to the uppermost Jordan and overlying Coon Valley.  Schuldt’s grain size and insoluble residue analysis was used to provide criteria for his separation of the Madison from the Jordan.  At this locality he placed the contact where a medium and coarse-sand dominated unit is overlain by units having 28 and 53 percent insoluble residue (Fig. 2).

The first 5 meters of section at Point Ann North appears as a massive sandstone unit.  Upon closer examination one can see that the sandstone is highly cross-stratified with medium to large-scale highly truncated trough cross-strata.  This lithofacies is widespread in the Jordan of Minnesota and Wisconsin and typifies the middle portion of the Jordan throughout the area.  Both Runkel (1994) and Byers and Dott (1995) interpret this facies as having developed during a relative shallowing of the Jordan seaway to a middle shoreface environment where storm-enhanced current energy regularly impinged the sandy ocean seafloor and molded the bottom into vast dune fields.  The dunes dominant migration direction, as indicated by the azimuth of the trough axes, varied from southeast to southwest.  The 5 to 8.5 meter portion of the exposure is characterized by coarser sand with abundant siltstone to very fine sandstone intraclasts.  This coarser-grained lithofacies is in the same stratigraphic position as the large-scale cross-stratified facies of both Runkel (1994) and Byers and Dott (1995), but meter-scale medium to high-angle cross-beds are not apparent at this locality.  Instead the strata are dominated by horizontal to large-scale low-angle stratification with common to abundant intraclasts intercalated with sets of smaller scale cross-strata.  Although not readily apparent, this style of stratification may well be part of larger-scale composite bed forms that are only clearly discernable across larger, better-exposed outcrops.  Alternatively, the strata are what they look like, horizontal to low-angle stratified sets of intraclast-rich coarse sandstone.  In either case they almost totally lack slack-water bioturbation features and represent deposition under a frequent and energetic current regime.  In other outcrops in town broad low-angle decameter wide truncation surfaces are present at this horizon.  All these features are consistent with the interpretation that these strata were deposited in an energetic upper shoreface environment similar to that envisioned by Runkel (1994) and Byers and Dott (1995) for the upper part of the Jordan in the areas of their studies.

The remainder of this exposure (units 9 to 16) is accessible along the steep and sandy talus slope.  It consists mainly of interbedded flasered to lenticular cross-stratified and biotubated sandstone sets that are suggestive of deposition under a tidal-current regime and perhaps represent deposits of a tidal flat.  Runkel (1994) mentioned that at one outcrop in Minnesota there appeared a lithofacies with similar features suggestive of tidal flat deposition.  Unit 12 is particularly interesting in that it consists of cross-stratified lenses, up to 2 meters wide, that are draped by partially Skolithos burrowed laminated finer sands and silts (Fig. 4).  These lenses could represent starved large ripples deposited on a tidal flat during spring tides with the finer-grained drapes having been deposited during the weaker neap tides.

 

Figure 4.  Composited graphic sections of Jordan Sandstone exposures from Point Ann North and South.

 

Unit 14 represents a change in sedimentation style from the strata below.  It corresponds to the high insoluble residue units in Schuldt’s section at 11 meters, the base of what he called the Madison Formation.  The lower part of the unit is dolomitic sandstone while the upper part consists of 3 to 5 thin beds of bioturbated, silty to fine-sandy dolomite.  This unit forms a local marker bed in this portion of the county and can be traced as far north as Yellow River State Forest.   Dr. James F. Miller, from Southwest Missouri State University, has processed samples of this unit from Point Ann South and from an outcrop adjacent to our next stop and has recovered the Cambrian conodonts Eoconodontus notchpeakensis and Proconodontus muelleri from both sets of samples.  This is interesting in that although the unit is a dolomite it contains Cambrian, not Early Ordovican conodonts, and thus represents the only dolomite from the upper Jordan formation of the outcrop belt that is demonstrably Cambrian in age.  The large Cambrian-Ordovician hiatus demonstrated by Runkel et al. (1999) must lie above this dolomite, and preliminary work suggests it may reside at the unit 6-unit 7 contact (Fig. 5) in the Point Ann South section, but further work remains before this conjecture is conclusive.

We will not visit the section at Point Ann South but as figure 3 illustrates the section continues upward through well-exposed Coon Valley strata into the pure dolomites of the Hager City Member.  The Coon Valley here is similar to many other Coon Valley sections and is dominated by sandy peloidal and oolitic grainstones, several types of stromatolite boundstones, and dolomitic sandstone.  Cherts occur in the upper half and two locally traceable chert horizons occur near the top of the member.  The lower contact of the Coon Valley is placed at the base of unit 7 where highly intraclastic sandy dolomites sharply overlie variably stratified sandstone (Fig. 5).  A quartz and metasediment pebble lag, similar to that described by Runkel (1994), has not been identified at this suggested contact, and only one indeterminate conodont element with Ordovician affinities has to date been recovered from unit 15 by Jim Miller, so the contact may yet be adjusted. 

 

The “A” Street Section

 

From the grain terminal we’ll walk over to “A” Street and view an almost identical Jordan section to that at Point Ann North but one that is more readily accessible (Figs. 6 and 7).  This section is now maintained as a small city park but is actually owned by the state.  The lower portion of the section is composed of medium to large scale trough cross strata with minor to abundant intraclasts.  The sandstone is friable but stands well vertically because it is very weakly cemented by minute amounts of silt size dolomite rhombs.  These two characteristics made the unit ideal for excavation with simple hand tools by the early settlers of McGregor, and numerous “caverns” and storage rooms were excavated during the 19’th century (McKay, 1997).  Towards the western end of the exposure is the remains of an old brick lined cistern that was dug into the friable sandstone.

The excavations don’t trend higher in the section than units 4 and 5, the distinctive recessive thin beds at the base of the main ledge.  Those units display some rather rare clay draped form symmetrical ripples and overturned to contorted small cross strata, and contain a rather high concentration of small quartz pebbles near the base.  The concentration of small pebbles at the base of unit 4 coupled with its’ sharp lower contact initially led to the hypothesis that this contact might represent the Cambrian-Ordovician boundary; that proved not to be the case.  Both unit 4 and 5 have been unsuccessfully processed for conodonts, however samples of the local dolomite marker bed of unit 8 have yielded the Cambrian conodonts Eoconodontus notchpeakensis and Proconodontus muelleri (Dr. Jim Miller email correspondance).

The occurrence of both symmetrical ripple lenses, clay flasers, and unidirectional cross sets in units 4 and 5 (Fig. 8) attest to deposition under waxing and waning orbital and asymmetrical current regimes, and the cross laminae and clay laminae deformation suggests rapid deposition followed by some type of loading.  Units 6 and 7 contain stratification very similar to units 10 through 13 at Point Ann North; multiple sets of small to medium scale cross strata or cross stratified lenses separated by variably Skolithos burrowed laminated finer sands.  Again, these structures are very suggestive of deposition under the ebb and flood of a tidal current regime, and may represent the spring and neap tide current deposits of a tidal flat.  So one interpretation of the depositional setting of this section would be that it represents the deposits of an energetic upper shoreface environment overlain by a tidal flat that prograded across the shoreface.  The overlying conodont bearing dolomite marker bed of unit 8 is silty to fine sandy and moderately bioturbated and must have been formed under distinctly different conditions.  These features suggest deposition under a substantially less energetic setting experiencing a significantly slower rate of sedimentation that allowed for disturbance of the substrate by burrowing infauna.  These conditions could have been created within a lagoon behind the tidal flat or more likely may represent deposits of quieter water conditions following a transgression of the seaway and deepening of the water column.

 

 

References

 

Byers, C.W., and Dott, Jr., R.H. (1995) Sedimentology and depositional sequences of the Jordan Formation (Upper Cambrian), northern Mississippi Valley, Journal of Sedimentary Research, v. B65, p. 289-305.

Calvin, S. (1895) Geology of Allamakee County, Iowa Geological Survey 3rd Annual Report 1894, v. 4, p. 35-120.

Davis, R.A. (1970) Lithostratigraphy of the Prairie du Chien Group in the Upper Mississippi Valley, in : Ostrom, M.E., Davis, R.A. and Cline, L.M., editors, Wisconsin Geological and Natural History Survey Information Circular no. 11, Cambrian-Ordovician geology of western Wisconsin, Geological Society of America Annual Meeting, Fieldtrip Guidebook, Milwaukee, Wisconsin, p. 35-44.

Hall, J. (1858) General Remarks on the Geology of the Northwest, and the Relations of the Formations to those of the East in: Report on the Geological Survey of the State of Iowa 1855-1857, V. 1., Pt. 1, p. 35-146.

Irving, R.D. (1875) Note on some new points in the elementary stratification of the primordial and Canadian rocks of south central Wisconsin, American Journal of Science, 3^rd series, v. 9, p. 441-442.

Keyes, C.R. (1893) Geological Formations of Iowa, Iowa Geological Survey 1st Annual Report 1892, v. 1, p. 11-144.

Leonard, A.G. (1906) Geology of Clayton County, Iowa Geological Survey Annual Report 1905, v. 16, p. 213-218.

McKay, R.M. (1997) McGregor’s 19^th century refrigerators, Iowa Department of Natural Resources Geological Survey Bureau, Iowa Geology, no. 22, p. 20-21.

Miller, J.F. and Runkel, A.C. (1998) Biostratigraphy of conodonts from Jordan Sandstone and lowermost Oneota Dolomite near Homer, Minnesota, in: Boerboom, T.J. and Lusardi, B.A., editors, Institute on Lake Superior Geology, 44^th Annual Meeting, Minneapolis, Minnesota, 1998, Proceedings 44, part 2, p. 119-120.

Nelson, C.A., (1956) Upper Croixan Stratigraphy, Upper Mississippi Valley, Geological Society of America Bulletin, v. 67, p. 165-184.

Norton, W.H. (1895) Thickness of the Paleozoic Strata of Northeastern Iowa, Iowa Geological Survey 2nd Annual Report 1893, v. 3, p. 167-210.

Norton, W.H. (1897) Artesian wells of Iowa, Iowa Geological Survey Annual Report, v. 6, p. 113-428.

Odom, I.E. and Ostrom, M.E. (1978) Lithostratigraphy, petrology, and sedimentology of the Jordan Formation near Madison, Wisconsin, in: Odom, I. E. et al., editors, Lithostratigraphy, petrology, and sedimentology of Late Cambrian-Early Ordovician rocks near Madison, Wisconsin, Wisconsin Geological and Natural History Survey Field Trip Guidebook no. 3, p. 23-45.

Ostrom, M.E. (1964) Pre-Cincinnatian Paleozoic cyclic sediments in the Upper Mississippi Valley, Kansas Geological Survey Bulletin, no. 169, v. 2, p. 381-398.

Ostrom, M.E. (1965) Cambro-Ordovician stratigraphy of southwest Wisconsin, Guidebook to the 29^th Annual Tri-State Field Conference, Wisconsin Geological and Natural History Survey Information Circular, no. 6, 57 p.

Ostrom, M.E. (1967) Paleozoic stratigraphic nomenclature for Wisconsin, Wisconsin Geological and Natural History Survey Information Circular no. 8.

Owen, D.D. (1852) Lower sandstone of the Upper Mississippi, in: Report of a geological survey of Wisconsin, Iowa and Minnesota, Philadelphia p. 48-58.

Raasch, G.O. (1951) Revision of Croixan Dikelocephalids, Illinois Academy of Science Transactions, v. 44, p. 137-151.

Raasch, G.O. (1952) Oneota Formation, Stoddard Quadrangle, Wisconsin, Illinois Academy of Science Transactions, v. 45, p. 85-95.

Raasch, G.O. and Unfer, Jr., L. (1964) Transgressive-regressive cycle in Croixan sediments (Upper Cambrian), Wisconsin, Kansas Geological Survey Bulletin, no. 169, v. 2, p. 427-440.

Runkel, A.C. (1994) Deposition of the uppermost Cambrian (Croixan) Jordan Sandstone, and the nature of the Cambrian-Ordovician boundary in the Upper Mississippi Valley, Geological Society of America Bulletin, v. 106, p. 492-506.

Runkel, A.C. (2000) Sedimentology of the Upper Cambrian Jordan Sandstone: a classic cratonic sheet sandstone deposited during regression in a “typical” marine setting, in: Havholm, K.G., Mahoney, J.B. and Runkel, A.C., Stratigraphy and sedimentology of Cambrian strata in west central Wisconsin and southeastern Minnesota, Great Lakes Section of the Society for Sedimentary Geology Fieldtrip Guidebook for the 30^th Annual Field Conference, Eau Claire, Wisconsin, p. 43-46.

Runkel, A.C., Miller, J.F., McKay, R.M., Shaw, T.H., and Basset, D.J. (1999) Cambrian-Ordovician boundary strata in the central midcontinent of North America, Acta Universitatis Carolinae-Geologica, v. 43, p. 17-20.

Schuldt, W.C. (1943) Cambrian strata of Northeast Iowa, Iowa Geological Survey Annual Report, v. 38, p. 379-422.

Smith, G.L., Byers, C.W., and Dott, Jr., R.H. (1993) Sequence stratigraphy of the Lower Ordovician Prairie du Chien Group on the Wisconsin Arch and in the Michigan Basin, American Association of Petroleum Geologists Bulletin, v. 77, p. 49-67.

Twenhofel, W.H, Raasch, G.O., and Thwaites, F.T. (1935) Cambrian strata of Wisconsin, Geological Society of America Bulletin, v. 46, p. 1687-1743.

Ulrich, E.O. (1924) Notes on new names in the table of formations and on physical evidence of breaks between Paleozoic Systems in Wisconsin, Wisconsin Academy of Science Art and Letters Transactions, v. 21, p.71-107.

Winchell, N.H. (1873) The Potsdam sandstone, Minnesota Geological and Natural History Survey, 1st Annual Report 1872, p. 68-80.

Winchell, N.H. (1874) The Jordan sandstone, Minnesota Geological and Natural History Survey, 2’nd Annual Report 1873, p. 147-155.

     __________________________________________________

from

Anderson, R.R. (ed.), 2000, The Natural History of Pikes Peak State Park, Clayton County, Iowa: Geological Society of Iowa Guidebook 70, p. 13-23.