Wisconsin Geological and Natural History Survey

Cramer, B.D., Brett, C.E., Melchin, M.J., Männik, P., Kleffner, M.A., McLaughlin, P.I., Loydell, D.K., Munnecke, A., Jeppsson, L., Corradini, C., Brunton, F.R. & Saltzman, M.R. 2011: Revised correlation of Silurian Provincial Series of North America with global and regional chronostratigraphic units and δ13Ccarb chemostratigraphy. Lethaia, Vol. 44, pp. 185–202. Recent revisions to the biostratigraphic and chronostratigraphic assignment of strata from the type area of the Niagaran Provincial Series (a regional chronostratigraphic unit) have demonstrated the need to revise the chronostratigraphic correlation of the Silurian System of North America. Recently, the working group to restudy the base of the Wenlock Series has developed an extremely high-resolution global chronostratigraphy for the Telychian and Sheinwoodian stages by integrating graptolite and conodont biostratigraphy with carbonate carbon isotope (δ13Ccarb) chemostratigraphy. This improved global chronostratigraphy has required such significant chronostratigraphic revisions to the North American succession that much of the Silurian System in North America is currently in a state of flux and needs further refinement. This report serves as an update of the progress on recalibrating the global chronostratigraphic correlation of North American Provincial Series and Stage boundaries in their type area. The revised North American classification is correlated with global series and stages as well as regional classifications used in the United Kingdom, the East Baltic, Australia, China, the Barrandian, and Altaj. Twenty-four potential stage slices, based primarily on graptolite and conodont zones and correlated to the global series and stages, are illustrated alongside a new composite δ13Ccarb curve for the Silurian. Conodont, graptolite, isotope, New York, Ontario, series, Silurian, stage.

Large‐scale thrusting by the glacial ice occurred in many parts of the Interior Plains of North Dakota, Alberta, and Saskatchewan in late Wisconsinan time. Thrust features are especially prominent in parts of North Dakota. Many of the topographic features of the glaciated landscape of North Dakota formed, either entirely or in part, by the thrusting mechanism, and many individual ice‐thrust features have been recognized. The ice‐thrusting process was also related to fluting by the glacier, and, as a result, drumlins and other types of fluted features are commonly closely associated with ice‐thrust topography. Thrusting by the glacier was dependent primarily on groundwater conditions beneath the ice; thrusting occurred only where hydrologic conditions were appropriate. Most of the individual ice‐thrust features that have been recognized are located over discrete aquifers, and the sizes and shapes of the thrust features are dependent upon the sizes and shapes of the aquifers.

Field measurements of debris‐bed friction on a smooth rock tablet at the bed of Engabreen, a hard‐bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500 kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed‐normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1. A small exponent ( n < 2) is likely because stresses in ice are small and flow is transient. Numerical calculations of the bed‐normal drag force on a sphere in contact with a flat bed using n = 1 show that this force can reach values several hundred times that on a sphere isolated from the bed, thus drastically increasing frictional resistance. Various estimates of basal friction are obtained from this model. For example, the shear traction at the bed of a glacier sliding at 20 m a −1 with a geothermally induced melt rate of 0.006 m a −1 and an effective pressure of 300 kPa can exceed 100 kPa. Debris‐bed friction can therefore be a major component of sliding resistance, contradicting the common assumption that debris‐bed friction is negligible.

Septic systems that are built in compliance with regulations are generally not expected to be the cause of groundwater borne disease outbreaks, especially in areas with thick vadose zones. However, this case study demonstrates that a disease outbreak can occur in such a setting and outlines the combination of epidemiological, microbiological, and hydrogeological methods used to confirm the source of the outbreak. In early June 2007, 229 patrons and employees of a new restaurant in northeastern Wisconsin were affected by acute gastroenteritis; 6 people were hospitalized. Epidemiological case-control analysis indicated that drinking the restaurant's well water was associated with illness (odds ratio = 3.2, 95% confidence interval = 0.9 to 11.4, P = 0.06). Microbiological analysis (quantitative reverse transcription-polymerase chain reaction) measured 50 genomic copies per liter of norovirus genogroup I in the well water. Nucleotide sequencing determined the genotype as GI.2 and further showed the identical virus was present in patrons' stool specimens and in the septic tank. Tracer tests using dyes injected at two points in the septic system showed that effluent was traveling from the tanks (through a leaking fitting) and infiltration field to the well in 6 and 15 d, respectively. The restaurant septic system and well (85-m deep, in a fractured dolomite aquifer) both conformed to state building codes. The early arrival of dye in the well, which was 188 m from the septic field and located beneath a 35-m thick vadose zone, demonstrates that in highly vulnerable hydrogeological settings, compliance with regulations may not provide adequate protection from fecal pathogens.

Abstract. Specific capacity data obtained from well construction reports can provide useful estimates of hydraulic conductivity (K). A simple computer program has been developed which can correct specific capacity data for partial penetration and well loss and, using an iterative technique, provide rapid estimates of K at hundreds of data points. The program allows easy data handling and is easily linked with existing statistical programs or contour mapping routines. The method was tested at two field sites in Wisconsin, one underlain by a sandy outwash aquifer, the other by fractured dolomite. In both areas, estimates of K from corrected specific capacity data agree reasonably well with data from pumping tests.

Pleistocene permafrost had a major but generally unappreciated effect on the landscape of Wisconsin, second only to glaciation. Evidence for continuous permafrost during the last part of the Wisconsin Glaciation includes ice‐wedge casts seen both in outcrop (generally in gravel pits) and as polygonal networks (on aerial photographs). Other important evidence includes fossil tundra organisms. Other features that are probably the result of permafrost in Wisconsin include talus cones, block streams, solifluction rubble at the base of most hillslopes, fluvial cobble gravel, gullies that are today inactive, lake‐ice collapse trenches, and ice‐walled‐lake plains. Permafrost caused accelerated regional erosion of the landscape; most topographic features formed before the last permafrost melted have been highly modified or even destroyed, whereas those formed after are much better preserved. In addition, the presence of permafrost influenced many glacial processes and land‐forms. Permafrost was present until about 14000 yr BP in the southern part of the state to about 10000 yr BP in the northern part.

Abstract To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m × 1.6 m × 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7–12 days, the glacier regelated downward into the prism to depths of 50–80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore-water pressure (70–100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity.

Abstract Rapid flow of the Des Moines lobe of the Laurentide ice sheet may have been related to its unlithified substrate. New reconstructions of the lobe, based on moraine elevations, sediment subsidence during moraine deposition, and flow-direction indicators, indicate that the lobe may have been ∼3 times thicker than in previous reconstructions. Nevertheless, implied basal shear stresses are <15 kPa, so internal ice deformation was not significant. Instead, the lobe likely moved by a combination of sliding, plowing of particles through the bed surface, and bed shear. Consolidation tests on basal till yield preconsolidation stresses of 125–300 kPa, so effective normal stresses on the bed were small. A model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed, resulting in too small a shear traction on the bed to shear it at depth. Consistent with this prediction, measurements of orientations of clasts in basal till yield a weak fabric, implying pervasive bed shear strain less than ∼2, although some stronger fabrics have been reported by others. We infer, tentatively, that movement was principally at the bed surface by plowing.

Aquitards protect underlying aquifers from contaminants and limit recharge to those aquifers. Understanding the mechanisms and quantity of ground water flow across aquitards to underlying aquifers is essential for ground water planning and assessment. We present results of laboratory testing for shale hydraulic conductivities, a methodology for determining the vertical hydraulic conductivity (K(v)) of aquitards at regional scales and demonstrate the importance of discrete flow pathways across aquitards. A regional shale aquitard in southeastern Wisconsin, the Maquoketa Formation, was studied to define the role that an aquitard plays in a regional ground water flow system. Calibration of a regional ground water flow model for southeastern Wisconsin using both predevelopment steady-state and transient targets suggested that the regional K(v) of the Maquoketa Formation is 1.8 x 10(-11) m/s. The core-scale measurements of the K(v) of the Maquoketa Formation range from 1.8 x 10(-14) to 4.1 x 10(-12) m/s. Flow through some additional pathways in the shale, potential fractures or open boreholes, can explain the apparent increase of the regional-scale K(v). Based on well logs, erosional windows or high-conductivity zones seem unlikely pathways. Fractures cutting through the entire thickness of the shale spaced 5 km apart with an aperture of 50 microns could provide enough flow across the aquitard to match that provided by an equivalent bulk K(v) of 1.8 x 10(-11) m/s. In a similar fashion, only 50 wells of 0.1 m radius open to aquifers above and below the shale and evenly spaced 10 km apart across southeastern Wisconsin can match the model K(v).

Subsurface heterogeneity in hydraulic properties and processes is a fundamental challenge in hydrogeology. We have developed an improved method of borehole dilution testing for hydrostratigraphic characterization, in which distributed temperature sensing (DTS) is used to monitor advective heat movement. DTS offers many advantages over conventional technologies including response times in the order of seconds rather than minutes, the ability to profile temperature synoptically in a well without disturbing the fluid column, sensitivity to a wider range of flow rates than conventional spinner and heat pulse flow meters, and the ease of interpretation. Open-well thermal dilution tests in two multiaquifer wells near Madison, Wisconsin, provided detailed information on the borehole flow regimes, including flow rates and the locations of inflows from both fractures and porous media. The results led to an enhanced understanding of flow in a hydrostratigraphic unit previously conceptualized as homogenous and isotropic.

Probably the most important mechanism of glacial erosion is quarrying: the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments. Although evidence indicates that erosion rates depend on sliding speed, rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice‐bed separation. To study quarrying in real time, a granite step, 12 cm high with a crack in its stoss surface, was installed at the bed of Engabreen, Norway. Acoustic emission sensors monitored crack growth events in the step as ice slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water was pumped to the lee of the step several times over 8 days. Pumping initially caused opening of a leeward cavity, which then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically. With repeated pump tests this crack grew with time until the step's lee surface was quarried. Our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. Natural basal water pressure fluctuations should also concentrate stresses on rock steps, increasing rates of crack growth. Stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size, which may help explain the effect of sliding speed on erosion rates.

ABSTRACT Floristic quality is a rapid assessment metric designed to evaluate the closeness of the flora of an area to undisturbed conditions. To be useful for a variety of monitoring applications it has to be calibrated for local conditions. Based on data from 554 lakes, this paper develops a floristic quality metric for Wisconsin lake plant communities and calibrates it for ecoregional and lake type differences. The Northern Lakes and Forests ecoregion had the highest floristic quality with flowages having the highest number of species and lakes having the highest average coefficient of conservatism ([Cbar]). Floristic quality in lakes and flowages in the North Central Hardwoods region and the Southeastern Till Plain region were not significantly different and were combined into a single group. Their floristic quality was intermediate between the Northern Lakes and Forest groups and the final group that was a combined Driftless Area and Mississippi River Backwater lake group. When applied to a limited number of examples the floristic quality index appears to give reasonable results but more trials of use under well documented disturbance conditions is needed to determine its sensitivity.

ABSTRACT We have developed a method to calculate ground‐ water recharge rates using the mass‐balance equation, water‐ table elevation data, estimates of hydraulic conductivity, and aquifer thickness data, and have applied this method to produce a map of the recharge and discharge patterns for a ground‐water basin in central Wisconsin. This recharge mapping method is simplified using a modified computer program, the USGS Modular Groundwater Flow Model (McDonald and Harbaugh, 1984). The modeled recharge pattern compares favorably with a recharge map based on field observations. Because recharge rates are extremely sensitive to hydraulic conductivity, the magnitudes of the calculated rates are less reliable than the patterns of recharge and discharge areas. However, introducing stream discharge data constrains the model to produce net recharge rates averaged over the basin which agree with estimates of the basin yield. Because the method is insensitive to the position of lateral boundaries, it can be used to map recharge over areas within basins that are not physically bounded. Recharge maps made with this method can be used to design ground‐water monitoring networks and as frameworks for interpreting geochemical or potentiometric data.

Wisconsin Geological and Natural History SurveyMadison, Wisconsin

Research Article| September 01, 2015 Stratigraphic correlations using trace elements in apatite from Late Ordovician (Sandbian-Katian) K-bentonites of eastern North America Bryan K. Sell; Bryan K. Sell § 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA †Current address: Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005, USA; bksell@umich.edu. §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Scott D. Samson; Scott D. Samson § 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Charles E. Mitchell; Charles E. Mitchell § 2Department of Geology, State University of New York at Buffalo, Buffalo, New York 14260, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Patrick I. McLaughlin; Patrick I. McLaughlin § 3Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Alan E. Koenig; Alan E. Koenig § 4U.S. Geological Survey, Central Mineral and Environment Resources Science Center, Denver, Colorado 80225, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Stephen A. Leslie Stephen A. Leslie § 5Department of Geology and Environmental Science, James Madison University, 395 South High Street, MSC 6903, Harrisonburg, Virginia 22807, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Search for other works by this author on: GSW Google Scholar Author and Article Information Bryan K. Sell †Current address: Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005, USA; bksell@umich.edu. § 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Scott D. Samson § 1Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Charles E. Mitchell § 2Department of Geology, State University of New York at Buffalo, Buffalo, New York 14260, USA Patrick I. McLaughlin § 3Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, USA Alan E. Koenig § 4U.S. Geological Survey, Central Mineral and Environment Resources Science Center, Denver, Colorado 80225, USA Stephen A. Leslie § 5Department of Geology and Environmental Science, James Madison University, 395 South High Street, MSC 6903, Harrisonburg, Virginia 22807, USA §E-mails: bksell@syr.edu; sdsamson@syr.edu; cem@buffalo.edu; pimclaughlin@wisc.edu; akoenig@usgs.gov; lesliesa@jmu.edu. Publisher: Geological Society of America Received: 24 Aug 2014 Revision Received: 12 Dec 2014 Accepted: 17 Feb 2015 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2015 Geological Society of America GSA Bulletin (2015) 127 (9-10): 1259–1274. https://doi.org/10.1130/B31194.1 Article history Received: 24 Aug 2014 Revision Received: 12 Dec 2014 Accepted: 17 Feb 2015 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Bryan K. Sell, Scott D. Samson, Charles E. Mitchell, Patrick I. McLaughlin, Alan E. Koenig, Stephen A. Leslie; Stratigraphic correlations using trace elements in apatite from Late Ordovician (Sandbian-Katian) K-bentonites of eastern North America. GSA Bulletin 2015;; 127 (9-10): 1259–1274. doi: https://doi.org/10.1130/B31194.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The early Late Ordovician sedimentary rocks of eastern North America contain a relatively large number (>100) of widespread heavily altered tephra layers (K-bentonites). These beds represent an intense period of subaerial volcanism that occurred from ca. 455 to 449 Ma. The sedimentary rocks that contain these K-bentonites display complex regional lithostratigraphic relationships ranging from clastic foreland basin facies to cratonic carbonate platform facies. Accurate correlation of these ancient ash-fall beds is essential for testing chronostratigraphic hypotheses that attempt to connect these different tectono-sedimentary provinces. Despite the relatively thorough study of a few of these K-bentonites over the past several decades, the full stratigraphic potential of these beds has yet to be realized. To test the utility of the apatite trace-element K-bentonite correlation method on a larger scale, we studied over 200 K-bentonite samples from the Mohawkian Stage of eastern North America and statistically compared our results with previous studies on the same suites of K-bentonites. Electron microprobe (EPMA) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) results show that apatite trace-element data provide unique bed discriminators. Each of the K-bentonite layers exhibits unique and reproducible trends in Mg, Cl, Mn, Fe, Ce, Y, and other trace-element concentrations in apatite. Statistical evaluation of results from our apatite analyses suggests correlations for 12 K-bentonite beds, providing a significant improvement in stratigraphic resolution. The stratigraphic relations indicated by these new K-bentonite fingerprints provide a rigorous means by which to evaluate some previous interpretations of biostratigraphic, chemostratigraphic, and sequence stratigraphic studies in eastern North America. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| December 01, 1985 Correlation of late Wisconsin glacial phases in the western Great Lakes area JOHN W. ATTIG; JOHN W. ATTIG 1Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705 Search for other works by this author on: GSW Google Scholar LEE CLAYTON; LEE CLAYTON 1Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705 Search for other works by this author on: GSW Google Scholar DAVID M. MICKELSON DAVID M. MICKELSON 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN W. ATTIG 1Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705 LEE CLAYTON 1Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705 DAVID M. MICKELSON 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1985) 96 (12): 1585–1593. https://doi.org/10.1130/0016-7606(1985)96<1585:COLWGP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation JOHN W. ATTIG, LEE CLAYTON, DAVID M. MICKELSON; Correlation of late Wisconsin glacial phases in the western Great Lakes area. GSA Bulletin 1985;; 96 (12): 1585–1593. doi: https://doi.org/10.1130/0016-7606(1985)96<1585:COLWGP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Late Wisconsin glacial phases of the Superior Lobe in Wisconsin and adjacent Minnesota are tentatively correlated across northern Wisconsin and adjacent Michigan to phases of the Green Bay and Lake Michigan Lobes. Spanning the period from about 26,000 yr B.P. to about 9500 yr B.P., 7 phases of the late Wisconsin Laurentide Ice Sheet are discussed, but only 2 of these are precisely dated.Ice advanced out of the basins of Lakes Superior and Michigan and stabilized during the St. Croix-Hancock phase, the most extensive late Wisconsin phase in most of the area, between about 18,000 and 15,000 yr ago. Following the St. Croix-Hancock phase, the general wasting of the ice was interrupted by stillstands or readvances of the ice margin during the intermediate and Mountain phases. The ice then wasted into the Superior basin before readvancing, over a landscape containing ice masses from earlier phases, during the Winegar-early Athelstane phase. This phase is believed to have occurred just prior to growth of the Two Creeks Forest in eastern Wisconsin. The ice then wasted back and advanced three more times during the Marenisco-late Athelstane, Porcupine (about 11,000 yr B.P.), and Marquette (about 9900 yr B.P.) phases. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Chippewa and Wisconsin Valley Lobes of the Laurentide Ice Sheet reached their maximum extent in north‐central Wisconsin about 20 000 years ago. Their terminal positions are marked by a broad area of hummocky topography, containing many ice‐walled‐lake plains, which is bounded on the up‐ice and down‐ice sides by ice‐contact ridges and outwash fans. The distribution of these ice‐disintegration landforms shows that a wide zone of stagnant, debris‐covered, debris‐rich ice separated from the active margins of both lobes as they wasted northward during deglaciation. Accumulation of thick, uncollapsed sediment in ice‐walled lakes high in the ice‐cored landscape indicates a period of stability. In contrast, hummocky disintegration topography indicates unstable conditions. Thus, we interpret two phases of late‐glacial landscape evolution. During the first phase, ice buried beneath thick supraglacial sediment was stable. Supraglacial lakes formed on the ice surface and some melted their way to solid ground and formed ice‐walled lakes. During the second phase, buried ice began to melt rapidly, hummocky topography formed by topographic inversion, and supraglacial and ice‐walled lakes drained. We suggest that ice wastage was controlled primarily by climatic conditions and supraglacial‐debris thickness. Late‐glacial permafrost in northern Wisconsin likely delayed wastage of buried ice until after about 13 000 years ago, when climate warmed and permafrost thawed.

Glacial hummocks associated with the Superior Lobe in western Wisconsin are stagnant‐ice features composed of melt‐out till, meltwater‐stream sediment, and flow till. The greater proportion of melt‐out till in these hummocks than in hummocks described elsewhere suggests that a model of extensive, supraglacial reworking of supraglacially released debris does not apply to the western Wisconsin hummocks. Interpretation of melt‐out till in hummock exposures is based on its strong fabric oriented parallel to regional ice‐flow direction. Other features of this melt‐out till include poorly developed stratification (color banding and discontinuous thin sandy lenses), and minor faulting, both of which support a melt‐out origin. We suggest that as stagnant, debris‐rich ice began to melt, supraglacially released debris was deposited as flow till and meltwater‐stream sediment (with some debris‐flow sediment and lake sediment), but as the thickness of supraglacial debris increased, debris melting out at depth was stabilized, allowing features characteristic of melt‐out till to be retained. Because the supraglacial debris was sandy and the stagnant ice was likely at the pressure‐melting point, the supraglacial debris was well drained and did not readily fail and flow. Debris volume in the glacier generally was greater at the glacier margin, but lateral and longitudinal variations within this zone were caused by thrusting, freezing‐on, or ice‐margin fluctuations, which in turn resulted in variations in hummock relief. Ice‐walled‐lake plains are commonly associated with the hummocks and developed where debris volume was small.

Abstract Particle fabrics of basal tills may allow testing of the bed‐deformation model of glacier flow, which requires high bed shear strains (&gt;100). Field studies, however, have not yielded a systematic relationship between shear‐strain magnitude and fabric development. To isolate this relationship four basal tills and viscous putty were sheared in a ring‐shear device to strains as high as 714. Fabric was characterized within a zone of shear deformation using the long‐axis orientations of fine‐gravel and sand particles and the anisotropy of magnetic susceptibility (AMS) of small (∼5·8 cm 3 ) intact samples. Results indicate that till particles rotate toward the plane of shearing with long‐axis orientations that become tightly clustered in the direction of shear (0·78 &lt; S 1 &lt; 0·94 for three‐dimensional data). These strong, steady‐state fabrics are attained at shear strains of 7–30, with no evidence of fabric weakening with further strain, regardless of the specific till or particle‐size fraction under consideration. These results do not support the Jeffery model of particle rotation, which correctly describes particle rotation in the viscous putty but not in the tills, owing to fluid‐mechanical assumptions of the model that are violated in till. The sensitivity of fabric development to shear‐strain magnitude indicates that, for most till units where shear‐strain magnitude is poorly known, attributing fabric variations to spatial differences in other variables, such as till thickness or water content, will be inherently speculative. Attributing fabric characteristics to particular basal till facies is uncertain because shear‐strain magnitude is unlikely to be closely correlated to till facies. Weak or spatially variable fabrics, in the absence of post‐depositional disturbance or major deviations from unidirectional simple shear, indicate that till has not been pervasively sheared to the high strains required by the bed‐deformation model. Strong flow‐parallel fabrics are a necessary but insufficient criterion for confirming the model. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Abstract We present the first study of the distribution, genesis and paleoenvironmental significance of late Pleistocene loess in northeastern Wisconsin and adjacent parts of Michigan's Upper Peninsula. Loess here is commonly 25–70 cm thick. Upland areas that were deglaciated early and remained geomorphically stable preferentially accumulated loess by providing sites that were efficient at trapping and retaining eolian sediment. Data from 419 such sites indicate that the loess was mainly derived from proglacial outwash plains and to a lesser extent, hummocky end moraines within and near the region, particularly those toward the east of the loess deposits. Most of the loess was transported on katabatic winds coming off the ice sheet, which entrained and transported both silt and fine sands. The loess fines markedly, and is better sorted, distal to these source regions. Only minimal amounts of loess were deposited in this area via westerly winds. This research (1) reinforces the observation that outwash plains and end moraines can be significant loess sources, (2) provides evidence for katabatic winds as significant eolian transport vectors, and (3) demonstrates that the loess record may be variously preserved across landscapes, depending on where and when geomorphically stable sites became available for loess accumulation.