1 Clays and Clay Minerals, Vol. 44, No. 4, , GEOLOGIC CONTROL OF SEVERE EXPANSIVE CLAY DAMAGE TO A SUBDIVISION IN THE PIERRE SHALE, SOUTHWEST DENVER METROPOLITAN AREA, COLORADO J. D. GILL, l M. W. WEST, 1 D. C. NOE, 2 H. W. OLSEN, 3 AND D. K. MCCARTY 4 Michael W. West and Associates, P.O. Box 555, Morrison, CO Colorado Geological Survey, Denver, CO 3 Colorado School of Mines, Golden, CO 4 Montana State University, Boseman, MT Abstract--Shortly after construction of a subdivision in the southwest Denver metropolitan area in 1986, a portion of the subdivision built directly on steeply-dipping strata of the Pierre Shale began experiencing damaging differential movements, causing house foundations to fail and pavements to warp and crack. This formation is a Late Cretaceous marine clay-shale composed predominantly of fluvial mixed-layer illite/smectite and quartz. During deposition of the shale, periodic and explosive volcanism generated thin beds of bentonite, consisting initially of volcanic ash and subsequently altered to nearly pure smectite. Some of these bentonite beds were exposed in a trench adjacent to the subdivision and perpendicular to the strike of the steeply-dipping strata. The thickest bentonite beds correlated well with linear heave features that these beds parallel the bedrock strike throughout the subdivision were mapped via severely deformed pavements. Mineralogical data show the bentonite bed that correlates with the worst damage within the subdivision consists of about 62% smectite by weight with mixed-layer illite/smectite expandability of 92%. By comparison, a sample of the typical silty claystone, which is fluvial mixed-layer illite/smectite mixed with detrital quartz from the adjacent strata, had about 23% smectite by weight with 70% to 90% illite/smectite expandability. Geotechnical tests for swell potential show that samples of 2 bentonite beds swelled 39% to 43% compared to 2% to 8% for samples of the typical silty claystone. It is proposed that differential swell resulting from stratigraphically-controlled differences in clay mineralogy and grain-size is the primary factor controlling extreme damage for this geologic setting. Key Words--Bentonites, Expansive clays, Pierre Shale, Smectites INTRODUCTION Rapid suburban growth near the foothills of the southwest Denver metropolitan area is causing pressure to build in areas where the underlying sedimentary bedrock formations have been deformed into steeply-dipping strata that outcrop at the ground surface in some areas and are covered with surficial soils elsewhere. Residential developments on one of these steeply-dipping formations, the Pierre Shale, have experienced substantially higher damage rates than similar developments near the generally fiat-lying Denver Formation that underlies most of the Denver metropolitan area to the East (Thompson 1992). It is generally recognized that damage within the Pierre Shale is caused by differential ground deformations that generate linear heave features in response to developmentinduced changes within the natural soil cover and subsurface hydrologic conditions. This paper is based on an investigation carried out by the authors during litigation concerning the cause(s) of damaging heave features for a subdivision that was constructed during 1986 on steeply-dipping strata of the Pierre Shale. Since construction, these heave features have grown continuously and become locally extreme. They have caused lightly-loaded drilled-pier foundations to fail and pavements to crack and buckle (Figures 1, 2 and 3). For some of the streets, the heaves exhibit over 12 in. of differential vertical displacement. For several years, construction crews have been repairing structures and pavements that have been, and are continuously affected. The scope of the authors' investigation included mapping the nature and distribution of damaging heave features throughout the subdivision, correlating these heave features with the steeply-dipping strata within the stratigraphic interval of the Pierre Shale beneath the subdivision, correlating this stratigraphic interval with previous geologic studies of the Pierre Shale and obtaining new data regarding the composition and swell potential of the strata beneath the subdivision. This paper presents the results of these studies and discusses their significance concerning the conventional design and construction approach to minimize damage from expansive soils that has been employed throughout the Denver metropolitan area for several decades. BACKGROUND GEOLOGY The subdivision is located southwest of Denver, CO near the foothills of the Rocky Mountain Front Range in the southwest corner of the Denver Basin (Figure 4). The 5400 foot thick Pierre Shale strikes N 17 ~ W, is parallel to the mountain front and dips steeply 70 ~ Copyright , The Clay Minerals Society 530
2 Vol. 44, No. 4, Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 531 Figure 1. Two largest linear humps in subdivision street pavement. Note failure of street drainage. NE (Scott 1963b) as a result of the Laramide compressional folding and faulting during the early to mid Eocene epoch. The outcrop width at the subdivision is 5800 ft. Stratigraphically, the subdivision is underlain by claystone and minor siltstone strata overlying the Hygiene Sandstone Member. The Pierre Shale and geologically equivalent formations comprise an extensive sedimentary deposit within the western interior of the United States and Canada formed during the Cretaceous Interior Seaway between 69.5 and 81.5 m.y. ago (Cobban et al. 1994). Clay minerals dominate the constituents of the shale ranging from 50 to 75% (Schultz 1978). Mixed-layer illite/smectites make up the majority of the clay minerals. However, smectites are concentrated within bentonite beds, pervasive for certain stratigraphic intervals. The composition of the clay minerals and grain size within the shale vary with changes in the depositional environment. Fluvial mixed-layer illite/smectites mixed with detrital quartz derived from volcanic source terrain to the west predominate the majority of the shale. However, airborne pyroclastic material settled into the interior seaway during volcanic eruptions. During major eruptions, the deposition rate of volcanic air-fall sediment far exceeded that of the fluvial sedi- ment, thereby forming discrete layers of concentrated volcanic ash with little or no detrital quartz. This ash subsequently altered into nearly pure smectite clay and thus formed bentonite beds. Some of these layers are recognizable as "air-fall" deposits with preserved phenocrysts, sharp contacts with the underlying shale, and gradational contacts with the overlying shale. During minor eruptions, the deposition rate of the air-fall sediment was slower. This allowed the air-fall sediments to be extensively transported and mixed with minor amounts of detrital quartz forming bentonitic beds that typically overlie discrete bentonite beds. These are gradational with the surrounding shale such that they contain more smectite clay and less quartz. Discrete bentonite beds represent less than a few percent of the total shale thickness. The corresponding percentage for bentonitic beds is poorly understood because they are more difficult to recognize. The discrete beds are generally very thin (< 2 in.), although occasional beds having thicknesses of several inches to a foot have been reported (Gill and Cobban 1966; Schultz et al. 1980). After deposition and alteration, smectite diagenesis, that is illitization, has not affected the Pierre Shale in Colorado (Schultz 1978), but some bentonites seem to have been almost completely replaced by calcite (Schultz et al. 1980).
3 532 Gill et al. Figure 2. Linear hump trending into driveway and house. Figure 3. Buckled curb and gutter. Clays and Clay Minerals
4 Vol. 44, No. 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 533 Figure 4. Location and geologic setting of study area. Key: pc = Precambrian core of Front Range, PPf = Pensylvanian- Permian Fountain Fm., Kp = Late Cretaceous Pierre Shale, and TKda = Cretaceous-Tertiary Dawson Arkose. The stratigraphic section underlying the subdivision spans 1800 ft of Pierre Shale strata beginning at the top of the Hygiene Sandstone Member and continuing upward into claystones and minor siltstones. Figure 5 compares the stratigraphic relationship between this section and 2 other reference sections of the Pierre Shale. Ammonite fossils of Didymoceras nebrascense and Didyrnoceras stevensoni (Cobban 1994) found at the subdivision established the age of the underlying shale at approximately 73 to 76 m.y. old (Cobban et al. 1994). This section of shale, deposited during the Bearpaw Transgression, correlates with the upper shale unit of the Pierre of the Northern Black Hills in Montana and the Unnamed shale Member of the Pierre at the Red Bird Section in Wyoming. Both sections contain documented bentonite beds. The Red Bird Section has 13 bentonite beds including the 36-ft thick Kara Bentonitic Member (Gill and Cobban 1966) and the northern Black Hills section has the Monument Hill Bentonitic Member (Schultz et al. 1980). The chart shows the propensity for this section of the Pierre to contain bentonite beds correlating to explosive volca- nism from 76 to 73 + m.y. ago in the Elkhorn Mountains, Boulder batholith region of Montana (Gill and Cobban 1973; Robinson et al. 1968). Figure 5 also illustrates that the section beneath the subdivision is relatively thicker than its northern counterparts indicating higher depositional rates for the Denver area. Higher depositional rates probably enhanced the preservation of the air-fall deposits by allowing less time for the sea-floor ash deposits to be disturbed during rare, storm-generated, turbulence within the Interior Seaway. Before site grading, Quaternary surficial geologic deposits covered bedrock within the subdivision to depths ranging from 3 to 19 ft. The predominant natural soil was wind-deposited (loess) silty clay with minor patches of Verdos, Slocum and Piney Creek alluvium (Scott 1963a). During site grading, the natural soil cover was drastically changed by excavating the central portion of the subdivision, exposing bedrock, and placing the excavated soils along the east and west sides of the subdivision (Figure 6). The fill consisted of a mixture of loess and reworked claystone.
5 " I 534 Gill et al. Clays and Clay Minerals STRATIGRAPHIC COMPARISON (BEARPAW TRANSGRESSION) 4- ~ 3- -~2- ~_ E x-4z BIOSTRATIGRAPHIC r'~ ~1 all' ZONE (Scott ~ ~,* ~ "~ Cobbu iai I,,J,. "r" fj) ~eu=(~s ~(~bccu= F ---~--~-~ I e. -.~'~"~. ~.~. 7-~---~r -" =~ Fossils,r; subdivision/ I~= I ~= ~=~ Bld)wlc~'Irr o,~...= It~nvenst~ll... ~ II--J. < SUBDIVISION RED BIRD NORTHERN ELKHORN NEAR DENVER SECTION BLACK HILLS MOUNTAINS COLORADO WYOMING MONTANA MONTANA / IHIr9 H]qllenm_l len~/- ~ ILl n. Ssdblrd I I8~.ndst~ $andstan I ~- ~" Silty a,z,.,au,= rrego,.~,,~,.~ ----~ [Member I" ~ ~ I Member '\\ (Gill & Cobban, (Schultz, Tourtelot, (Gill & Cobban, Gill & Boerngen, 19]'3) 1980) \\ r,/) rt ~\ \\ X-.IZ < \\ \\ [L::[;(/) "\\xx\\ 'm._~, \\\, _<lf_;s~,=. ~ :,,, =_ Zz< ulu <1-< ~ nlw mz~ r X-JZ O~ < I Fo~ m.s r --;,1 --~ R-g--o--,,_-:---H~... ~ ~ EXPLO81VE Beorpaw ---Z--Z ~--75 VOkCANISMTratosgrssM~ Regression EXPLANATION SCHEMATIG PI.OT Thlckn~. af Bentonite layers nat OF BENTONITE represented, lines Indicate depth Only. UEDS Most Rent~nYtes <9.5 f~ ~hlck.... Silty Upper Transition Figure Blostratlgraphlc zone Stratigraphic comparison of Pierre Shale during Bearpaw Transgression. METHODS During litigation, the authors mapped the differential heaves revealed from the street and sidewalk pavements. They also mapped the surficial geology and logged the bedrock stratigraphy beneath the subdivision from a 25 ft-deep trench adjacent to the subdivision and perpendicular to bedrock strike. The strata within the trench were correlated with pavement damage patterns for the subdivision. A profile of relative displacement across the largest heave feature was obtained with a string-line method. Samples of selected strata were analyzed for mineralogy and mixed-layer illite/smectite (US) expandability, cation exchange capacity (CEC) and geotechnical properties including air-dried moisture content, Atterberg limits, grain-size distribution and the percent swell upon inundation. For mineralogy and mixed-layer illite/smectite expandability, the % expandable smectite layers in US, X-ray diffraction (XRD) pattems were collected on random and oriented samples using an automated digital-step scanning Siemens XRD system employing CuKa radiation, a graphite monochrometer and 1 ~ divergence and receiving slits. Random sample prepa- rations were made by using a side-loading sample holder after ground bulk rock was passed through a < 74 txm sieve to ensure comparable results between samples, and to obtain representative hulk mineralogy determinations. Oriented preparations were made by using a millipore filter peel transfer onto glass slides after a < 1.0 ixm suspension was collected by centrifugation. After oriented mounts were X-rayed in an airdried state, samples were glycolated within a glycolwater vapor atmosphere at 60 ~ overnight and then scanned over the same angular range. When possible, mixed-layer US expandahilities were determined by the ~ method of Moore and Reynolds (1989) utilizing the I/S (001)/(002) and (002)/(003) reflections. For cases where peak interferences from other phases made this difficult, I/S expandability was determined from the correspondence of the I/S (002)/(003) peak position with calculated (002)/(003) peak positions from the NEWMOD computer program (Reynolds 1985). The given expandability values are usually within -5%, but because of discrete illite interference, this error could be slightly higher (McCarty and Eberl 1992).
6 Vol. 44, No: 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 535 Figure 6. Damage distribution in pavements and site geology. Cation exchange capacities (CEC) were measured from NH 4- saturated specimens using the ammonia electrode method of Busenberg and Clemency (1973) after cation exchange with a known weight of 1M NH4C1 solution. For selected bulk rock specimens, the weight percent of smectite was estimated using the CEC of a < 1 txm specimen from a bentonite consisting of pure smectite as a reference. We assumed that the other minerals within the sample had a CEC of 1 meq/100 g, and was a linear relation between smectite content and CEC (McCarty and Eberl 1992). Geotechnical properties were run on bulk samples of 2 silty claystone strata and of the 2 thickest bentonite beds correlating to the largest heave features. The bulk samples were crushed until the material passed the number 40 sieve, spread out to a uniform depth of approximately 1A in., and allowed to air-dry at room temperature for 4 d. Then moisture contents (ASTM D-2216), Atterberg limits (ASTM D-4318), hydrometer method panicle size distribution (ASTM D-422) (Am. Soc. for Testing and Mat. 1994) and activity index (liquid limit / < 2 ~m fraction; Lambe and Whitman 1969) were determined for these materials. Swell-test specimens were also prepared by compacting the remolded and air-dried materials to a dry density of 89 pounds per cubic foot (pcf). These specimens were placed into one-dimensional consolidom- eters, loaded via steps to a surcharge of 800 pounds per square foot (psf), inundated with water and monitored to determine the resulting percentage of swell (ASTM D-4546). RESULTS Heave features within pavements mapped during 1992, are shown in Figure 6. These features follow linear trends that are parallel to bedrock strike from street to street across the subdivision. By projecting these linear trends to the trench adjacent to the subdivision, the heave features were correlated with the bentonite beds from the steeply-dipping Pierre Shale. The locations of 37 different bentonite beds within the trench are plotted in Figures 5 and 6, The majority of the bentonite beds were very thin, less than 1 in. thick; however, a 12 in. thick, an 8 in. thick and several 3 to 4 in. thick beds were found. The bentonite beds were generally lighter in color than the surrounding olive-gray silty shale strata (Figure 7) and ranged from light gray to yellow. Within the bentonites, fibrous calcium carbonate and selenite crystals were commonly found within a matrix of smectite clay, which was generally soft and felt waxy (free of silt) and sometimes contained fine-grained phenocrysts. For the thinnest beds, the smectite clay was difficult or impossible to recognize by eye. For the thicker ben-
7 536 Gill et al. Figure inch thick bentonite bed "A'" correlated to pavement damage in Figure 8. tonite beds, the smectite was typically slickensided in a tight cuspate, anastimosing manner. The slickensides did not appear to line up along a continuous plane, rather they cross-cut each other, possibly due to repeated cycles of intense swelling, shrinking and plastic flow of the smectite. Comparison of the linear heave trends with the bentonite beds exposed within the trench (Figure 6) reveals a strong correlation of the 2 largest pavement heaves, which are traceable across 4 streets, with the 2 thickest bentonite beds. Another linear heave trend correlates with a cluster of thinner beds. Several of the thinner (less than 1 in. thick), isolated, bentonite beds did not correlate with specific pavement damage. Some minor linear heaves could not be correlated to discrete bentonite beds. Differential displacements for the asphalt pavement across the 2 largest heaves, measured in 1994, are compared with the underlying stratigraphy correlated from the trench 150 ft along strike to the south (cross sections A-A' and B-B', Figure 6) in Figure 8. These sections differ in that the street in Section B-B' was Clays and Clay Minerals built directly upon excavated bedrock whereas surficial soils overlie the steeply-dipping bedrock in Section A-A'. In Section B-B' the vertical differential displacements associated with the 2 thickest beds (A and D) exceed 12 in. and show a distinct asymmetry with the steeper side of the humps being on the west (downsection) side, and that the width of the humps can exceed that of the associated bentonite beds. Similar vertical differential displacements were not found in Section A-A' where surficial soils overlie the steeplydipping bedrock. The XRD and CEC data indicate differences in mineralogy of the < 11xm fractions of the bentonite beds, bentonitic beds and the adjacent silty-claystone strata (Figure 9). This figure also shows the bentonite bed is monomineralic, the bentonitic bed shows traces of kaolinite, and the adjacent silty claystone strata contains kaolinite and quartz. The clays in the 4 bentonite beds contained > 90% US expandabilities, whereas, clays in the adjacent silty-claystone strata generally contained lower percentages ranging from 70% to 90%. The CEC for the < 1.0 p,m fractions of smectite clay from 2 bentonite beds were measured at 50.5 and 53.6 meq/100 g. Using a mean value of 52 meq/100 g as a reference value relating to 100 wt% smectite, bulkrock percent smectite was estimated for bed D and a specimen from the adjacent silty claystone (McCarty and Eberl 1992). The bulk-rock specimen from bentonite bed D (Figure 8) contained nearly 3 times more smectite than the silty claystone sample, having 62% compared to 23% smectite (Figure 8), derived from bulk rock CEC values of 31.8 meq/100 g and 12.7 meq/100 g, respectively. The geotechnical test results show the bentonite beds have a much higher affinity for water and swell potential than the silty claystone (Table 1). Under equal air-dried conditions, bulk-rock samples of bentonite beds A and D had 13.7% to 15.1% water by weight, about 10% more than 3.6% to 5.2% found for bulk-rock samples of the surrounding silty claystone. The liquid limit (LL) and the plasticity index (PI) values for the bentonite were 1.5 to 3 times higher than those for the shale. Swell tests on the bentonites resulted in 39% to 43% swell, which are substantially higher than values of 2% to 8% for the silty claystone. DISCUSSION The strong correlation of linear heave features with bentonite beds within the steeply-dipping Pierre Shale, and the relatively strong affinity of the bentonite beds for water, compared with the silty claystone strata between the bentonite beds. This strongly indicates that many of the linear heave features are caused by more swelling of the bentonite beds compared with the adjacent strata. Steeper-to-the-west (down-section) asymmetry heave features with their widths wider than the actual bentonite beds, are attributable to the bentonite
8 Vol. 44, No. 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 537 Figure 8. Correlation of pavement damage with bentonite beds and respective %I/S and wt% smectite. beds that typically have a sharp contact with the underlying shale and are gradational with the overlying shale. Shale immediately overlying bentonites is often bentonitic. The data presented on Figure 8 illustrates this for beds A and C by showing relatively sharp increases of deformation and % US followed by graduai decreases for both of these factors while progressing upward through the geologic section. It follows that the dominant control on damaging vertical differential deformations in this geologic setring is stratigraphically-controlled differences within clay mineralogy and grain size that govern the affinity of the strata to water and hence their swell potential and magnitudes. This condition differs substantially from that in the generally fiat-lying Denver Formation that underlies most of the Denver metropolitan area to the east. For fiat-lying geologic strata, stratigraphic differences occur vertically and hence do not contribute directly to differential vertical deformations. Rather, the dominant source of such damaging deformations is development-induced changes within subsurface moisture conditions that vary horizontally in response to infiltration and evapotranspiration changes. It needs to be recognized that, in steeply-dipping geologic strata, development-induced changes in subsur- face moisture conditions are also to be expected. However, the results of this study strongly suggest that the effects of this factor were overshadowed by stratigraphic differences of clay mineralogy and grain size. The subdivision was developed with the conventional design and construction approach for minimizing damage from expansive soils that has been employed throughout the Denver metropolitan area for several decades. The essence of this approach is to avoid damaging vertical deformations by supporting light structures on drilled piers anchored at depth into claystone bedrock, and to isolate structures from assumed uniform swelling of surficial materials by maintaining voids beneath horizontal structural beams and structural floors. Thompson's (1992) statistical study shows this conventional approach has been far less successful with steeply-dipping strata than with flat-lying strata. This study indicates a major deficiency of this approach in that it does not consider stratigraphically-controlled horizontal variations in the subgrade. He also shows the severity of damage within steeply-dipping strata decreases as the depth of overburden increases. The results of this study are consistent with his findings in that the linear heave features were substantial primar-
9 538 Gill et al. Clays and Clay Minerals,,d IS 17,ooA i/s 17.o~ i/s a.5ox i/s ~6~X I/s 3.z7X %j~ I/s e.so~, i/s s.e+x i/s t/s 3.s+/, I QOl I/Sool/oo 2 I/Soo2/oo 3 I/s 3.3+A 2 I 5 I 8 I 1~ (Cu-K=) I BENTONITE aed i/s eo g2z BENTONITIC BED lts RO a7x TYPICAL SHALE BED I/S RO vodes from 7QZ to 90~ I/S - ILLITE/SMEClaTE I - plite K - KAOLINITE Q - QUARTZ Figure 9. XRD patterns of bentonite bed, bentonitic bed and typical shale bed. (<1 p,m, glycolated). ily where the alluvial overburden had been removed and structures were constructed on the exposed claystone bedrock. It is of particular interest that the bentonite beds in this study could be correlated with biostratigraphic zones that represent geologic time horizons and are mapped extensively in the western interior of the United States and Canada (Cobban et al. 1994). This suggests that the biostratigraphic zones, which have already been mapped for the Front Range area (Scott and Cobban 1965) can be used as guidelines for locating bentonites and bentonitic layers. However, because the thicknesses of the Pierre Shale and the biostratigraphic zones are not constant, supplementary studies will generally be necessary for defining the stratigraphy for site-specific investigations. For this purpose, trenching appears to be the best available approach. CONCLUSIONS Residential developments on steeply-dipping Pierre Shale in the Denver metropolitan area are vulnerable to damaging differential deformations arising from stratigraphically-controlled differences within clay mineralogy and grain size. These differential deformations are usually expressed in terms of linear heave ridges that are parallel to strike and underlain by bentonite beds that are areally extensive throughout the Western Interior of the United States for rocks deposited into the Cretaceous Interior Seaway. At the site investigated in this study, the bentonite beds consist of more than 60% smectite compared with approximately 20% smectite for the adjacent silty-claystone strata. Geotechnical tests confirmed that the bentonite beds have a much higher affinity for water and swell potential than the silty-claystone strata. These conditions differ substantially from those for flat-lying geologic strata, such as the Denver Formation that underlies most of the Denver metropolitan area to the east. Within fiat-lying geologic strata, strati- Table 1. Geotechnical test results. EQUILIBRIUM AIR-DRIED MOISTURE CONTENT (%) ATTERBERG LIMITS LL P] CLAY FRACTION (% <.002 ram) ACTIVITY INDEX SWELL INDEX TEST (REMOLDED) % SWELL ON SATURATION (800 psf surcharge) CATION EXCHANGE CAPACITY (meq/100 g) bulk sample, divalent cations dominant Silty claystone number of specimens Bentonite number of specimens (2) (2) (10) (2) (10) (2) (10) (2) (12).97-1,10 (2) 2-8 (2) (2) 12.7 (1) 31.8 (1)
10 Vol. 44, No. 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 539 graphic differences occur vertically and hence do not contribute directly to differential vertical deformations. Rather, the dominant source of such damaging deformations of development-induced changes is subsurface moisture conditions that vary horizontally in response to infiltration and evapotranspiration changes. It is suggested that the biostratigraphic zones, which have already been mapped for the Front Range area, can be used as guidelines for locating bentonites and bentonitic layers within the steeply-dipping beds of the Pierre Shale. However, because the thicknesses of the Pierre Shale and the biostratigraphic zones are not constant, supplementary studies will generally be necessary for defining the stratigraphy of steeply-dipping beds for site-specific investigations. For this purpose, trenching appears to be the best available approach. REFERENCES American Society for Testing and Materials Moisture Content (D-2216), Atterberg limits (D-4318), Particle size distribution (D-422) and Swell (D-4546). In: Annual Book of Standards. Philadelphia, PA: ASTM. 4.08: Busenberg E, Clemency CV Determination of the cation exchange capacity of clays and soils using an ammonia electrode. Clays & Clay Miner 21: Cobban WA, Merewether EA, Fouch TD, Obradovich JD Some cretaceous shorelines in the western interior of the United States. In: Caputo MV, Peterson JA, Franczyk KJ, editors. Mesozoic systems of the Rocky Mountain Region, USA. p Cobban WA Personal communication. Regarding fossil identification, United States Geological Survey, Box 25046, Mail Stop 966, Denver, CO Gill JR, Cobban WA In: Kier PM, editor, new echinoid from the Cretaceous Pierre Shale of eastern Wyoming. The Red Bird section of the Upper Cretaceous Pierre Shale in Wyoming. U.S. Geological Survey Professional Paper 393-A. 73 p. Gill JR, Cobban WA Stratigraphy and geologic history of the Montana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota. U.S. Geological Survey Professional Paper p. Lambe TW, Whitman RV Soil mechanics. New York, NY: John Wiley & Sons. 553 p. McCarty D, Eberl D Written communication. Mineralogical and Chemical analyses of samples. Moore DM, Reynolds RC X-ray diffraction and the identification and analysis of clay minerals. New York: Oxford University Press. 332 p. Reynolds RC NEWMOD computer program for the calculation of the one-dimensional X-ray diffraction patterns of mixed-layer clays. Available from author, Dept. Earth Sciences, Dartmouth College, Hanover, New Hampshire, Robinson GD, Klepper MR, Obradovich JD Overlapping plutonism, volcanism, and tectonism in the Boulder batholith region, western Montana. In: Coats RR, Hay RL, Anderson CA, editors. Studies in volcanology--a memoir in honor of Howel Williams. Geological Society of America Memoir. 116: Scott GR. 1963a. Quaternary geology of the Kassler quadrangle, Colorado. U.S. Geological Survey Professional Paper 421-A. 70 p. Scott GR. 1963b. Bedrock geology of the Kassler quadrangle, Colorado. U.S. Geological Survey Professional Paper 421-B. p Scott GR, Cobban WA Geologic and biostratigraphic map of the Pierre Shale between Jarre Creek and Loveland, Colorado. U.S. Geological Survey Miscellaneous Field Studies Map MF-482, scale 1:62500, 2 sheets. Schultz LG Mixed-Layer Clay in the Pierre Shale and Equivalent Rocks, Northern Great Plains Region. U.S. Geological Survey Professional Paper 1064-A. 27 p. Schultz LG, Tourtelot HA, Gill JR, Boerngen JG Composition and Properties of the Pierre Shael and equivalent rocks, Northern Great Plains Region. U.S. Geological Survey Professional Paper 1064-B. B 1-B 114. Thompson RW Performance of foundations on steeply dipping claystone. Proceedings of the 7th International Conference on Expansive Soils, August 3-5, 1992, Dallas, TX. 1: (Received 26 July 1994; accepted 8 November 1995; Ms. 2546)