Geology

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/4925

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Petrology and structure of the pre-Tertiary rocks of Lummi and Eliza Islands, Washington
(1980) Carroll, Paul R.
Exposed on Lummi Island are two groups of pre-Tertiary rocks everywhere in fault contact with each other: a 1300 meter thick section of turbidite sandstone and mudstone, and rocks largely of igneous affinity. The latter rocks consist of pillow lavas with associated radiolarian chert dated as Middle to early Late Jurassic, and, in a relatively small, isolated exposure, an intrusive complex containing rocks dated by Pb/U methods as 160 + 3 my.
Active tectonics in the central Tien Shan, Kyrgyz Republic
(2001) Thompson, Stephen C., 1970-
Fault slip rates show that late Quaternary contraction is distributed across the Kyrgyz central Tien Shan, and not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north-south shortening on thrust and reverse faults. In a region that spans almost two-thirds of the north-south width of the central Tien Shan, slip rates on eight faults in five basins range from ∼0.1 to ∼3 mm/yr. Fault slip rates come from faulted and folded river terraces and from trench-wall exposures. Radiocarbon, infrared-stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. A major episode of regional river incision occurred ∼13.5--15.7 x 103 cal yr B.P., coincident with abrupt warming of global climate at the end of the last glacial period. The penultimate major river incision occurred ∼141 ka, coincident with the end of the penultimate glacial period. The sum of late Quaternary rates of shortening resembles current rates of north-south shortening measured using Global Positioning System (GPS) geodesy since 1992. This similarity suggests that deformation is concentrated along major fault zones near range-basin margins. Such faults, separated by essentially rigid blocks, accommodate most of the shortening in the upper crust.Paleoseismic data from offset river terraces and trench exposures of faults in the northern foreland and intermontane basins provide evidence ∼2--4 m of displacement during earthquakes, and include a ∼3.5 m displacement from the 1885 M ≈ 6.9 Byelovoda earthquake. The range of displacement is similar to the observed surface displacement after the 1992 Ms = 7.3 Suusamyr earthquake. A preliminary source map of high slip-rate faults across the Kyrgyz central Tien Shan indicates that a 2--4 m event should occur every 60--70 years. This result is generally consistent with the historical record.
Influence of mass wasting on bed-surface armoring, lag formation, and sediment storage in mountain drainage basins of western Washington State
(2006) Brummer, Christopher Jon
This dissertation uses field studies and analyses of digital topography in western Washington State to investigate (1) hydraulic controls on the spatial variation in bedsurface armoring, (2) grain-size controls on the formation of resistant "lag" deposits, and (3) grain-size and lithologic controls on the dispersion rate and residence time of sediment in mountain channels. Field and topographic analyses document systematic downstream coarsening of the median bed-surface grain size in channels with a drainage area < 1 km2 and a subsequent shift to the conventional pattern of downstream fining at a drainage area of about 10 km2. The grain-size maximum corresponds with the maximum unit stream power and the inflection in the drainage area-slope relation thought to represent the transition from debris flow-dominated channels to fluvially dominated channels. The results suggest that basin-wide trends in bed-surface armoring are hydraulically controlled by systematic variations in unit stream power and should therefore be common in headwater channels where debris-flow processes set the channel gradient. Results of a four-year case study of channel recovery from blockage by a landslide dam indicate that, in addition to hydraulic controls, trends in bed-surface armoring are forced by the temporary accumulation of lag deposits arising from the reworking of mass-wasting deposits. Results indicate that the caliber of supplied sediment relative to the flow competence of the receiving channel can influence the rate of sediment-pulse dispersion and the residence time of sediment storage. Measurements of sediment volumes stored in recent and ancient landslide dams in western Washington indicate that the coarsest deposits retain > 80% of initial debris after 103 years, exhibit long-term incision rates that are within one to two orders of magnitude of regional bedrock-lowering rates, and create backwater lakes that provide sediment "capacitance" to mountain drainage basins. Results of this research have practical implications for understanding the residence time of sediment storage in mountain river systems, the disruption of sediment routing, the enhancement of fluvial relief through the shielding of bedrock surfaces, and the timescale of channel recovery following natural and anthropogenic disturbances.
Late Holocene climate change and calving glacier fluctuations along the southwestern margin of the Stikine Icefield, Alaska
(2001) Viens, Robert J
The retreat of large tidewater- and lake-calving glaciers, as well as nearby land-based glaciers, in southeastern Alaska during the middle to late Holocene was primarily triggered by increases in summer temperature. Shakes, LeConte, Patterson, and Baird glaciers, located along the southwestern margin of the Stikine Icefield in southeastern Alaska, experienced two or three major periods of advance and retreat during this period. Historical, stratigraphic, and dendrochronological evidence suggests that these periods of advance culminated approximately 3,500--3,300, 2,700--2,200, 1,100--900, and 220--110 years ago in the study area. Comparison with previously published regional records from glaciers located along the coast of northwestern North America suggests a general synchrony in the timing of ice advance across the region. Regional intervals of ice maxima date approximately 3,000--1,900, 1,500--900 and 250--100 years ago, and encompass three of the main periods of advance represented in the study area.To determine a regional cause of glacier synchrony, glacier chronologies were compared to local and regional climate and climate proxies. Summer temperature fluctuations in the study area for the past four centuries were derived from trey-ring-width time-series from Crystal Mountain near Petersburg. Previously published precipitation and summer temperature values, inferred from palynological studies, provide a record of climate change for the last 10,000 years. Throughout southeastern Alaska, periods of glacier retreat for both calving glaciers and land-based glaciers tend to correlate with periods of warming summer temperature.The collective data imply that the geologic record left by calving glaciers, like that left by land-based glaciers, has the potential to serve as an important climate-proxy record in a region where few such records have been studied. Furthermore, such a relationship helps further to quantify calving glacier dynamics and improve prediction of calving-glacier response to human-induced global warming.
Depositional environments, basin evolution and tectonic significance of the Eocene Chumstick Formation, Cascade Range, Washington
(1988) Evans, James Erwin
The Chumstick Formation represents a Paleogene, humid-tropical, alluvial-fan system which filled a wrench-fault basin in Washington State. Chumstick deposition was characterized by stream-flow processes; low ($$1 m/k.y.) accumulation of sublithic feldspathic sediments; and extensive vegetation which imparted bank stability even in proximal facies. Coarse-grained fluvial facies near major fault zones consist of laterally stacked braid-bar deposits with an overall sheet geometry. The bulk of the basin fill consists of vertically stacked multistory-channel deposits, sandy overbank deposits, and lacustrine deposits.Paleovegetation studies demonstrate the presence of broad-leaved, evergreen taxa typical of humid-tropical climatic conditions. The Chumstick flora can be grouped according to depositional sites as "upland" or "lowland" forest in either floodplain or channel-margin settings. Paleosols include entisols, inceptisols, (incipient Cca or K horizon development) and histosols. Evidence for periodic (seasonal?) dryness includes xerophytic flora, pedogenic calcretes, and episodically reoccupied fluvial channels with mud-draped trough cross-bedding.Three major phases of deposition can be discerned in the Chumstick Formation. Phase 1 ($>$51 Ma to about 42 Ma) consisted of westward-flowing streams in the western sub-basin, with no evidence for relief in the Leavenworth fault zone (LFZ). Phase 2 (about 42 Ma to 40 Ma) resulted from dextral faulting on all three fault zones. West-derived sediment sources resulted from uplift at two transpressive restraining bends in the LFZ. The eastern sub-basin opened as a transtensional step-over between the Eagle Creek (ECFZ) and Entiat (EFZ) fault zones. Tectonic disruption of drainage resulted in axial drainage systems which flowed SE, parallel to the LFZ and ECFZ, and a phase of internal drainage (lacustrine) in the eastern sub-basin. Phase 3 (about 40 Ma to 38 Ma) shows no relation to faults and may indicate tectonic quiescence. The basin was deformed in a zone of dextral transpression between about 38 Ma and 34 Ma. The stratigraphic thickness of the Chumstick Formation is about 12 km, however thermal maturity and geophysical data suggest an actual basin thickness of about 2 km. This discrepancy in thickness is explained by a conveyor-belt sedimentation model.
Anisotropic lattice thermal diffusivity in olivines and pyroxenes to high temperatures
(2002) Harrell, Michael D., 1968-
The anisotropic lattice thermal diffusivity of three olivines (Fo 0, Fo78, and Fo91), one orthopyroxene (En 91), and one clinopyroxene (Di72He9Jd3Cr 3Ts12) have been measured via impulsive stimulated light scattering, permitting the calculation of their lattice thermal diffusivity tensors to high temperatures. For Fo0 olivine, measurements extend from room temperature to 600°C, for Fo78 to 900°C, and for Fo91 to 1000°C, all in steps of 100°C. The orthopyroxene also was taken in steps to 1000°C, while the clinopyroxene was measured at room temperature. A limited set of room-temperature measurements to 5 GPa on a fourth olivine (Fo89) is also included. Diffusivities have been combined with calculations of density and specific heat to determine the lattice thermal conductivity tensors. An earlier theory that explains the observed behavior in terms of a positive lower bound on the phonon mean free path is discussed, and the data are used to constrain a model of thermal conductivity at high temperature. The relative contributions of optic and acoustic modes are evaluated from analysis of published dispersion curves. Five conclusions are reached: First, the anisotropy of lattice thermal conductivity remains essentially unchanged over the observed range of temperatures, indicating that anisotropy remains significant under upper-mantle conditions, and, in regions displaying preferred alignment, may account for observed lateral variations in the geotherm. Second, thermal conductivity departs significantly from earlier predictions of its temperature dependence; this may be understood in terms of a phonon mean free path that cannot diminish below 1.75 times the mean interatomic spacing. Third, for olivine, the optic modes have group velocities that are approximately one-third those of the acoustic modes, and do not dominate lattice conduction despite their greater number. Fourth, impurity scattering is significant along the olivine Fe-Mg solid solution series, but is not appreciable near the endpoints and therefore likely does not play a major role in the upper mantle. Fifth, the historic underestimation of lattice thermal conductivity at temperature has led to an overestimation of radiative conductivity; radiative transport, although significant, plays an even smaller role in the upper mantle than has heretofore been assumed.
Experimental studies of deposition by debris flows: process, characteristics of deposits, and effects of pore-fluid pressure
(1996) Major, Jon J
This study examines deposition by experimental debris flows (to 15 m$\sp3$) composed of mixtures of gravel (to 32 mm), sand, and loam using the 95-m-long, 2-m-wide, 31$\sp\circ$ U.S. Geological Survey debris-flow flume. It examines the depositional process, relations between flow kinematics and deposit character, and fluid pressure in debris at, and following, deposition. These data permit evaluation of competing hypotheses regarding debris-flow deposition.Experimental debris flows invariably developed surges; deposits developed abruptly on a 3$\sp\circ$ runout slope as sediment transported by shallow $(<$10 cm deep) surges accumulated. Finer source debris formed thinner deposits. Deposits developed by a combination of forward pushing, vertical accretion, and lateral shunting of previously deposited debris. Unsaturated debris flows deposit sediment dominantly through forward pushing and sourceward horizontal accretion; deposits are lobate, have large aspect ratios $(\geq$0.5), and commonly exhibit arcuate surface ridges. Saturated debris flows progressively deposit sediment primarily through vertical accretion from successive surges; deposits are lobate, but elongate, have small aspect ratios $(\leq$0.3), nearly flat surfaces, and lack prominent surface ridges. Observed progressive accretion is contrary to commonly assumed en masse sedimentation by debris flows.The depositional process is recorded primarily by deposit morphology and surface texture and is not faithfully registered by interior sedimentary texture. Homogeneous internal textures can be interpreted as the result of deposition by a single surge. Individual debris flows as well may leave little distinctive signature in the sedimentary record. Superposed deposits from similar yet separate flows could not be distinguished without the aid of an artificial marker horizon. These results show that methods of estimating flow properties from deposit thickness or from relations between particle size and bed thickness are in error. Relations between sediment composition and deposit thickness are incompatible with deposition by a simple viscoplastic material.Experimental debris flows deposited sediment despite measured basal fluid pressures that were lithostatic to nearly lithostatic. These data refute hypotheses that propose uniform fluid-pressure dissipation as a control on deposition. Modeling and laboratory analyses of gravity-driven consolidation reveal that characteristic pressure-dissipation times in quasistatic debris exceed surge periods and durations typical of debris flows. Numerical simulations of transient changes in fluid-pressure and effective-stress fields in 2-dimensional quasistatic domains reveal that excess fluid pressures remain elevated, and effective stresses depressed, everywhere except adjacent to margins in wide thin bodies for time scales that exceed durations typical of debris flows. Observed deposition, measured fluid pressures, and modeling results suggest that debris-flow deposition is controlled by a balance between a diminishing driving stress and locally increasing resisting stresses along flow margins rather than by a uniform bodywide increase of effective stress.
Fluvial recovery following basin-wide sediment loading at Mount Pinatubo, Philippines
(2005) Gran, Karen Bobbitt
The June 1991 eruption of Mount Pinatubo, Philippines, was the second largest of the 20th century, emplacing 5--6 km 3 of pyroclastic-flow material and creating record high sediment yields on rivers draining the volcano. This thesis explores landscape response to and recovery from sediment loading in the 1991 eruption from the drainage basin scale to mobility of individual clasts on the bed. Basin-scale erosion patterns were studied on the Pasig-Potrero and Sacobia Rivers for one decade following the eruption using digital elevation models, multispectral imagery, geomorphic terrain maps, and field observations. In the latter half of the decade, channel recovery was examined more closely through a multi-year field campaign on five east side basins measuring flow and channel parameters, grain size distributions, roughness, sediment mobility, and sediment transport. The details of bed coarsening and sediment transport under conditions similar to those at Mount Pinatubo were explored through physical experiments modeling future conditions as sand is depleted.Sediment yields on the Pasig-Potrero and Sacobia Rivers declined exponentially for the first decade following the eruption. Decay coefficients were inversely correlated with initial sediment loading. Erosion of pyroclastic flow deposits from valley incision, widening, and extension dominated sediment yields as early as 1991. Ten years later, sediment yields remained 20 times higher than pre-eruption levels. In the most disturbed basins, mainstem channels were braided and highly mobile. Coarse clasts moved independently over a smooth bed of sand and fine gravel. As sediment yields declined, selective transport preferentially removed sand and pumice, starting in the upper basin, causing downstream fining and gradients in pumice content. As sand content on the bed declined, coarse clasts interacted, leading to cluster formation and decreasing clast mobility. As sand content declines, bed reorganization into coarse and fine patches enables transport rates to remain high over a wide range of sand contents. In the near future, sediment yields on the Pasig-Potrero and Sacobia Rivers should remain elevated rather then declining to pre-eruption levels, sending an additional 14 million m3 of sediment downstream in the second decade after the eruption.
Debris flow fans: process and form
(1994) Whipple, Kelin X
Lithologic, climatic, and tectonic influences on debris-flow-dominated alluvial fan (debris-flow fan) morphology are elucidated through a combination of field mapping, map and aerial photo analysis, and a theoretical analysis of the controls on debris-flow confinement within channels. Fan size in Owens Valley, California, is controlled primarily by source drainage area and local subsidence rate, as seen elsewhere. However, spatial variability in subsidence rates is shown to be important, and may largely control fan area-drainage area relationships. Depositional patterns on debris-flow fans are governed by interactions between debris flows and channels, which are products of fluvial incision and not debris-flow scour. Channel geometry (i.e., size and gradient) is set by competition between fluvial sediment supply, stream power, and the rate of channel filling by debris flows. The channel-debris flow interaction is controlled by channel geometry, flow volume, flow hydrograph form, and flow rheology. The frequency distributions of these debris-flow properties determine fan morphology. I focus on the role of debris-flow rheology, which is sensitive to debris granulometry and water content. Abundance of boulders and debris silt-and-clay content are important, lithologically controlled properties. Hydrologic conditions associated with debris-flow initiation are hypothesized to be the most important climatically controlled variable. Numerical simulation of open-channel flow of Bingham fluids is utilized to quantify lithologically dependent differences in debris-flow rheology and to analyze the channel-debris flow interaction on fans in a range of field settings. Differences in source lithology exert the dominant influence on debris-flow fan morphology in Owens Valley. Debris-flows from the most fines-rich sources (meta-volcanic source rocks) are more viscous at the same water content (i.e., initiated under the same conditions) than granitic-source debris flows and, consequently, are poorly channelized. As a result, massive overbank deposition near the fan head is common on these fans and they develop steeper radial profiles. Similarly, enhanced overbank deposition leads to the formation of prominent channel-margin levees.
Geology of the south and east slopes of Mount Adams volcano, Cascade range, Washington
(1976) Hopkins, Kenneth Donald
Nature and origin of variations in late-glacial and Holocene atmospheric 14C as revealed by global carbon cycle modeling
(1990) Braziunas, Thomas F
Simulations with a global box-diffusion $\sp$C model indicate that the millennium- and century-scale atmospheric $\Delta\sp$C variations during the Holocene are more likely explained by fluctuations in $\sp$C production rate (Q) than by changes in air-sea CO$\sb2$ exchange rate (F) or internal ocean mixing (parameterized as an "eddy diffusivity" K$\sb{\rm z}$. The $\sp$C reservoir model deconvolves histories for each of these three processes that are compatible with a 96-yr bi-decadal atmospheric (tree-ring)$\Delta\sp$C record assuming alternative pre-Holocene $\sp$C conditions. Holocene microparticle concentrations in ice cores and dust grain sizes in marine sediment cores disagree with the model-derived global wind speeds necessary to explain (through F variations) the millennium-scale trends in atmospheric $\Delta\sp$C. Alternately, foram $\sp$C data do not support the history in the oceanic ventilation index generated by millennium-scale K$\sb{\rm z}$ variations. Coral $\sp$C data for recent centuries conflict with the marine $\Delta\sp$C history associated with century-scale variations in F or K$\sb{\rm z}$ but are consistent with changes in $\sp$C production rate.The $\sp$C production rates derived theoretically from an 11,000-yr record of averaged global dipole moments strongly correlate with the Q history required to explain tree-ring $\Delta\sp$C. Several pre-Holocene Q histories were calculated from limited dipole moment data available for the past 30,000 yrs and do not contradict $\sp{234}$U/$\sp{230}$Th-calibrated coral $\sp$C measurements. Relative variations in Greenland ice-core $\sp$Be concentrations (reflecting changes in $\sp$Be production) over the past 9000 yrs also correlate strongly with tree-ring Q fluctuations except for a 4500-3500 BC discrepancy.Simulations of transient variations in Q, F, and K$\sb{\rm z}$ supplement previous studies of alternative steady-state $\sp$C situations. The modeling of combined climate and production rate scenarios (i.e. F + K$\sb{\rm z}$, F + Q, K$\sb{\rm z}$ + Q) incorporates "feedback" effects which depend on the instantaneous marine $\sp$C profile and atmosphere/surface ocean $\sp$C disparity.Spectral analyses of the 9600-yr tree-ring Q history generally produce power at or near harmonics of a 420-yr cycle. Century-scale Q periodicities may reflect harmonic or nonsinusoidal solar processes. Residual $\sp{14}$C production and bi-decadal sunspot numbers from AD 1700 to 1840 define a preliminary history of relative sunspot numbers back to 7730 BC. "Fine-tuned" Q periodicities are used to forecast future solar behavior.
The role of oceanic lithosphere in inter- and intra-volcano geochemical heterogeneity at Maui Nui, Hawaii
(2004) Gaffney, Amy Michelle
The volcanoes of Maui Nui (West Molokai, East Molokai, Lanai, West Maui, Haleakala and Kahoolawe) record Hawaiian magmatism at ∼1--2 Ma. These volcanoes nearly span the compositional range erupted from all the Hawaiian volcanoes over the past 5 My, and erupt lavas representing both the Kea and Koolau compositional endmembers of Hawaiian lavas. Using lavas from these volcanoes, we evaluate the role that oceanic lithosphere, both ancient recycled lithosphere in the Hawaiian plume and modern Pacific lithosphere, plays in the generation of geochemical variability on inter- and intea-volcano scales. We also present trace element models that explicitly address the petrologic complexities of melting eclogite (derived from ancient oceanic lithosphere) in the plume. Trace element, major element and isotope compositions of Lanai are consistent with the origin of these lavas in large degree (∼70%) melts of ancient upper oceanic crust (basalt + sediment) that mix with plume-derived Haleakala-type melts. Trace element and isotope compositions of West Maui and East Molokai are consistent with an origin in ancient depleted oceanic lithosphere that has been re-fertilized with moderate-degree melts (20--40%) of associated crustal gabbro. The physical mechanisms through which the oceanic lithospheric components melt and mix within the plume lead to the generation of isotopically homogeneous Kea-type lavas and isotopically heterogeneous Koolau-type lavas.Stratigraphically-controlled sequences of late shield-building stage lavas from West Maui volcano show age-dependent compositional variability distinct from that seen in shield-stage lavas from any other Hawaiian volcano. These distinctions are defined by 206Pb/204Pb 207Pb/204Pb variation as well as 87Sr/ 86Sr correlation with 206Pb/204Pb, 187Os/188Os and trace element compositions. The 87Sr/86Sr-206Pb/204Pb and 87Sr/86Sr-187Os/188 Os variation in the deep lavas is orthogonal to the all-Hawaii variation, indicating that it is not the result of mixing between components normally sampled by Hawaiian shield-stage magmas. We compare our West Maui data to observed compositions of Pacific oceanic basaltic and gabbroic crust, and predicted compositions for 2 Ga basaltic and gabbroic oceanic crust. The observed fine-scale compositional variability in the stratigraphically deep West Maui lavas is consistent with 10--15% mixing of small degree (2%) partial melts of the Pacific gabbroic oceanic crust with plume-derived, Kea-type magmas.
The use of streambed texture to interpret physical and biological conditions at watershed, reach, and subreach scales
(1998) Buffington, John M
Physical controls on bed-surface grain size and consequent implications for aquatic habitat are examined at a variety of spatial scales. Data compiled from laboratory studies show that reach-average median surface grain size (D50) varies inversely with sediment supply, while field studies of forest gravel-bed rivers conducted here demonstrate that D50 varies directly with bed shear stress as modified by hydraulic resistance due to channel walls, bars, and wood. Textural response is evaluated relative to a theoretical prediction of competent D50. I hypothesize that surface textures are altered through size-selective erosion or deposition caused by imbalances between sediment supply and transport capacity. Moreover, changes in surface texture that smooth or roughen the bed should alter both the boundary shear stress and the critical shear stress of grains moving over the bed, thereby creating a feedback process between bed-surface texture, sediment flux, and supply-transport state. At subreach scales, my field studies demonstrate that bed surfaces are composed of textural patches, the number and frequency of which varies directly with the magnitude and frequency of hydraulic roughness, due presumably to greater spatial divergence of transport capacity and sediment supply forced by the roughness elements. Bed-surface textures were classified using a new procedure that combines visual and quantitative measurements of patch grain-size distributions and offers reasonable statistical discrimination of differences in both mean grain size and variance between textural patches.I further examined the correspondence between bed shear stress and surface grain size in two forest pool-riffle channels using a theoretical shear-stress partitioning model. Reach-average velocities calculated from the model are within 7--8% of those measured in the field using a salt-tracer, providing support for the model. Observed values of D50 are in close agreement with those predicted from modeled bankfull bed stresses, indicating that surface grain size of the sites is in quasi-equilibrium with bankfull channel hydraulics.Greater textural variation at subreach scales creates a broader diversity of aquatic habitats for animals that rely on specific substrate sizes. Furthermore, textural fining caused by bar and wood roughness has the potential to create usable salmonid spawning-gravels in channels that otherwise would be too coarse. Coupling field measurements and theoretical predictions via digital elevation models, I propose a watershed-scale framework for assessing the potential influence of hydraulic roughness on salmonid spawning-habitat availability. Application of the model indicates that textural fining caused by bar and wood resistance can control more than 90% of the potential spawning habitat in mountain drainage basins studied here, with bar roughness most important for lower-mainstem reaches (41--73% of the potential habitat), and wood roughness important for upper-mainstem channels (25--53% of the potential habitat).
Glacier dynamics and the development of glacial landforms in the eastern Puget lowland, Washington
(1984) Booth, Derek B
In the Late Pleistocene, the Puget lobe of the Cordilleran ice sheet covered the Puget lowland in western Washington to an average depth of 1000 m in the Skykomish-Snoqualmie region and abutted against the western front of the Cascade Range.Reconstructions of the extent, altitude, and mass balance of the Puget lobe lead to an estimate of its sliding velocity and basal water flux. Lobe dimensions are inferred from ice limits and flow-direction indicators. An equilibrium-line altitude of 1220 m is calculated for the maximum of the Vashon advance. Resultant average sliding velocities range up to 600 m/a and average meltwater discharges up to 2500 m('3)/s.Deeply incised valleys in the eastern lowland reflect considerable erosion primarily by subglacial meltwater. Many of these channels were continuously occupied by water, whereas a broad, submarginal channel carried highly variable discharge as a result of episodic, catastrophic drainage of ice-marginal lakes. The analysis of subglacial water flow is expanded to include the effects of sliding ice, providing a physical model for the development of subglacial fluvial landforms.Morainal embankments of glacial, fluvial, and lacustrine sediments fill the mouth of each alpine valley emerging into the east-central lowland. They reflect subglacial sedimentation at and near the grounding line of the Puget lobe adjacent to ice-dammed lakes. Analysis of the physical behavior of ice and water, particularly by detailed reconstruction of the hydraulic potentials, provides geologic insights regarding the location and internal composition of these embankments.Basal stress conditions control both depositional processes and physical properties of subglacial sediments. Beneath rapidly sliding ice, till, which averages 5-10 m in thickness in the lowland, can accrete only by the accumulation of sediments released by basal meltout. High pore pressures are expected under the glacier except within a few km of the margin, irrespective of subglacial tunnels or till properties. These pore pressures are responsible for low till strength and consequent deformation of subglacial sediments by moving ice, and for overconsolidation values significantly less than the maximum overburden.

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