In Glacier Bay, southeastern Alaska, meltwater streams carry high sediment loads into Qords. Sea-floor sediments in the proximal marine areas of glacier-fed deltas exhibit prominent layering predominantly composed of: (1) couplets of coarse laminae grading to fine laminae, (2) solitary massive to graded layers, and (3) microlaminae. Sediment layers form from pulsations in sediment influx. Our investigations have shown that sedimentation rates vary as a result of cyclic and noncyclic processes on several time scales. Semi-diurnal and diurnal tidal stages, diurnal discharge fluctuations, fortnightly neap-spring cycles, and a distinct melt season may result in a temporal hierarchy of sea-floor sediment stratification. During the melt season, graded couplets are produced on the order of 1 to 2 per day by suspension settling from sediment pulsations due to reentrainment of delta plain sediment and encroachment of stream channel mouths during ebb of the macrotidal prism. Diurnal discharge fluctuations appear to play a lesser role in sediment layer production. Sequences of relatively thin fine and thick coarse layers are deposited during neap and spring tides, respectively. Sediment gravity flows from the delta front occur mainly during lower low spring tides when delta plains are completely exposed and fluvial bed load is transported directly to the delta plain edge. Massive to graded layers result, as well as graded couplets from small sediment gravity flows. Microlaminae may be produced in short intervals by several processes such as minor transport fluctuations in plumes and other transporting currents. The processes and sedimentation products described here are probably not restricted to temperate glacial marine deltas, but could be expected in other regions characterized by high tidal amplitudes and sediment influxes.

Climate is the first-order control on fluctuations of glacial tidewater termini. However, factors affecting relative water depth such as eustasy, isostasy, and sediment yield are important second-order controls on termini fluctuations. Deposition of grounding-line systems, which include ice-contact deltas, morainal banks, and grounding-line fans, can change relative water depth and consequently alter the stability of tidewater termini independent of climate. Therefore, the rate at which sediment accumulates in these grounding-line systems can control the rate of terminus movement. Modern temperate glaciers at Glacier Bay, Alaska, on average produce about 10 6 m 3 /yr of sediment, most of which accumulates at grounding lines. Sediment volume decreases logarithmically downfjord from a grounding line. In Glacier Bay, readvance rates that average about 10 m/yr are one to two orders of magnitude slower than retreat rates. Rates of glacial advance may be a function of sediment yield (determined by drainage area and rates of glacial erosion, transportation and debris release), total sediment volume of grounding-line systems (determined by fjord width, fjord depth less the maximum water depth for terminus stability, and angle of repose of sediment making the system), and sediment dispersal patterns (determined by process of debris release and dispersion, type of sediment, and marine currents). Consequently, average rates of advance are slower than retreat rates because of the time required for sediment to accumulate as a bathymetric high at a grounding line, which effectively decreases relative water depth and increases terminus stability. Retreat rates are faster because they occur during terminus instability in deep water and cannot be altered by sediment accumulation rates until quasi-stability of the terminus is caused by other means, such as bedrock pinning-points or a decrease in ablation area. As marine-ending glaciers expand, total sediment delivered to a grounding line increases; as they retreat, sediment volume decreases because of the change in size of the drainage basin. Grounding lines are the major depocenters receiving this sediment, and resulting lithofacies packages and geometries are primarily controlled by grounding-line movement over a glacial advance-retreat cycle. During glacial minima, fjords trap sediment while the continental shelf and slope are starved. During glacial advances and retreats, fjords are eroded while the continental shelf receives sediment and the slope may remain starved. At glacial maxima, fjords and commonly the shelf are eroded, and the continental slope receives sediment. Because grounding-line advance and retreat may or may not coincide with eustatic sea-level changes, the sedimentary record may vary depending on environmental settings and the magnitude of glacial cycles.

G. K. Gilbert, as a member of the Harriman Alaska Expedition of 1899, studied and described nearly 40 glaciers, many of which reached the sea and produced icebergs. Gilbert’s maps and photographs from marked locations are still being used to record glacier fluctuations, as at Columbia Glacier. Noting that some termini were stable or advancing but that others were retreating rapidly, he suggested that a general change in climate, perhaps related to a change in ocean temperature, might cause such local differences in behavior. This conclusion was remarkably prescient, but it is now known that terminus stability is also involved. Gilbert’s discussions of the processes of glacier flow adjustment to an uneven bed, glacial erosion (including erosion below sea level), and variations in the rate of iceberg calving are remarkably modern and relate to one of the most important problems in glaciology today—the role of a water layer in coupling a glacier to its bed.