Figure 10. Shore-perpendicular topographic profiles west (left) and east (right) of the Fairweather fault at Icy Point. Profile locations shown in Figure 4. Topographic profile L–L’ (left) crosses Terrace A west of the Fairweather fault. Higher emergent shorelines on Terrace B only occur west of the fault (Figure 4). The elevation of the highest beach ridge crest on Terrace A is equivalent to the highest beach ridge crest on the Palma Bay barrier beach located east of the Fairweather fault. Topographic profile M–M’ (right) crosses the barrier beach fronting Palma Bay east of the Fairweather fault and shows shorelines uplifted in historic times due to glacial isostatic adjustment (’LIA beach ridge’) caused by loss of ice mass in Glacier Bay near the end of the Little Ice Age (LIA). Also shown on profile M–M’ is a channel (’Stump Slough’) incised into alluvial deposits that records the inception of incision linked to glacial rebound.

Landward of the LIA-age beach ridges east of the Fairweather fault, cutbank exposures in a tributary slough of Kaknau Creek (Stump Slough, Figure 10) record centuries of alluvial flooding that drowned and buried spruce trees in a coastal forest. Channel incision accompanying post-LIA isostatic rebound has exposed growth-position, subfossil trees. The stratigraphically lowest sub-fossil trees exposed in Stump Slough died near the turn of the 13^th century between 1290 and 1310 CE. Typically, the outer rings of the stumps were too narrow to provide enough materal for ¹⁴C dating, so we sampled larger interior rings. To estimate the time the tree died, we used a Bayesian model that incorporates ¹⁴C dates of the 16^th and 26^th rings sampled from stump #34 exhumed in the slough channel (Table 2, Figure 8). Alluvial sediment buried another stand of trees at 1710–1730 CE, entombed 4 m higher in the cutbank, based on dating rings 29 and 79 sampled from stump #33 (Table 2, Figure 8). The higher trees lie ~1 m below the top of the cutbank, which marks the level of highest alluvial aggradation. After 1730 CE, channel incision exposed the buried forest stratigraphy in the slough cutbank (Figure 10).

We interpret the Terrace A geomorphology and stratigraphy of Stump Slough in the context of RSL fluctuations caused by isostatic adjustment during the LIA (Mann and Streveler, 2008). The LIA began around 1300 CE, when ice advanced down Glacier Bay and into Icy Strait (McKenzie and Goldthwait, 1971) causing regional isostatic depression and a high-sea stand (Motyka, 2003; Larsen et al., 2005). We attribute the highest beach ridge crests on Terrace A and in the barrier complex facing Palma Bay as shorelines recording the LIA high-sea stand (Figure 10). Regional isostatic rebound and RSL fall initiated in 1750–1800 CE when the retreat of marine-based ice in Glacier Bay began to accelerate. Along the coast of Palma Bay, the stratigraphy exposed in Stump Slough records the drowning of spruce trees in 1290–1310 CE caused by RSL rise in the beginning of the LIA, which raised the base level of Kaknau Creek. The highest buried stumps in the profile show that the creek continued to drown trees as late as 1710–1730 CE—just a few decades before the sudden retreat of Glacier Bay ice caused isostatic rebound.

4.5 Terrace B: A sequence of uplifted Holocene shorelines

Terrace B is a composite, time-transgressive surface that steepens toward the sea and is etched by 9 to 12 shorelines. These shorelines, mapped on lidar DEMs and verified in the field, create a stair-stepped profile in the lower half of Terrace B and represent paleo-sea cliff and shore platform junctions (Trenhaile, 1972; Kelsey, 2015). Each higher, older shoreline has the same general planform as the one below (Figure 11). The highest shoreline follows the base of the sea cliff separating Terraces B and C. The shorelines record episodes of sudden shoreline emergence along an uplifting coast west of the Fairweather fault (Figures 9 and 11). In the field, we confirmed that these shorelines (sea-level indicators) mark the paleo-sea cliff and shore platform junctions (shoreline angles) and provide an indicative meaning of approximately mean high water (MHW) (e.g., Kelsey, 2015).

The longshore preservation of individual shorelines varies across Terrace B. Near Icy Point, the southern part of Terrace B preserves 9–12 shorelines that climb to a maximum elevation of about 50 m. We map these shorelines as continuous landforms based on their geomorphic expression and elevation (Figure 11). In the central part of Terrace B, few shorelines are preserved, and a ~40 m high uplifted sea cliff, probably eroded during the LIA, backs Terrace A. The northern extent of Terrace B includes 9–10 shorelines expressed as prominent paleo-sea cliffs and more subtle shore-parallel, slope breaks and ridges. The oldest and highest shorelines along the northern part exceed the elevation of the highest shoreline in the southern part of Terrace B and reach a maximum elevation of ~70 m (Figure 9), where the Finger Glacier fault vertically displaces Terrace B (Figure 4).