Geology

Geologic and Physiographic Setting

The Nulato Hills are characterized by rolling hills and ridges with only a few alpine summits reaching above 3000 ft. elevation (915 msm) (Figure 2). The highest peak is VABM Gross at 3697 ft. (1127 msm) on the northeast margin of the region. Numerous incised, V-shaped drainages produce a well-developed reticulate drainage pattern on the landscape. The entire area was unglaciated during the Quaternary period, although a few perennial snow banks may have persisted at the highest elevations (W.W. Patton, Jr., pers. comm.). The Nulato Hills would have been centrally located within Beringia, the broad, exposed land bridge and biological refugium linking northwestern North America and Asia during the Quaternary full glacials.

The regional country bedrock consists primarily of Cretaceous sedimentary rocks which have been deformed by a complex northeast trending fault and fold system (Patton et al. 1985, 1994, 1996). There are numerous Tertiary and late Cretaceous volcanic intrusions scattered throughout the region which have altered the sedimentary rock along their respective contact zones. Bedrock, and select geomorphic features, varied among the several areas visited during the survey. Plant species composition is often strongly influenced by slope stability, by the occurrence of calcareous rock, and by the relative abundance of fines in the substrate (Parker 1989), hence bedrock at the alpine sites visited are described here in more detail.

The most massive alpine locality visited was Debauch Mountain in the north central area of the Nulato Hills (Figure 3). Bedrock here includes fine-grained to conglomerated volcanic graywacke and fine-grained mudstones (Patton et al. 1994). Active blockslopes in the vicinity of the summit were composed predominantly of blocky debris, but fines were abundant, and often deep, in many widely scattered pockets. Bedrock was primarily of volcanic origin and the substrate was non-calcareous.

VABM Chiroskey, an isolated, dark-colored massive dome south of the Unalakleet River and east of the Chiroskey River, is composed of a complex of numerous small volcanic intrusives and the associated altered sedimentary country rock (Patton and Moll-Stalcup 1996, Patton et al. 1994). As on Debauch Mountain, active blockslopes in the vicinity of the summit are mostly composed entirely of boulders, but localized pockets of deep fines with scattered cobbles on the surface exist. This site is generally non-calcareous, although pockets of calcareous substrate may exist where sedimentary rocks are exposed.

Alpine ridges and screes visited above the North Fork Unalakleet River drainage south and east of Debauch Mountain seemed to be composed of more fine-grained and platy bedrock relative to the alpine sites noted near the summit. Screes here often consisted of deep, well-weathered fines with scattered coarser fragments. These sites may lie within the fluvial and marine sandstone and shale that is mapped at the regional scale for this vicinity (Patton et al. 1994). The substrate, where tested, was non-calcareous.

The subalpine ridge systems on both sides of the South River valley are composed of sandstones, shales, and pebble conglomerates (Patton and Moll-Stalcup 1996, Patton et al. 1994). The active blockslopes exposed at several sites along these ridges were rich in fines and often had a partial mulch of cobbles. The bedrock exposed here originated from shallow marine deposits and is known to contain marine mollusk fossils. The substrate was calcareous at our site (ridges between South R. and Rabbit Cr.) and the species composition reflected this.