Juneau Nature

Natural & cultural history of Juneau & Southeast Alaska

Juneau Nature - Natural & cultural history of Juneau & Southeast Alaska

Auke2 geology

This is the content of the 2nd in a 3-part sign array on Auke Lake Trail. See also the other 2 panels: Auke1 intro and Auke3 biocultural.

1geo

Geology   Hillsides surrounding Auke Lake are glacially rounded bedrock knobs. Gentler terrain is blanketed with surficial deposits (stippled) including: 1) fine marine sediments from times of higher sea level (pale green); 2) gravel-cobble alluvial fan where streams meet the northern lake shore (tan); 3) colluvial or landslide deposits (pink) on steep slopes. Poorly-drained, fine marine sediments support small trees, while alluvial and colluvial deposits (especially those on slopes sheltered from southeast winds) support larger trees.
Bedrock from Ford & Brew. 1973. USGS Misc. Field Studies Map MF 527.
Surficial geology from Miller. 1975. Misc. Investigations Series I-885.

Rock, sea & ice

To make sense of the topography of Auke Bay and Lake, it helps to visualize the comings and goings of ancient rock masses, mile-deep glaciers, and migrating seas. The landscape’s basic structure is set by NW-SE bedrock alignment, reflected on a grand scale by the Gastineau Channel fault traversing the entire CBJ, and on a fine scale by NW-SE fissures in rock outcrops. Successive ice ages exploited the weakness of fault zones, carving Lynn Canal, rounding off the “soft” rocks such as slate, and leaving the hardest granite and gneiss (Mendenhall Towers, Stroller White) high and jagged. Finally, the bedrock “skeleton” was fleshed-out with loose, surficial deposits (stippled on map) of marine silt, glacial boulders, and stream-carried gravel.

2-20000

Glaciation series: 20,000 yrs before present (BP) At the height of the Great Ice Age, only Stroller White Mtn, 5,150 feet, stood free of the ice. The gigantic Lynn-Seymour Glacier excavated Auke Lake on a NW-SE axis, parallel to the “grain” of the bedrock (compare fault alignment).

3-14000

14,000 BP  Lynn-Seymour glacier had melted, but proto Mendenhall Glacier still calved into the sea. Erosive forces were briefly directed at Auke Lake from the northeast.

4-9000

9,000 BP The glacier had receded well upvalley from today’s position. Land was slowly rising from millennia of depression under mile-deep ice, but relative sea level was still 200 feet above today’s. Humans had occupied Southeast Alaska since at least 10,3000 BP; were some of them hunting seals in Mendenhall Bay?

big tree in woods

The largest Sitka spruce measured near Auke Lake grows on steep colluvial deposits (pink on geology map) in a position sheltered from periodic gales. It’s 170 feet tall and more than 500 years old. Even at 10 feet above ground level, well above the root swell, this majestic tree is 5 feet in diameter

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