Juneau Nature

Natural & cultural history of Juneau & Southeast Alaska

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

Surficial geology

Here’s a surficial geology map for the Juneau area. I used most of the geologic surface types of R.D. Miller, USGS, 1972, Some types were collapsed into broader categories: especially the many varieties of colluvial and raised-marine landforms. On the other hand, unit boundaries have been considerably adjusted and fine-tuned from 2013 DEM-generated bare-earth. Miller’s original map is available as a pdf from ADNR.
Zoom with your mouse roller. Click on any of the color coded units for a pop-up listing landform type, generating agent, and geologic age. For a legend, open >>, upper left. To view this map in ArcGISonline, select view larger map.

‘Flesh’ atop the ‘bones’ of bedrock geology

Surficial geology is the study of loose, unconsolidated material overlying the bedrock foundation. These layers were deposited, rearranged and eroded by glaciers, fluctuating sea level, streams, landslides, and bulldozers.

Surficial versus bedrock geology

Surficial versus bedrock geology

While bedrock geologists tend to think in time-frames of millions of years, the surficial story is usually faster paced, taking place over millennia and mere centuries. On the map above, I’ve divided landforms into 5 age groups (geol_age field):

  • triassic: applied only to a couple bare-bedrock roches moutonees near Mendenhall Glacier (for map of bedrock types, go here)
  • early holocene:  marine, glacial and alluvial formations dating back to shortly after the great ice age, ~10,000 years ago
  • neoglacial:  dating to just the last few millennia
  • little ice age:  mostly formed since peak of the last glacial advance in the mid-1700s
  • anthropocene:  built or excavated features mostly since 1950.

Two sub-studies of surficial geology addressed so far on this site are:

Glacial history

Glacial rebound