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Pacific Northwest Geologic Mapping and Urban Hazards

 

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Geomorphology and surface processes
Bainbridge Island
Subglacial surfaces

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Geomorphology and surface processes

The landscape, carefully observed, tells the story of its own making. Glacially scoured upland, diffusionally softened ridgecrest, landslide, alluvial river bottom--collectively they record a sequence of processes and events that led to the present landforms. The hills are not unchanging!

An effective way to read the landscape is to make a geomorphic map that shows the distribution of different surface units. The map guides and documents interpretations of the age and genesis of these units. Geomorphic maps are not geologic maps, as they do not show the distribution of rocks and sedimentary deposits. Where surface units correspond to underlying rocks or sediments, the geomorphic map is a useful proxy for a geologic map. The geomorphic map can also show parts of Earth history that the geologic map is blind to: geologic maps are limited by the depositional record, whereas geomorphic maps can demonstrate history that leaves an erosional record only.

Traditionally, geomorphologists (geologists who study the shape of the land) examined the landscape with their own eyes in the field and indirectly via air photos and topographic maps. This has been an imprecise process because these tools, especially in wooded areas, have not fully shown the shape of the land. Transferring interpretations from a field view or a photo to a base map is tedious and often imprecise. Lidar topography--accurate, highly detailed, fully geo-referenced, and digital--will revolutionize interpretation of the landscape.

Bainbridge Island geomorphic map
Click to enlarge.

The image at left is part of a draft geomorphic map of Bainbridge Island, by R. Haugerud, that identifies different surfaces on the basis of their morphology, position, and relative geometry.

Yellow areas are large, deep-seated landslides. This map provides the most complete and consistent inventory of large landslides yet created for any part of the Puget Lowland.

During the last ice age, a thick sheet of ice flowed south across the Puget Lowland. At the latittude of Seattle and Bainbridge Island, the ice sheet was as thick as 3,000 feet. Flowing ice made distinctive N-S flutes, or scour marks, that are visible on the map fragments here and on a 290 kB shaded-relief image of Bainbridge Island.

Bainbridge Island geomorphic map enlargement (43 KB).
Bainbridge Island geomorphic map enlargement (168 KB).
Shaded-relief image of Bainbridge Island (290 KB).

Bainbridge Island geomorphic map Click to enlarge Click to enlarge Click to enlarge Click to enlarge
Click any part of image to enlarge.

4 kinds of subglacial surface are evident on Bainbridge. Glacially fluted surface (unit gf) is typical of most of the Puget Lowland. Locally, transverse ribs (some identified with red lines at left) underlain by less-easily-eroded conglomerate indicate that the fluting is developed on bedrock (unit gbr, glaciated bedrock). In places the fluting is disrupted and the surface appears pockmarked (unit gp). Elsewhere flutes are modified by transverse ripples (unit gc).

Profiles of these surfaces are shown at lower right.

The vertical axis on this Correlation of Map Units diagram from the Bainbridge geomorphic map shows relative age with oldest at the bottom. Ages are assigned largely on the basis of geometric relations between mapped units.

Correlation of map units
Click to enlarge.

The vertical axis on this Correlation of Map Units diagram from the Bainbridge geomorphic map shows relative age with oldest at the bottom. Ages are assigned largely on the basis of geometric relations between mapped units.

For more images like these visit the Puget Sound LIDAR Consortium website.

This site is maintained by the Pacific Northwest Urban Corridor Geologic Mapping Project, part of the Geology, Minerals, Energy and Geophysics Science Center

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