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    National Historical Park Alaska


A view of the soils in the park.

NPS Photo

Soils in the park exhibit the relative ages of each of the landforms. The upland terrace and lowlands have soils with the greatest development, suggesting they are the oldest landforms (Spodosols). Typically these are well drained shallow soils. Spodosols have a well developed B horizon which is a subsurface layer of iron and/or humus accumulation. One of the other soils found in the lowlands belong to the order Andisols which are formed in volcanic ash. The third soil in the lowlands is a Histosol which is basically organic material over bedrock at depths less then 20 inches.

Next in relative age are the uplifted beaches, stream terrace (old floodplain), and present floodplain. The floodplain has greater soil development but is lower then the stream terrace, suggesting it is the younger formation. They all belong to the soil order Inceptisols. They are less developed then Spodosols in the upland terrace and lowlands, having a slightly developed B horizon (cambic horizon). The name Inceptisols implies a soil at its inception or beginning. Greater soil development in the floodplain may be due to the increase in flooding of the area and the growth of red alders which has the ability to fix atmospheric nitrogen, making the site more productive and hence able to accumulate more organic matter.

The youngest landforms are the estuary and the beach meadow. The beach meadow is actually part of the uplifted beach, but is still influenced by high tides during storm and extreme tidal activity. Soils in these landforms belong to the soil order Entisols. They are the most recent soils in development and usually have A horizons (mineral surface layer enriched with organic matter) overlying C horizons (layers of parent material or the case of the park, layers of sand and/or gravel reworked by stream and tidal influences).

Pleistocene-aged mafic-tephra deposits above 40 feet msl are wide spread in the Sitka area and average five feet thick. This represents volcanic eruption from the Mount Edgecombe volcano complex. A second ash fall occurred around 4500 years ago and can be seen in as a layer in the park’s upland soils.

Did You Know?

Map of Alaska and the continental United States

With 570,374 square miles, Alaska is twice the size of Texas and 1/5 the size of the rest of the United States. It stretches 2,400 miles east-to-west and 1,420 miles north-to-south.