The majority of the park overlies surficial deposits derived primarily from graywacke, schist, and phyllite. These surficial deposits include alluvium on Indian River's floodplain, estuary, and stream terrace; ablation till on the lateral moraine; and beach sands and gravel on the uplifted beach and uplifted beach meadow. The source rocks for all these deposits are the steep mountain sideslopes and cirque walls, located at the head of Indian River and its tributaries, which were formed during local alpine glaciation.

 

Deglaciation occurred sometime before 10,000 years ago. The land mass associated with the park was under water during the marine transgressions resulting from deglaciation. Since then, the beach deposits have been worked many times by wave action and Indian River. Deglaciation was followed by isostatic rebound, the upward movement of a land mass responding to the removal of the thick mass of ice from the glacier. It is estimated that the total rebound to present in the Sitka area has been approximately 35 feet. Rebound is now occurring in the Sitka area at approximately 0.13 inches per year.