Geomorphology of coastal Olympic National Park

The surficial geology of coastal Olympic National Park (OLYM) was surveyed from 2012-2016 to help meet the goals of the Natural Resources Challenge Inventory Program of 1998. Coastal OLYM contains a rich collection of landforms created by the intersection of a variety of processes. The influence of glaciation and active tectonic processes on coastal processes makes this unit unique in NPS managed coastlines. This coastline is a stunning landscape created by the interaction of dynamic tectonic, coastal, glacial, mass-wasting and fluvial processes of erosion and deposition. These processes created 802 distinct landforms, from the rare (two tombolos) to the plentiful (hundreds of sea stacks) and the inviting (74 sand beaches) to the treacherous (26 headlands). Glacial landforms, including moraines, ice-walled lake plains and drift mantled bedrock hills are the most extensive landforms. Second in abundance are glacial outwash terraces deposited during the ice age at the mouths of the Hoh and Quillayute rivers. Landforms common on most coastlines that we did not observe, due to the Olympic coast’s active tectonics and high wave energy, include long barrier islands, deltas at the mouths of large rivers and extensive sand dunes. We identified three distinct sections of coastline, where certain types of foreshore landforms are associated with backshore and tectonic conditions. In the northernmost section, from Shi Shi Beach to Cape Johnson, the Cordilleran ice sheet (CIS) inundated the coast, bringing erratic rocks from Canada and shaping the hills and valleys. Thrust faults are also common near the coastline in this section. These faults are linked to the formation of headlands, sea stacks and wave cut platforms. Point of the Arches stands out due to its many sea stacks, an unusual collection of folded thrust faults and igneous rocks of Mesozoic age. The sedimentary bedrock along most of the coastal zone is much younger. From Cape Johnson to Hoh Head, glacial deposits are absent or severely eroded, giving this section of coast a slightly different geomorphology although the thrust faults (and associated landforms) remain. Coastal bluffs in this area are mostly composed of intensely weathered bedrock that is failing by deep-seated slumping, particularly where undercut by winter waves. Beaches are smaller and crescent shaped, tending to be coarser-textured. South of Hoh Head to Kalaloch, the coastal bluffs consist of layers of alpine glacial outwash and occasional beds of peat and lacustrine deposits of silt and clay. No nearshore thrust faults have been mapped in this section of coastline and there are few wave-cut platforms. Instead, long wide sandy beaches are the dominant landform. Along the entire coast backshore zone, coastal bluffs are 37% bedrock and 41% glacial drift, with approximately 15% of the backshore area composed of floodplains and terraces, concentrated near the mouths of the Hoh and Quillayute rivers. Many of the coastal bluffs are failing due to wave undercutting and pervasive chemical and physical weathering under several hundred inches of rain that falls annually and mild coastal temperatures. We identified 45 individual landslides, but many of these are features that include many other smaller landslides, or those that have coalesced and were not distinguishable as individual failures. Glacial landforms such as moraines and outwash terraces also occur throughout the backshore zone and an unusual landform called an ice-walled lake plain occurs west of Ozette Lake. These features form where large masses of the ice sheet stagnated in and near the lake basin and underlie the modern-day prairies. In addition to sea stacks and wave cut platforms, the foreshore area has a diverse collection of landforms including 174 beaches. The largest and most common beaches are composed of sand and are found adjacent to bluffs composed of glacial outwash from Kalaloch north to Hoh Head, and fronting the...

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