A study conducted along the Lincoln County, Oregon coast
delineated the exact location of 153 landslides. The landslides were
found through the interpretation of aerial photographs. Each landslide
was verified in the field and its class (ancient, historic, recent),
type (slump, debris slide or flow, planar slide), and status (stable,
potential, active) was determined. Each slide was numbered and mapped
at scales of 1:4800 and 1:62500.
The study area was divided into seven homogeneous reaches based on
lithologic and morphologic differences. The number of slides occurring
in the differing geologic units was then analyzed.
It was found that most landslides occur in conjunction with siltstone
materials of Tertiary age. Areas with similar morphology but of
sandstone materials are less likely to have landslides. Structurally,
landslides occur in areas with steeply dipping Tertiary bedding planes
or in areas with steep slopes such as river valleys.
A large portion of the landslides in the study area occur at contact
zones between Tertiary and Quaternary materials. Most of these
slides are ancient, stable slides and are larger than other slides in
the study area. The only active ancient landslides in the coastal
zone take place in the Nye mudstones near Beverly Beach State Park.
Those slides in the study area classed as historic and recent are
predominantly active or likely to become active in the near future.
Only 11 of these slides remain stable.
Landslides will continue to be a geologic hazard in Lincoln County,
Oregon. However, knowledge of landslide location, lithology and morphology
will reduce their impacts.