What are some important clues that indicate that a slope has ...

[yoroshambo]

What are some important clues that indicate that a slope has experienced landslide activity in the
  distant past? What will be an ideal response?

Question 2

Why do the rainfall thresholds for debris flows and landslides for California semiarid regions and
  Contra Costa and Marin Counties in Central California vary? What will be an ideal response?

[fraziera112]

Answer to Question 1

Landslide areas commonly have irregular (hummocky) terrain, depressions with or without standing
water, amphitheater-like overall shape, an absence of through-going stream channels, concentrations of
water-loving plants, and spring lines along their margins. These features stand out in contrast to
adjacent hill slope areas. In a broad sense, landslide affected slopes are flatter than surrounding areas.
Ironically, this has attracted development in some areas because of reduced grading costs

Answer to Question 2

Semiarid areas have little vegetation and abundant loose surface debris. There are few root systems to
retain the weak soils. Less rainfall Intensity/duration is needed to start a debris flow or landslide. The
differences between Contra Costa and Marin Counties are due to topography (Marin is steeper) and
geology (Marin has less cohesive rock).