8.2. Cliff Profiles

Question 1

Strata (stratum - plural)

Answer 1

a layer of sedimentary rock or soil, or igneous rock that was formed at the Earth’s surface with internally consistent characteristics that distinguish it from other layers.

Question 2

Factors affecting cliff profiles

Answer 2

1) Lithology (Rock type)
2) Rock structure (lots of cracks, horizontal/vertical beds)
3) Sub-aerial processes (rate and type of weathering)

Question 3

Vertical Cliff Profiles (Horizontal Strata)

Answer 3

• Tend to produce stable, steeply-sloping or vertical cliff profiles.
• The cliffs retreat parallel to the cliff face by undercutting and rockfalls.
• The direction of which the strata develops can either be vertical or horizontal
• Vertical cliffs form where the rocks are resistant to
  erosion, such as hard rocks like granite, sandstone.

Question 4

Vertical Cliff Profiles (strata dipping seaward with near vertical joints)

Answer 4

• Cliffs with strata dipping seaward with near vertical joints produce vertical cliff profiles
• The cliffs also retreat parallel to the cliff face by some undercutting and many rockfalls.
• The sections of cliff tend to topple over.
• These cliffs are less stable.
• The strata dips seaward.

Question 5

Steeply dipping profile

Answer 5

• strata dip steeply seaward
• where dip lies seaward then the cliff face is very unstable (the angle of orientation encourages mass
  movement) .
• Removal or collapse of rock towards the cliff base leads to rock slides.
• The cliffs retreat parallel to the cliff face by sliding.

Question 6

Concave/Convex Cliff Profile

Answer 6

• Inward-dipping strata
• Rocks are weathered quickly
• Mass movement occurs by slumping and flows
• The input of material is due to weathering and mass movements
• Cliff face → Material being released from the cliff base

Steps:

1) Slope angle declines to produce a low-angled cliff
2) Inputs of the material from the cliff exceed the outputs by marine process
3) Marine erosion removes materials at a slower rate than the supply from the upper cliff. However, erosions by waves at the cliff base causes continued instability in the cliff
4) Marine removal offshore

Question 7

Slope-over-wall cliff profile

Answer 7

• The underlying strata (hard rock) dip seaward and overlying loose rock (periglacial deposits or glacial deposits) slide forwards (by solifluction) over the top, especially if they are undercut by the hard rock strata.
• The cliff is unstable.
• Broken-up rocks and soil produced by free-thaw weathering
• Large angular blocks from landslides/rockfalls that are still too large to have been removed by wave action.

Question 8

What balance of erosion and sub-aerial processes (weathering) results in vertical cliff profiles?

Answer 8

Input of material (weathering/mass movement ie. Sub-aerial processes) from cliff face
< output material from cliff base (marine erosion)

Question 9

What balance of erosion and sub-aerial processes (weathering) results in convex cliff profiles?

Answer 9

Input of material (weathering/mass movement) from cliff face
= output material from cliff base (marine erosion)

• Moderate strength rocks. Mass movement by sliding
  and rockfalls

Question 10

What balance of erosion and sub-aerial processes (weathering) results in concave cliff profiles?

Answer 10

Input of material (weathering/mass movement ie. Sub-aerial processes) from cliff face
> output material from cliff base (marine erosion)

• Rocks are weathered quickly. Mass movement occurs by slumping and flows.