What is sedimentology?

Sedimentology is the study of the sequence of deposits and the processes thatcause the formation of those deposits. These deposits comprise the uppermostpart of the Earth’s crust.

One of the basic tenets of sedimentology is that newer deposits overlie olderdeposits. Geologists call this superposition.

You can always remove older deposits, but the deposits that remain alwaysremain in the order that were produced.

The pattern of deposits and its order is sometimes called stratigraphy.

New

OldOlderOldest

Ground surface

How do you tell the difference between deposits?

- Grain-size distribution

- Stratification

- Lithology

- Radioisotopes

- Magnetism

Common stratigraphy found in theuppermost sediments of the Puget Sound

Soil

Till

Clean sand

Clay

“Bedrock”

20,000-25,000 yearsago

Formation of the Lawton Clay

Glacier

Shallowglacial

lake

Glacier

Shallowglacial

lake

Drainage to Grays Harbor

20,000-18,000 yearsbefore presentFormation of theEsperance Sand

Outwash at an advancing glacial front

~18,000 years ago

Formation of the Vashon Till

Glacier

Glacier

Freshwaterlake

18000-16000 yearsbefore present

Glacial retreat - no deposits

The last ~6000 years

Modern conditions

15000-6000 years agoGlacial rebound and sea-level rise

Beaches or “the nearshore” can be divided into two broad categories

Constructionalsand/material comes from elsewhere

Degradationalmaterial is removed

What does this mean for the shoreline?

beach beach

old surface

waterlevel

waterlevel

The most common result...

Foreshore, or “beach face”, covered mostlywith loose gravel

Some sand andremnant cobbles on

low-tide terrace

Receding bluff

In deep water drape of sandand mud OR exposed

till/bedrock

low-tide line(MLLW)

Material removedby wave action

Post-glacial surface

Bluff face comprised of till