Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 29

3

2

1

FIGURE 2.6
Concept of unconformities. Unconformities identified by black line. (From Hutton, J., Trans. R. Soc. Edinb. , 1,
209, 1788.)

• Following erosion, sediments identified within the zone labeled as 2 were depos-
ited on top of the folded and tilted rock layers, creating a nonconformity (bold line
between zones 1 and 2).
• After the layers identified as 2 were deposited, they were uplifted, exposed, and
eroded to create a second erosion surface.
• Following a rise in sea level, sequence 3, a marine limestone was deposited, creat-
ing a disconformity (bold line between zones 2 and 3).

2.3 Sedimentary Processes: From Removal to Rock Formation
Sedimentary deposits make up only 5% of the Earth's crust (Figure 2.7a), but account for
approximately 75% of the rocks exposed at the earth's surface (Figure 2.7b) (Flint and

Sedimentary Sedimentary

Igneous and metamorphic

Igneous and metamorphic

(a) (b)

FIGURE 2.7
(a) Sedimentary deposits within Earth's crust. (b) Sedimentary deposits at Earth's surface.