Chapter 9 – Geologic Time
-Scottish geologist, James Hutton realized at Siccar Point that rock layers exposed at Siccar Point require Eartch to
have a long and protracted history. Siccar Point is famous for a boundary or contact that separates two chapters in
Earth’s history. Contact is from right to left and below it are grey sandstone and shale, whose beds are nearly
vertical. There were vertical grey beds below and inclined red beds above
-Hutton’s insight was that you might be able to use modern processes to interpret events that had occurred in the
Earth’s past. This is called uniformitarianism, and was the key step in the development of geology as a science. It is
often stated as “the present is key to the past” meaning that processes that occur now have happened in the past.
-Hutton realized that the grey sandstone below the contact must have been tilted and eroded before red
sandstone was deposited across upturned layers. This contact surface represented an ancient erosion surface, now
called an unconformity.
-He concluded that the gray rocks below the unconformity represented a mountainous landscape that had been
eroded away (ruins of an earlier world).
-He also said that this process would take a very long time in comparison to the life span of a human and so the
earth was very old.
-The first strategy to determine the ages of geologic units, features and events is to determine the age of one rock
relative to another, using a series of commonsense approaches collectively called relative dating. Geologists then
try to assign actual numbers of years to the relative chronology, using analytical dating methods or numeric dating.
-Fossils allow us to compare ages of different rock layers and to construct the geologic timescale.
-There are 5 main principles of relative dating: Principle 1: Most Sediments are Deposited in Horizontal Layers –
Original horizontality is the notion that most sediments and volcanic units are deposited in layers that were
originally horizontal. Exceptions are the face of a sand dune or undersea slopes of a delta
-Principle 2: A Younger Sedimentary or Volcanic Unit is deposited on Top of Older Units- When a layer of
sediment is deposited, any rock unit on which it rests must be older, this is the principle of superposition concept.
The oldest rocks in a sequence would be the bottom left. This is not always true.
-Principle 3: A Younger Sediment or Rock Can Contain Pieces of an Older Rock- When a rock or deposit forms, it
may incorporate pieces or clasts of older rock.
-Principle 4: A Younger Rock or Feature Can Cut Across any Older Rock or Feature-Many rocks are crosscut by
fractures (joints and faults), so the rocks were there before the fractures formed. Dikes, Sills, and veins can also
intrude into or across pre-existing rock units, also showing cross cutting relations.
Principle 5: Younger Rocks and Features Can Cause Changes Along Their Contacts with Older Rocks- Magma
comes into contact with pre-existing rocks when it erupts onto the surface or solidifies at depth. Magma can bake
adjacent rocks, or fluids from the magma can chemically alter nearby rocks. These are called contact effects and
indicates that the magma is younger than the rocks that were altered.
How do We Study Ages of Landscapes? -Most landscapes form when rocks form and are then eroded. It typically includes the deposition of a sequence of
sedimentary layers, lithification into rocks and later erosion of the rocks. It all starts with deposition of new
sedimentary units on top of older metamorphic and igneous rocks. Most sediments are deposited horizontally.
Through time, depositional environment changes and different sedimentary layers accumulate, with each younger
layer deposited on top. Layers are lithified and at some point deposition stops. Weathering and erosion then
begin. If region is uplifted or seas withdraw, the area can begin to be eroded by rivers, streams, glaciers and wind.
Erosion is faster around river or canyon cutting downward and top layers of sediment are removed. Erosion by
river cuts downward, carving deeper canyon. Canyon widens as small drainages erode outward from the main river
and as the steep canyon walls move downhill in landslides. The combo of downcutting, widening, development of
subsidiary drainages, called tributaries sculpts a deeper, wider, and more intricate canyon.
How do we infer the Age of a Landscape Surface?
-To investigate when a landscape surface formed, we try to find a rock unit or other geologic feature that was
-Lava flows show a younger formation on top of an older surface
-Sometimes, we can’t tell the age of a landscape directly. EG. A terrace in a river (raised surface) was formed in the
past, before river was eroded down to present level.
-A landscape develops more soil if it remains undisturbed by erosion and deposition... therefore a lot of soil = old
-The age of a landscape surface must be younger than any rocks on which it is carved.
-Many rocks develop rock or desert varnish which consists of iron oxide and manganese oxide derived from
windblow dust. This is another way to tell the age of a rock. The darker the varnish, the older the rock.
-Desert pavement forms when stones become concentrated on the surface over time. Finer materials wash or
blow away and move down into the soil while larger clasts and pebbled remain on the surface. This means this
formation is older. Exposed stones even get desert varnish over time.
- A form of isotopic dating can determine how long stones have been on the surface because they accumulate
cosmic rays. The more, the older.
What is the Significance of an Unconformity?
-Erosion surfaces can be buried and preserved beneath later deposits. These buried erosion surfaces are called
unconformities and can represent a gap in geologic time. There are three types of unconformities: Angular,
nonconformities and disconformities.
-If underlying rocks are tilted before formation of the erosi