Comprehensive Geology and Hydrology: Dating, Glaciers, Streams, and Groundwater

Relative dating

Places events/rocks in order without assigning an actual age (e.g., 'older than' or 'younger than').

Numerical dating

Provides an actual age in years, using radiometric methods.

Law of Superposition

In undisturbed strata, oldest rocks are at the bottom; youngest are at the top.

Law of Original Horizontality

Sedimentary layers are deposited horizontally; if tilted/folded, deformation happened after deposition.

Conformable contact

Continuous deposition with no missing time.

Unconformable contact (unconformity)

A gap in the geologic record due to erosion or non-deposition → shows missing time.

Half-life

Time for half of parent isotopes to decay to daughter isotopes.

Age calculation

More daughter = older rock.

Closed system

No parent/daughter gain or loss.

Known decay constant

A necessary condition for reliable radiometric dating.

Accurate measurement of parent/daughter ratio

A necessary condition for reliable radiometric dating.

Glacier Formation

Snow accumulates → compacts → recrystallizes into ice → begins to flow under gravity.

Quaternary Glaciations

Occurred every ~100,000 years over the last 500,000 years.

Glacial periods

Sea level falls (water stored as ice).

Interglacial periods

Sea level rises (ice melts).

Crustal Loading

Heavy glaciers depress crust.

Isostatic Rebound

Crust rebounds after melting → can cause postglacial lakes & uplifted terrain.

Erosional landscapes

U-shaped valleys, striations, cirques (rugged, mountainous).

Depositional landscapes

Moraines, drumlins (gentle hills, fertile soils).

Erratic

Large boulder dropped by ice.

Till

Mixed sediment from glacier.

Outwash

Meltwater stream deposit.

Glacial lake sediment

Thin layers of fine silt/clay.

Loess

Wind-blown silt.

Esker

Sinuous ridge of sand/gravel.

End moraine

Ridge at glacier terminus.

Watershed

Land area draining into one stream.

Stream Energy & Sediment Transport

High velocity → erosion; Moderate velocity → transport; Low velocity → deposition.

Braided stream

High gradient, variable discharge, wide shallow channels, coarse sediment.

Meandering stream

Low gradient, relatively constant discharge, one sinuous channel, fine sediment.

Point bar

Inside curve, deposition.

Oxbow lake

Forms when a meander is cut off.

Recurrence Interval (RI)

(n + 1)/m

100-year floodplain

Area with 1% chance of flooding in any given year.

Climate change impact on flooding

Heavier rainfall.

Urbanization impact on flooding

Less infiltration, more runoff.

Porosity

How much water a rock can hold.

Permeability

How easily water flows through it.

Confined aquifer

Beneath impermeable layer.

Unconfined aquifer

Open to surface.

Pollution risk of confined aquifer

Lower.

Pollution risk of unconfined aquifer

Higher.

Groundwater flow

Moves from high hydraulic head to low head.

Cone of depression

Forms when water is pumped faster than recharge.

Oxford, OH aquifer

Sand and gravel (glacial outwash deposits).

Base flow

Stream flow sustained by groundwater discharge.

Supply issues in Midwest water

Overuse, drought, agriculture.

Quality issues in Midwest water

Nitrate & pesticide contamination.

Solutions for Midwest water issues

Conservation, improved irrigation, buffer strips, groundwater monitoring.

Unconformable contact

Surface representing erosion or non-deposition — missing geologic time.

Principles of Relative Dating

Superposition, original horizontality, cross-cutting relationships, inclusions, faunal succession.

Faunal succession

Fossil sequences are predictable through time.

Radioactive Decay

Radioactive isotopes decay from parent → daughter isotopes at a constant rate.

Numerical Ages

Used to date igneous and metamorphic rocks (not sedimentary).

Short half-life

Good for young materials.

Long half-life

Good for ancient rocks.

Example of Half-Life

If parent = daughter and half-life = 2.5 million years → rock is 2.5 million years old.

Age equation

t=("number of half-lives")×("half-life").

Determining Age

If ¾ of the parent has decayed, that's 2 half-lives → age = 2 × half-life.

Conditions for Reliable Radiometric Dating

Closed system - no parent/daughter lost or added.

Known decay constant (λ)

Must be measurable.

Accurate measurement

Isotope concentrations precisely determined.

Glaciers

Moving masses of ice formed by accumulation and compaction of snow.

Formation of Glaciers

Form when snow accumulates > melts over years.

History of Glaciations

Several glacial-interglacial cycles, roughly every 100,000 years.

Milankovitch cycles

Earth's orbit and tilt changes.

Last Glacial Maximum

Most recent glaciation peaked ~20,000 years ago.

Glaciations & Sea Level

During glaciation: Water stored as ice → sea level falls.

Erosional Glacial Landscapes

Ice carves land.

Depositional Glacial Landscapes

Ice drops sediment.

Glacial Abrasion

Rocks in ice scrape bedrock → striations and polish.

Plucking

Ice freezes onto rock fragments and pulls them away.

Interpreting Flow Direction

Striations: Parallel scratches → show glacier flow direction.

Ohio's Watersheds

Great Miami, Little Miami, Whitewater, Mill Creek Watersheds

Water Velocity

High velocity → erosion; Intermediate velocity → transport; Low velocity → deposition

Cut bank

Outer curve → erosion

Wide valleys

Low gradient, older rivers, slower erosion

Steep valleys/canyons

High gradient, active downcutting

Flood Recurrence Interval

RI=(n+1)/m; 100-year floodplain: 1% chance of flood in any given year

Groundwater & Surface Water Relationship

Connected systems: groundwater discharges into streams (base flow)

Groundwater Movement

Driven by gravity and pressure (hydraulic head); Flows from high head → low head

Major Water Supply Issues

Overuse, drought, uneven distribution; Solutions: Conservation, efficient irrigation, groundwater recharge

Major Water Quality Issues

Agricultural runoff (nitrates, pesticides), industrial pollution; Solutions: Riparian buffers, wastewater treatment, fertilizer control

Groundwater Flow Direction

Water flows from higher to lower water table elevation; Perpendicular to contour lines