STRUCTURAL GEOLOGY AND

STREAM EROSION

Streams tend to erode valleys along the fastest and least resistant paths to the sea:

This habit of preferential erosion forms landforms and drainage patterns that reflect the structure and lithology of the underlying bedrock.

Drainage Pattern	Rock Structure
Trellis/annular	Folded strata Dendritic Horizontal strata, massive plutonic rocks
Parallel	Inclined strata
Rectangular	Faulted and jointed rocks
Radial	Plutonic and structural domes, volcanic cones

Horizontal strata:

form broad, low-relief plateaus, buttes and mesas
dissected by deep, steep-walled canyons
resistant strata form steep cliffs
nonresistant strata form gentle talus-covered slopes
drainage pattern is dendritic whenever there is no underlying structure, or rectangular whenever the bedrock is faulted and jointed

Inclined strata:

the uplifted edges of the resistant strata form strike ridges (also called hogbacks)
strike ridges have gentle dip slopes and steep scarp slopes with dip and scarp streams that drain into....
strike streams that erode strike valleys into shale (noresistant) outcrops and which drain into a large...
main stream which cuts through water gaps in the strike ridges
streams assume a trellis drainage pattern

Folded strata:

resistant strata form prominent strike ridges
nonresistant strata are eroded into strike valleys
anticlines and synclines have straight and V-shaped strike ridges and trellis drainage patterns
domes and basins form circular strike ridges and annular drainage patterns

Faulted and jointed strata:

the preferential erosion of faults and joints forms straight stream valleys and (where there are two fracture sets) rectangular drainage patterns
the traces of normal faults form planar fault scarps
the downthrown and upthrown blocks of normal faults form grabens and horsts
strike-slip faults offset surface features and juxta-pose rocks of different lithology and topography

Plutonic rocks:

usually highly resistant to weathering and erosion (compared to the surrounding country rock)
plutonic rock bodies form steeply-sloping topo highs: domes, dike ridges and palisade cliffs
country rocks form topo lows and gentle slopes
drainage pattern can be dendritic (no structural control), rectangular (when rocks are fractured) or radial (typical of domes and laccoliths)

Volcanic rocks:

fissure eruptions create lava plateaus that erode into steep buttes, mesa, and inverted valleys
shield volcanoes are dome-shaped; cinder cones are small but steep; and stratovolcanoes have gentle lower slopes but high, steep summits
volcanic cones have radial drainage patterns
central vents of active volcanoes are surrounded by circular craters and large collapse calderas
erosional remnants of volcanoes include volcanic necks and radial dikes that fed lava to vents