Woody Plants BI237
Week 2
TOPOGRAPHY
Topography also goes by the name physiography.
Both refer to physical geography -- the surface features of an area.
Within a particular region, topography (together with soil) can be an excellent predictor
of vegetation.
In describing the topography of an area, several questions must be addressed:
A. SPECIFIC LANDFORM (mountain, river valley, particular glacial feature, etc.)
B. ELEVATION (affects temperature and rainfall, if in mountains)
C. SLOPE
1. GRADIENT
If level, water doesn't flow laterally, only down.
Could be wetland or extremely dry, depending on soil type.
If sloping, lateral movement of water. Soil stays moist.
If very steep, may have soil erosion.
2. POSITION ON SLOPE
If high, tends to be windy, sunny, and dry.
If low, tends to be sheltered, moist, and to accumulate eroded soil.
3. ASPECT
In N. Hemisphere, S-facing slopes get most direct sun in summer. Driest.
SSW, SW also hot and dry (afternoon sun hotter than morning sun).
North and east-facing slopes are coolest.
Woody Plants BI237
Week 2
Woody Plants BI237
Week 2
GLACIAL GEOLOGY IN MAINE
THE TIMING:
There have been numerous glacial and interglacial periods over the past 1.5 million years.
Our current landscape has been shaped by the most recent glacial period, which reached a
peak about 21,000 years ago.
THE EXPANSIONS:
During the times of glacial expansion, the glaciers were thousands of feet thick.
Their enormous weight caused them to "flow" across the landscape, dragging all loose
materials -- including boulders -- along with them.
Softer rocks were gouged away and even hard rocks are left with grooves from materials
being dragged across them.
THE "RETREAT":
As the earth warmed, the glaciers "retreated". They didn't actually flow backwards, but
(usually) melted in place, dumping whatever debris they had accumulated.
THE RESULT:
The glaciers have left their legacy in more than just scratched bedrock. All of the
landforms and soils in glaciated regions have been affected. The landforms can be
divided into two categories, depending on whether they were moved by ice or by water.
MOVED BY ICE:*
All of the sediments moved by the glacier have a mix of particle sizes, from rocks,
through sand, silt and clay. This generally makes them good sites for plants because the
soils are moist yet well-aerated.
Till: accumulations of unsorted, unstratified mixtures of clay, silt, sand, gravel, and
boulders. This is mostly what gets left behind, whatever the glacier happened to pick up.
Drumlins: streamlined hills whose long axis is parallel with the direction of ice flow.
Made of compacted till. Tear-drop shaped, with the blunt end towards the direction from
which the ice flowed.
Moraine: a mound, ridge, or other distinct accumulation of glacial till. These were
constructed parallel to the ice margin in places where the glacier was still actively
flowing and conveying debris to its terminus.
Kettle: A pond formed by the melting of an ice block in place.
MOVED BY ICE WATER:*
Esker: a sinuous ridge of sedimentary material (typically gravel or sand) deposited by
streams that cut channels under or through the glacier ice. Why gravel or sand? Because
Woody Plants BI237
Week 2
these streams were running so quickly that they were able to carry fine particles such as
silt and clay in the current. Only the heavier sands and gravels sank to the bottom.
Eskers can be dozens of feet high and dozens of miles long.
Outwash: Also carried by glacial meltwater, and characteristically sandy, but forming a
plain rather than a narrow hill. Some have subsequently been moved by wind and
deposited as dunes on the east sides of river valleys such as the Androscoggin and Saco.
MOVED INTO SEA WATER:
The glaciers were so heavy that they actually pushed down on the continental crust,
causing the coast to sink into the sea. Much of coastal Maine and the large river valleys
like the Kennebec were totally under water. The glaciers met the water's edge.
Delta: Deposit formed as sand- and gravel-laden meltwaters entered the ocean.
Presumscot formation: Silts and clays that had been held in suspension in the glacial
meltwaters dispersed over what was at that time the ocean floor.
By about 11,000 years ago, the crust rebounded, bringing the sediment-covered ocean
floor up above sea level.
Soils of the Presumscott formation have a finer texture than do the glacial tills. They
tend to support more nutrient-demanding mesic-site species, whereas the glacial till is
more likely to be colonized by slightly less nutrient-demanding species. Sandy soils on
deltas or eskers tend to be very dry, and support such xeric-adapted species as red pine.
*(Definitions adapted from the Maine state geology website (see course website for link).