Vegetation imagination

Can people look at plants and guess what rocks are underneath?

White Pine on a ridge=>well-drained soil=>bedrock, sand or cobbles=>if not bedrock, could be an esker! Maybe it’s possible, but first here is the background and logic:

Maine’s rolling hills and deep blue lakes owe their shape to their underlying rock type and erosion over time.  Maine’s bedrock rock types are diverse (see bedrock geology map of Maine) ranging from volcanics to fossil beds.  Granite makes up many of the mountains along the Appalachian Trail in Maine, but a course-grained granite also underlies Maine’s deepest lake (Sebago Lake).  Basically, the bedrock forms the obvious topography (hills, lakes, flatland).

Surficial geology (deposits resting on top of bedrock) influence what plants will live there by determining the availability of water.  For example, a sandy, well-drained ridge (like an esker) will probably have large White Pine (Pinus strobus) trees or blueberries (Vaccinium), and other flora that prefer well-drained soil.  On the other hand, Larch trees (Larix laricina), sphagnum moss, Black Spruce (Picea mariana), and cranberries (also a Vaccinium) are seen in bogs and heaths where it is very wet (poorly drained).  These environments are there because of the geology!

Water flows underground, between the grains of sand and out into a lake and then back into the ground again.  A household well taps into the ground water, allowing houses even on hilltops to have running water. Water flow stops or slows down when it hits a mud layer, or goes quite fast through cobbles or sand.  Large mud layers underlie the bogs and heaths along southern Maine and were deposited by the OCEAN!

Story time: As the last great ice sheet melted back toward Canada, the sea followed it inland (the land was depressed from the weight of the ice sheet).  Thick, beautiful, grey-blue glacial-marine mud sank to the sea floor (now southern Maine) and created an impermeable layer (poorly drained).  The land rose out of the ocean and the sea level dropped, but the mud stayed on the land (Presumpscot Formation).  Ponds and bogs formed over these mud layers.  The Great Heath in Downeast Maine rests on glacial-marine mud behind an impressive raised delta.  The delta formed at the edge of the ice sheet and indicates the height of relative past sea-level.  To see Maine’s old coastline, follow the blue line on the surficial geology map of Maine and the light purple deposits are the glacial-marine mud.

Your turn: Once you know a few indicator species, you can figure out whether the ground below you is quite wet (high water table) or relatively dry (low water table).  Think about the geology of the area, does your story fit with the geologic maps?

For more information on Maine’s surficial geology.

One response to “Vegetation imagination

  1. Pingback: Natural sciences | Rock Paper Glacier!·

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