An erratic • Bands of tolerance (M6.4)
This large granitic boulder was transported here by glacial movement from an igneous rock formation in central Massachusetts. As such it is called a glacial erratic. In contrast, the large boulder on top of the slope is an outcrop of the local sedimentary bedrock.
Granite is formed from magma that has cooled and solidified within the crust of the planet, as described in Geology under Plate Tectonics. Even without a hand lens it is possible to see the crystalline structure of the erratic and the dark specks of mica distributed throughout the other mineral components of feldspar and quartz.
Look southwards to see how different plant species sort out in bands at different distances from the water's edge. The location of each band indicates the species' tolerance of inundation by brackish water from South Cove. Smooth cordgrass is partly submerged even at low tide. Carolina sea lavender, Saltgrass and the saltmarsh rush further inland are both exposed at low tide but are mostly submerged at high tide. Maritime marsh elder and the seaside goldenrod straddle the high tide line as they share the highest ground along the shore.