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As a result of the geologic baking, metamorphic rocks are cohesive, well stuck-together rocks. The heat and pressure of the Earth's interior causes the structure of the original rock - be it igneous, sedimentary, or previously metamorphosed - to recrystallize into a tight, interlocking pattern with few open spaces. This process fuses together the crystal grains of metamorphosed rocks, forming a strong structure akin to welded steel. (Physically, the process is similar to what happens when one squeezes snowflakes together into a snowball.) In contrast, the sediments of a sedimentary rock such as brownstone are held together by a mineral glue, cementing individual grains to each other. A mineral weld is much stronger than mineral glue, making the metamorphic rocks of the Uplands on the whole much more resistant to erosion than the crumbly brownstone of the Central Valley. This is the principal reason that the Uplands are uplands and the Central Valley is a valley.
Metamorphic rocks are not difficult to distinguish from igneous and sedimentary rocks. The immense pressures of the Earth's interior force
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These tombstones in the graveyard
of Hartford's Center Church
demonstrate the effect of rock type
on landscape. On the left stands a
stone carved from Central Valley
brownstone in 1806; on the right sits
a stone carved from Uplands
metamorphic rock (schist) in 1768.
Like the Central Valley, the
brownstone tombstone has been
eroded much more deeply - even
though it is younger - than the
metamorphic stone from the
Uplands. |
