Eagle Rocks, the cliffs at Balky Hill between Twisp and Winthrop, also known as the Bonner Lake
Section of Pipestone Canyon. The upper cliffs are about 60 million years old; they lie ‘unconformably’
on the darker rock below of the Buck Mountain Formation, 130 million years old. The intervening 70
million years of rocks are missing, eroded away and lost to any semblance of history.
These steeply uplifted marine sediments of the Twisp
Formation can be seen in the roadcut one mile
south of the Winthrop post office on Highway 20.
A granite cobble from the Methow River. It has had a hard
life; first melted at 1000 degrees and cooled miles under-
ground, then ripped off the parent rock by frost wedging,
then battered by glacier and ice while being transported
to the lowlands.
Andesite tends to be bluish or greenish in our area,
but often has telltale white feldspar ‘phenocrysts’
in it that managed to crystalize as the rock cooled.
Kame terraces in the Methow between Twisp and
Winthrop, formed as the last glacial advance was
melting, about 14,000 years ago.