click to enlarge Bear Cieri Thea Alvin’s home in Morrisville An astonishing stone corkscrew arch marks the spot along Route 100 where Thea Alvin put down roots — and rocks — in Morrisville. It took her two years to build the Helix, as she named it, which frames the Green Mountains in the distance through a series of arches that resemble a stretched-out Slinky. Her work can also be found farther afield, from Saint Michael’s College in Colchester to Duke University in North Carolina to the Piedmont and…