Isles of the Ancients is the longest running series.  The series has to do with Legends from northern Europe with a particular interest in north Holland and the Freisan Islands. Even though the materials have always been mixed, recent years the prominate material is bronze.

Isles of the Ancients : Echoes from the North Sea
Isles of the Ancients : Visitation
Isles of the Ancients: Coronation
Bombs and Esker comes from a series that combined classic geological formations with contemporary international political issues. 

Bombs and Esker
Fishing Upstream comes from a series that dealt with regional political issues that deal with the environment and the struggles over its use.  Some of the earlier installation work was its forerunner.

Fishing Upstream
The Cutting of the Last Great Fir at the North Fork
Beach & Elk
The First Snow On Lone Butte