Cassie Oldfield and Walter Shaws are forensic geologists who investigate crimes threatening lives and the land--a job that always leads to trouble.
Books 4 - 6 in the series:
SKELETON SEA: The geologists investigate a mystery at sea, uncovering a strange and deadly project in an undersea world where they don't belong. They must face their deepest fears in order to prevent this calamity.
RIVER RUN: There are plenty of ways to die in the Grand Canyon. The geologists investigate one a stranded raft on the canyon's Colorado River. Trying to find out what happened to the missing rafters, Cassie and Walter uncover a hellish scheme.
LANDS END: Deep beneath the streets of San Francisco, a murder victim is dumped at an archaeological dig. A note pinned to his shirt warns we will bury you. Investigating the murder, the geologists follow the mineral trail to the city's unexpected wild edge, where they learn the meaning of the ominous note.
All books in the series are standalone novels and can be enjoyed in any order.