Most folks are blissfully unaware of geologic
hazards until those threats hit home – literally, in the case of
rock falls or landslides.
David Lopez, senior research geologist with the Montana
Bureau of Mines and Geology, hopes to prevent that from occurring
in developing areas of Billings, Mont., where new residential construction
can be found in areas that put homeowners close to the edge of geologic
forces at work.
His work in helping revise and complete the geologic
map of the state made him aware of several hazard areas in his hometown
of Billings, and his passion for mapping led him beyond the project’s
He completed a set of maps showing three hazard areas
-- rock falls, landslides and bentonite or swelling clay.
One map shows the area’s geology and three companion
maps show the hazard areas, accompanied by photographs and explanations
to help non-geologists understand the documents and the potential
problems they illustrate.
Because of the maps, homeowners have a clearer understanding
of potential problems before building or buying a house.
“They are absolutely wonderful,” said Billings consultant
Betsy Campen. “David’s main love in life is mapping, and he has
done a great job here ... and a great service to Billings.”
The dramatic scenery provided by Billings’ landmark
Rimrocks is the area that has spurred upscale development in areas
threatened by rock falls and slides from the 80 million-year-old
Eagle Sandstone, which is a major gas-bearing formation in other
parts of the state.
Campen said her own home, built in 1982, once was
threatened when a huge, square block above and behind her house
split and teetered. The rock had to be blasted away, she said.
Lopez said that even when people are aware of the
potential dangers, often they are willing to gamble that their homes
will escape damage.
Billings is located in central Montana and not prone
to earthquakes, but temblors in the western or southwestern parts
of the state could trigger rock falls or other events in the area,
As an example of the far-reaching effects of geologic
events, Campen cited a 1940s earthquake in Yellowstone that closed
a hot spring miles away, effectively shutting down the tourist attraction
in the town of Joliet, near Billings.
A Public Service
Lopez got the idea for the hazard maps from a similar
project done by a friend in the Golden, Colo., area.
Not everyone was happy with his idea.
For example, when Lopez’ maps were being reviewed
before release to the public, Bureau lawyers expressed concern that
publicizing the information could lower property values in the affected
areas, which in turn could perhaps prompt lawsuits against the agency.
Others, however, raised the point that withholding
the information would be a public disservice – and also could result
in potential liability if damage occurred that might have been avoided,
The legal concerns were eased with minor editing
so that photos and text did not identify specific homes or addresses.
The photos “just show the facts without editorial comment,” Lopez
Even so, Lopez said the maps are “30 years too late”
for some residential areas. In some areas, the rock fall danger
could have been avoided by building just a block away from the base
of the cliffs, he said.
In another instance, the toe of a large landslide
was cut away to make room for sport fields. The slide has been reinforced
with large, rock-filled wire baskets, Lopez said.
The bentonite problem usually can be avoided by moving
the building site or digging out the clay, Lopez said.
While the swelling clay can cause considerable damage
to a structure, it also presents a more immediate risk of damage
Billings has had some “close calls,” such as when
a truck-size boulder rolled onto a city street as recently as 1994.
That and other incidents are pictured on the maps.
A rock fall “could cause the total wipeout of a house
... could cause death,” Lopez said.
The falls are continual. The upper Cretaceous sandstone
underlain by shale is constantly eroded and shifted by frost and
The potential rock fall area is a long but narrow
strip. The landslide area is wider but actually affects fewer homes,
he said. Several homes were built on the slides and have experienced
warping, he said.
Lopez came to the Billings area in 1982, working
in the oil industry until joining the Bureau in 1992. He is a 20-year
member of AAPG.
Lopez said he decided to produce the hazard maps
in 1994 while teaching at Rocky Mountain College in Billings. He
gave the project to a student, Marianne Sims, and supervised as
she developed it as her senior thesis. Lopez then reworked the explanations
and added photos for the Bureau version, sharing credit with Sims.