The Valles Caldera collapsed ~ 1Ma ago and the landscape in the northern part the of
the caldera has been subsequently modified by San Antonio Creek. Groundwater-fed
wetlands—called fens occur along the creek at the northern margin of the caldera. The
fens appear to be associated with pre-caldera volcanic bedrock barriers . The Summer
of Applied Geophysical Experience program has sought to understand the fen barrier at
one location since 2019 using several geophysical methods. This has included seismic
reflection and refraction, electrical resistivity tomography and transient electromagnetic
surveys, from which the geometry of the pre-caldera volcanic flows, sedimentary layers
and hydrothermal alteration of the volcanic layer have been interpreted.