That's right...a fault. Can't explain 'sudden' 20+ meter jump of basal gravel and the abrupt change from stacked to inset within the Bogus lavas any other way...yet. Any other ideas? Note that the jump coincides with the photogenic fault that is passed on the way to Birch Ck takeout and this reach of the river is in a zone of pervasively faulted rocks.
P. Kyle House, PhD., Surficial Geologic Mapping Specialist / Geomorphologist