Juvenile clast-rich layer. The basal J1 layer is the thickest layer and is comprised entirely of inflated fresh scoria that is commonly black. Scoria sags, reverse grading, and bomb horizons exist in J1 at the sites closest to the caldera, 6-8 km from the rim. J1 is 133 cm thick at the type section (Vermont Creek).
Physical volcanology of a sub-plinian and phreatomagmatic eruption at Okmok Volcano, Alaska: implications for explosive mafic volcanism