See the context of this sign.

The Cleetwood Flow

Before you is the only place in the entire wall where one kind of
lava extends continuously from rim to water's edge.

This rock mass was once molten lava which welled up to produce a
flow similar to, but smaller than, Llao Rock to the west. The flow came
from one of a series of vents which extended along the west, north,
and east sides of the mountain, and along which the mountain fractured
as it collapsed.

As the thick, pasty, docite lava moved slowly down the mountain, its
surface cooled and hardened rapidly. The molten lava beneath,
however, continued to flow. As a result the brittle glassy crust
and fissured deeply to form a glistening rugged crust.

For many years this was known as Diller's backflow. Joseph Diller,
first geologist to study the area, believed that the lava originated
farther up the mountain and was flowing when the summit was destroyed,
thus permitting molten rock to enter a crack and cool in this position.
This belief, though later shown to be in error, led Diller to conclude
that destruction of the mountain was by collapse, a conclusion in
agreement with that accepted today.

Cleetwood Cove is named for the boat used in 1886 in the first
scientific investigations on Crater Lake.

Don't miss the rest of our virtual tour of Crater Lake National Park in 1757 images.