AS 15-9960 - The "D" in "Detail"...

The enlarged view provided by this stereogram shows that there are at least 3 different types of material within the floor of the D-shaped depression (see AS 17-1672). A brighter annulus parallels the wall, and darker material fills the inner floor.
Within both areas numerous bulbous and slightly raised comical structures are easily distinguishable.
There are craters on the summits of many of the structures, suggesting that each one is probably an extrusive dome with a summit crater.
Many similar features on Earth are caused by the subterranean drainage of lavas following extrusion and release of pressure following the upward movement of a central plug.