At a certain point the stored water broke through the rim of Holden and created a lake 200–250 m deep.

[5] Water with a depth of at least 50 m entered Holden at a rate that 5-10 times the discharge of the Mississippi River.

[6][7][8][9] Terraces and the presence of large rocks (tens of meters across) support these high discharge rates.

There is evidence that instead of rain or snow, the water that formed the valleys originated under ground.

It forms alcoves and stubby tributaries; these features are visible in the pictures below of Nirgal Vallis taken with Mars Odyssey's THEMIS.