"Old" Story from:
"Mars Exploration Rover Landings"
Press Kit, January 2004

>An asteroid or comet impact perhaps as much as 4 billion years ago
>dug Gusev Crater. Many smaller, younger impact craters pock Gusev's
>150-kilometer-diameter (95-mile) floor. One of the largest branching
>valleys on Mars, likely carved by flowing water more than 2 billion
>years ago, leads directly into Gusev Crater through a breach in the
>crater's southern rim..... ...Water flowing down the valley would
>have pooled in Gusev Crater, dropping sediments there before exiting
>through a gap in the crater's northern rim. Comparable crater lakes,
>such as Lake Bosumtwi in Ghana, exist on Earth. Gusev's lake, if
>indeed it did exist, is now gone. But the floor of Gusev Crater may
>hold water-laid sediments that preserve records of the lake
>environment, of the sediments' highlands origins and of the
>sediments' river trip.
>
>As a potential complication, sedimentary layers may lie buried under
>later deposits from volcanic eruptions or wind-blown dust. If so,
>the best chances for finding sedimentary rocks may be in material
>thrown outward when younger craters were excavated by impacts that
>punched through the covering layers... ...Several small craters in
>and near the ellipse have likely stirred up rocks from underneath the
>top veneer of Gusev's flat floor. Whether they have dug deep enough
>to expose lake-related material if volcanic overburden is deep remains
>to be seen.

Gary