Kris,

I believe that you may have observed an orographic (water-ice) cloud over the Hellas impact basin (Hellas Planitia, 70 degrees West, -42.7 degrees). This massive impact basin is believed to have been formed approximately 3.9 billion years ago and reaches a depth of ~4.3 miles (~7 km; this is the depth of the crater whereas the rim reaches up to 5.6 miles (~9 km)). Orographic clouds form over Hellas during Martian Southern Summer and Autumn (Ls 180-270) as the minimal water vapor within the Martian atmosphere condenses over this basin. Albedo features (e.g. Zea Lacus and Peneus) have been detected over Hellas as observed by notable planetary astronomers such as Schiaparelli, Antoniadi, and many others in the past.

Links;
http://en.wikipedia.org/wiki/Hellas_Planitia
http://cseligman.com/text/planets/marsmaps.htm
http://geology.com/articles/highest-point-on-mars.shtml
http://www.dustymars.net-a.googlepages.com/marsobserverscafe (An excellent reference for observing the planet Mars by Jeffrey D. Beish)

I have attached an observation of Mars made on April 21, 1986 (10:25 U.T.) using a 12.5-inch F/6.2 Newtonian reflector (at an impressive magnification of 700x!). It shows orographic clouds over the Hellas basin.

Happy Mars Observing!

Carlos