On the Flame board Thomas Lee Elifritz has discussed his theory that many of the features seen on Mars are due to glaciers:

Ancient Martian Lakes? Perhaps.
Posted by Thomas Lee Elifritz (209.208.61.151) on 3/14/2002 5:17:07 PM
http://www.habitablezone.com/flame/messages/14458.html

This post relates to the new report that proposes the sedimentary layers observed by Malin and Edgett were actually caused by ash fallout from the Tharsis volcanoes:

Ancient Martian Lakes? Perhaps Not.
http://skyandtelescope.com/news/current/article_531_1.asp

Supportive of the Elifritz idea is research suggesting not only past glaciers in Tharsis but currently active glaciers in the Tharis region:

Does Mars Hide Vast Water Deposits
by Johann Helgason
Reykjavik - June 10, 2000
http://www.spacedaily.com/news/mars-water-00b.html

Then the sedimentary layers could have been due to jokulhlaups, catastrophic glacial outburst floods. There is abundant evidence of subglacial volcanism on Mars and this is known to result in extensive sedimentary deposits:

FIELD INVESTIGATIONS OF ICELANDIC JÖKULHLAUPS AS AN ANALOG TO FLOODS ON MARS.
Mars Polar Science 2000
http://www.lpi.usra.edu/meetings/polar2000/pdf/4098.pdf

Flooding and Ponding on Mars: Field observations and insights from the polar realms of the Earth.
Lunar and Planetary Science XXXI.
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/2067.pdf

What Lies Beneath: Scientist reports on the dramatic geology of Iceland’s subglacial lakes at AAAS symposium.
"An important aspect of jökulhlaups is the enormous amount of sediment these floods bring to the outwash plain and surrounding marine environment, a virtue of the active volcanoes they lie upon. One of the most significant finds of Russell and his colleagues is that the 1996 jökulhlaup deposited large quantities of sediment within the Skeidararjokull glacier. Surprisingly, this sediment was rapidly frozen into the glacier by flowing super-cooled water, an extreme example of a mechanism which may also operate more slowly at other times. This mechanism of sediment transport in glaciers and ice sheets could be crucial for the determining historic sediment flux and resulting landforms and the sedimentary record."
http://www.earthwatch.org/pubaffairs/news/russell.html

This last is interesting in that it discusses how the flows can occur at subzero temperatures, i.e., supercooled water - certainly relevant to the situation on Mars.

If the flooding occurred below or within the glaciers this could also explain why there are not clear sources for the water in the Malin/Edgett deposits.

Note also that the layering seen in Valles Marineris might also be due to flooding due subglacial volcanism. Some of the landforms seen there have recently been proposed to be due to this formation mechanism:

SUBICE VOLCANISM ON THE AZAS PLATEAU: A COMPARISON WITH POSSIBLE MARTIAN TUYAS.
Lunar and Planetary Science XXXIII (2002).
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1262.pdf

However, another report given at the 33rd LPSC while arguing for subglacial volcanism in Marineris still suggests the layering seen there is due to volcanic ash fall:

ARE THE VALLES MARINERIS GIANT VOLCANO-TECTONIC DEPRESSIONS?
Lunar and Planetary Science XXXIII (2002).
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1689.pdf

Bob Clark