Former glaciers in Tharsis, Mars may explain some features I found in the Tharsis region. They occur in these images:

Sample typical east Tharsis plains
http://ida.wr.usgs.gov/html/m03017/m0301739.html

The image shows ridges arranged in series along the edges of many of the craters. There are some ridges in the channel in the image as well:

http://ida.wr.usgs.gov/fullres/divided/m03017/m0301739a.jpg

Some also appear inside the craters:

http://ida.wr.usgs.gov/fullres/divided/m03017/m0301739d.jpg

They also appear in this nearby image:

North contact between flows and upland with arcuate troughs E Tharsis.
http://ida.wr.usgs.gov/html/e04018/e0401892.html

Note: as with all orbital imaging, you may have to rotate the image to properly perceive positive and negative relief.

I was led to look for MGS images in this area of Tharsis because it appears in the recent PNAS paper on liquid water on Mars as an area where liquid water might be stable on Mars for at least 20% of the time:

Use of spacecraft data to derive regions on Mars where liquid water would be stable
Proc. Natl. Acad. Sci. USA, Vol. 98, Issue 5, 2132-2137,
February 27, 2001
http://www.pnas.org/cgi/content/full/98/5/2132

Fig. 4 shows a diagram of areas where liquid water might be stable:

http://www.pnas.org/cgi/content/full/98/5/2132/F4

The areas in red have the capacity for liquid water for at least 20% of the time.

A possibility for the origin of the ridges is that they might be Rogen moraines, also commonly called ribbed moraines. As discussed on this page, Rogen moraines commonly occur in basins and valleys as appears to be the case in the Mars images:

Abstract
Rogenmoräner
Lyrén, P-E.
http://www.bmg.umu.se/samarbeta/Examensarbete_naturgeografi/Abstracts/Lyren_01.htm

Below is an image of a field of Rogen moraines in Ireland from this report:

Subglacial bedform geomorphology of the Irish Ice Sheet reveals major configuration changes during growth and decay.
CHRIS D. CLARK1* and ROBERT T. MEEHAN2
1Department of Geography and Sheffield Centre for Earth Observation Science, University of Sheffield, Sheffield S10 2TN, England
2Teagasc, Kinsealy Research Centre, Malahide Road, Dublin 17, Ireland
JOURNAL OF QUATERNARY SCIENCE (2001) 16(5) 483–496
http://www.shef.ac.uk/geography/staff/clark_chris/jqs16.pdf

The ribbed appearance characteristic of Rogen moraines is apparent in this image. Compare to the ribbed appearance of the ridges in the Mars images.

Bob Clark

Rogen moraines on Earth.

Ridges in Tharsis, Mars (image rotated to make clear positive and negative relief.)