One possibility for the Columbia Hills is that they are frost mounds:

=====================================
From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Could the Columbia Hills in Gusev, Mars be frost mounds?
Newsgroups: sci.astro, alt.sci.planetary, sci.geo.geology, sci.geo.mineralogy
Date: 2004-06-12 01:15:06 PST

The discovery of sulfates near surface in Gusev in amounts that got
higher as you approach the hills might support the idea the hills are
frost mounds:

From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Spirit rover may yet make a key discovery.
Newsgroups: sci.astro, alt.sci.planetary, sci.geo.geology,
sci.geo.mineralogy
Date: 2004-04-18 02:22:29 PST
http://groups.google.com/groups?selm=832ea96d.0404180122.63dddc88%40posting.google.com

Bob Clark
=====================================

However, look at this MGS image of the hills:

taken from this page:

Remote Sensing Tutorial Page 19-13a
Missions to Mars during the Third Millenium
http://rst.gsfc.nasa.gov/Sect19/Sect19_13a.html

The image makes the two prominant hills visible in the image appear to have once been conical but with now eroded eastern sides, and with central craters.
Since MER data shows water related geochemistry occurs in association with these hills, perhaps the hills were ancient maars.
See examples of maars here:

Coliseum Maar, Hopi Buttes, Arizona
http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/coliseum_maar.html

and near the bottom of this page:

Remote Sensing Tutorial Page 17-3
Tectonic/Volcanic Landforms
http://rst.gsfc.nasa.gov/Sect17/Sect17_3.html

Note the water eroded sides in some of these maars.

Bob Clark