Commentary by Nick Hoffman, Geophysicist Extraordinaire, regarding the phenomenon below...

It is well known that C02 changes phase from solid to
vapour at relatively low temperatures at the low martian
ambient pressure. The corollory of this is that if you
can heat it above that temperature, it will develop a
substantial pressure (if you confine it). For instance,
at -57 C the solid has an equilibrium vapour pressure of
5 bars which is enough to physically lift a lid of 100m
of solid CO2 ice or ~ 75 metres of regolith.

The polar icepack is only a few metres thick, so
pressures of ~ 0.1 bars will suffice to blow through the
"roof" and erupt as a geyser. This equates to a
temperature of 170K (~24 K warmer than the main snowpack
which condenses in equilibrium with the ~6.5 millibar
atmosphere). Carbon dioxide has a very steep ramp of
equilibrium vapour pressure with temperature whicj makes
effects like this very powerful. It also has low specific
heat capacity which makes it easy to warm (unlike water)
and low latent heat capacity which makes it easy to
melt/vaporise (unlike water).

This is NOT a sign of internal volcanic heat, but simply
a consequence of spring sunshine shining down through
relatively transparent CO2 snow and block ice and warming
the surface of dark dune material underneath, like Earth
sunshine melting snow on a black bitumen sidewalk from
the bottom up.

What we observe is not so much the active geyser, as the
trail of fine dust that has been jetted out of the
orifice and spread downwind. If the wind direction
changes, so does the streak direction so on different
days you get different trails, or sometimes the winds
swirl noticeably acroll the length of a frame and show
varying orientations - a dead giveaway for wind streaks
rather than shadows.

There are a couple of scientific papers published already
that discuss this phenomenon.

"Annual Punctuated C02 Slab-Ice and Jets On Mars" by H.H. Kieffer of the USGS -- Adobe Acrobat reader required

Robert Shepherd suggests this:

How about a really unusual optical illusion, that can be something entirely different if you look at it differently?

Try this one: It's a long slope cutting through many layers of thin rock. This rock fractures into rectangular blocks, and where it's exposed it looks like a pile of mah-jong tiles. The left and bottom edges of the rock are shadowed. The left edge forms what people are interpeting as a geyser or dust devil, lifting up from what seems like a horizontal streak on the ground, but's really the bottom edge.

The effect's not clear near the top of the image, but as you get near the bottom the "tiles" are larger and better-defined. Your small version also shows the effect better than the original-size, I think because the original has some blurriness that heightens the illusion of dust or gas.

...
BTW, a side effect of shifting to this perception is that you can now see a neat little "anomaly": Near the top of your sepia image, center, one of the blocks has a smaller block on top, and two look a lot like a building with an air-conditioning unit on top. he he he...

--Robert