Alluvial fans are fan-shaped deposits emerging from regions of steep topography. Alluvial fans on Mars are thought to be ancient and record past episodes of flowing water. This image shows part of one of those fans, which has been eroded.
The old stream channels now stand above the rest of the fan as ridges, mostly in the southern (bottom) part of the image. This can occur because the channel materials are more resistant to erosion; perhaps they had larger grains (gravel) or because minerals deposited from the water cemented together. [More at link]