Just as an early warning, we're going to be working pretty much just with channels here.
1) Start by creating a document. I'm going to work with a 1024x1024 greyscale with a black background. Doesn't really matter.
2)Now go to the Channels palette, unless you've modified things, it should be in the same little window with Layers and Paths. If you've rearranged things, I expect you should know where it's at. Now create a new channel. There's a couple ways to do this: you can hit the little curled-page document button at the bottom of the palette, or you can use the pulldown menu under the arrow at the upper right-hand corner of the palette and select New Channel.
3) Draw whatever shape you want. You can also paste in a grescale image. I'm going to draw the jolly happy white sperm as before. If I'm not careful, I could become associated with that.
4)Gaussian Blur that shape to make a nice greyscale gradation of it. There's another mungier method to gradate your shape, but I'll go into that later...
Edit: I added a bit of Add Noise... filter.
5)Use your new channel as a selection. One way to do this is with Select>Load Selection..., in combination with Select>Save Selection..., this will take for-bleeding-evah. Fortunately, there's a button down there with the new channel button at the bottom of the Channels Palette. It should be on the far left side and look like a dotted circle.
Digression)Let's stop for a moment and look at these buttons. On the far left is the dotted-circle Load Channel as Selection button we just looked at, next to that is a circle-in-box button for Save Selection as Channel(the inverse to the previous), to the left of that we say hi to the curled-page Create New Channel Button we've seen before, finally, on the far right is a button with a trashcan icon, this is the Delete Current Channel button(also, interestingly, the inverse to New Channel, cool...)
6) Go to the Select>Modify>Contract... menu. I don't know the shortcut for this, but it would be a great thing to know. Choose a size for your Dilation; smaller sizes will make smoother dilations that are more interpolative, larger sizes will get you there a lot quicker. A quick warning: while small radii tend to cut out circular chunks, larger radii start carving out octagons, by that time though, it's going to need a lot of munging to make it look good, anyway.
7) Depending on how large a radius you chose, repeat the Contract a couple times.
Save your modified selection as a new channel.
At this point, you can blur(or munge up) the new channel, if you like, and repeat from step five, or, if your happy with what you have, you can copy it paste it onto a layer and work with it.
I think you'll agree with me, that you have the start of what could be a good little ridge.
Thank You for your attention.