Hello dear. Sorry I'm late!
Here's your rock pile red-line; however it's more of a tutorial then a "red-line" lol. I figured going over your already existing drawing would be redundant in this particular situation.
Anyways, as you may know, I'm not very comfortable drawing objects or scenery, so I had to take the time to study them a bit first. I'm pretty happy with what I discovered and hope you are too!
Step 1:
Draw a bumpy blob. This blob should probably be darker in color, either gray or brown. Don't feel shy to make it less symmetrical then my example. There are plenty of rock piles in all shapes and sizes!
Step 2:
Add smaller, multi-colored blobs inside of your original blob. These blobs do not all have to be the same shape or size, since most rocks in real life are rarely like that. Colors to recommend are grays, silvers, browns, blues, and creams. Really depends on the vibe you are going for.
Step 3:
Using a darker color then your base blob, fill in some "dead-space" and do some light line art. Then proceed to outline the rock pile as a whole.
Step 4:
Add details! Some examples I used here are: small air bubbles (I'm not really sure what they really are, but you know how rocks have teeny tiny holes on them), some scuff lines, and some moss/dirt. Don't think too hard during this step. Just throw some stuff around.
Step 5:
An extra step I didn't add would simply be to shade the rocks. Add a lighter area where the sun hits, and darker shades in area of shadows.
I hope this helps in whatever you needed it for! Hopefully it's not too late ^-^;