Nice work! It looks like from the beginning of the challenge, you had a good grasp of how you were meant to focus on shadow shapes rather than outlines. I imagine having worked through this in the texture analysis exercise in Lesson 2 helped establish this as a baseline, but I can see that as you started, there was a certain lack of nuance. You were very focused on achieving clear shadow shapes, but the shadow shapes themselves were pretty simplistic, and somewhat clumsy. Not bad by any stretch, but certainly with a lot of room for refinement.

I feel that as you continued to work through the challenge, this improved a fair bit. Your observation skills grew considerably, and you were able to identify smaller, more specific pieces of information without losing track of the texture as a whole. You picked up on less noticeable textural forms, and made better use of their contributions to the shadows when building up your density gradient as well.

As you progressed, I can see that you were experimenting with different ways to approach and think about the shadows - some worked out nicely, and others were somewhat less effective. Overall however, this helped you yield a better overall grasp of how to play with these kinds of textures.

One that actually stood out from the rest was your attempt at tackling hair - specifically because of how it kind of receded back into some of the clumsier, more simplistic textures from the start of the challenge, despite being much further in and surrounded by much more complex attempts. I expect this is because you had particular challenge with it, and so since you were struggling to work through it, you slipped back into some more simplistic manners of looking at your reference.

Ultimately there are two problems with it. First off, you're drawing the outlines of each "lock" of hair (the form you get when a bunch of strands of hair group together), instead of the shadows. Secondly, you end up with a visible jump from the black bar at the far left of the gradient into the white of some of your strands of hair. To put it simply, because this one was especially challenging, you slipped back into forgetting about key elements of the exercise, and fell back into old patterns. This will happen - you're still in a stage where you need to be at least somewhat conscious of how you're approaching drawing things, and if your brain gets distracted by other difficulties, you're liable to forget those key instructions and techniques if you aren't more aware of what you're doing. Over time you'll find those things becoming more instinctual, and you are getting there - but not completely there yet.

I put together a quick demonstration of how to tackle hair. Notice how when you're drawing the shadows, there are basically two things that change. First of all, instead of getting "hair spikes" that look like >, you instead get their inverse.

*Edit: Unfortunately because of the angle brackets above, part of my critique got lost, so I'll try and summarize it again.

The other point is that when working this way, your line weight will behave more naturally and predictably - the closer you get to the point, the thicker the shadows will get. When working with every spike being an independent shape (rather than drawing the gaps between the spikes, which inherently involves taking two and drawing them together), you're able to transition from heavy shadows to lighter line weight much more easily.

The rest of your work is looking great, and continues to demonstrate an improvement in your observational skills, and a greater complexity to your textures. With that, I'll happily mark this challenge as complete. Congratulations!