You can get to them.
There is no item in the game that is impossible to get to.
I just answered your question, but I figure that you will want to know how to get to the item too.
I am doing this from my phone so I can't circle things for you, but I will explain.
I think that the item that you are talking about is item Q(Silver Powder), as there are stairs leading up to it, which can be visible. Other possibilities are items M and N, but I find this unlikely as they can be accessed easily.
So, to get to the Silver Powder, follow these steps(Note that it requires Surf):
- Enter the inner area of Pinwheel Forest.
- Go straight up, towards Start B
- Once you cross the bridge, turn in to the path on the right. Note that in BW you must have first encountered Cobalion in Mistralton Cave to advance on this path.
- Keep going right until you reach the small steps, and go down them, then continue right.
- When you can't go right any further, take the steps leading down.
- Take the steps leading up to the right and go down.
- Go left, then up, and take the stairs down.
- Now use Surf to ride on the water, and keep going left until you reach land.
- Then, you simply walk up the steps and retrieve your Silver Powder.
You probably could have done it yourself with the map, but meh.
Hope I helped.