What did I find this ammonite in? And need ID on calcite fossil.

[Lone Hunter]

Went to creek I frequent but went down to a part I haven't been to for awhile, had several good floods since.  Much to my surprise I rounded a corner and the bottom of the creek was pale yellow as far as I could see down. As you see in the picture there was shapes carved out by water, and it was very thick in places, but it was soft enough to break of pieces and it was grey underneath. So is this a layer of shale? If so why is it yellow? This may be natural in Eagle Ford and I've just never seen it in this form, just looking around I spotted this ammonite and just plucked it out. So now wondering if more fossils are deeper in this and how long it will take to erode down being so soft.  One interesting calcite rock I picked up not far away, it's a jumble of pieces of fossils but the center has these fine layers of sheets with delicate veining I'm thinking is something that may be recognized. 

[Lone Hunter]

Anyone?

[Plax]

The surface yellowing is probably just oxidization of the minerals in the rock. Fresh rock tends to be grayer than rock that has been out for a while. Sort of like iron oxidizing to rust. We see this in quarries and natural exposures here in NC. Quarried rock from much below the surface is usually gray and natural exposures of these same rock tend toward browns. Even quarried rock browns up with exposure in some instances.

[Lone Hunter]

Since this is shale, I think, would the same apply to it even though it's under water?

[Plax]

or just a biological coating os some sort.