Topanga State Beach, California

[Melbums]

I'm brand new here. My kids and I were at Topanga State Beach today and though we've been thousands of times before, this was the first time we found fossils! We found lots of rocks packed with small shell fossils, but then this one was much larger. The photos are all from the same one rock, different angles and sides. Any idea what it is (or maybe it's just a large shell)? Any idea how old it is?

Edited November 23, 2020 by Melbums
added two more photos

[MrR]

I live 30 minutes from Topanga beach. I do know that there are places in Old Topanga where people used to go to find fossils, but it is now off limits. The word is that Caltrans, or some other road entity, made the property owners post "No Trespassing" signs where the hill is eroding. So yes, there are areas in our locality that have fossiliferous formations that might produce such a thing. I wouldn't be surprised if one of the many large storms that got Topanga Creek running brought that rock from inland. Then again, who knows? Nice find though. I'll be interested to see what kind of ID's you get. Good luck.

 

[Darktooth]

Welcome to the forum!

[Plax]

I think I see a Trigonia in there but perhaps the age of the rocks precludes this guess.

[Fossildude19]

It may be extremely difficult to get any ID on these shells, as most are in cross section.  

 

[jhw]

I've done a fair amount of collecting in old topanga canyon. Strangely there's not a whole lot of info. out there on the subject. It is a fairly rich site though especially for marine mollusks. I think they've identified some 200+ species. The area is considered to be middle miocene. As said your rock shows crosscuts of assorted shells. I think the one might be a crosscut of one of the spinier gastropods. Here's a short list from our natural history museum here in L.A. https://research.nhm.org/ip/topanga-formation/ and a random shot from the forum sometime back. Whole Turritellas are fairly common and fun to find. Here's my best find as far as those go.