Attn: Itcairborne
I dont know of any in the immediate Seal Beach Vacinity. I am not sure but I think bike riding may be allowed at gun grove park (either it is allowed or many people just do it anyway). Yes gum grove is super small, but fun nonetheless if you want something local. Other than that the closest place I know of to mountain bike is the fullerton loop.
Fullerton Loop - Fullerton - Bike Site - Mountain Bike Trails, Bicycle Information for - Bike Shops in - SoCalMtB.com
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