For years, I believed the demise of the Jaredites at Hill Ramah (Ether 15:29) was connected to the abandonment of La Venta ca. 400 BC. This was based on a naive assumption about the relationship between the Jaredites and Olmec civilization. This incorrect correlation caused a chronological problem. The Limhi explorers left the city of Nephi, traveled to Hill Ramah, and returned with artifacts ca. 121 BC (Mosiah 8:8-9). These 43 intrepid explorers found bones of men and beasts littering the ground. Skeletal ramains completely decompose in as few as 10 - 15 years in damp, warm, shady, aerobic conditions. It is virtually inconceivable that osteological material would still be visible on the surface of the tropical or temperate earth after more than 100 years elapsed time. Jaredite bones are such an important motif in the Nephite record we find them mentioned in Omni 1:22, Mosiah 21:26-27, and Alma 22:30 in addition to the Mosiah 8 reference cited above.
Coriantumr survived the annihilation at Hill Ramah (Ether 15:31-32), joined the people of Zarahemla (the Mulekites) for 9 moons, carved his history on a stone stela that the Mulekites could not read, (Omni 1:20-21) and died among these Mulekite strangers (Ether 13:21).
Mosiah I became king over the combined Nephite/Mulekite nations before Zeniff's reign which began ca. 200 BC. (Mosiah 9:1). So, a date of 210 BC for the Jaredite demise at Hill Ramah may be in the ballpark. The Book of Mormon explicitly says the bones Limhi's explorers found were dry (Mosiah 21:26).
So, 210 BC for the final Jaredite battles may work, but 400 BC is entirely out of the question.
What caused the Olmec to abandon their capital, La Venta, ca. 400 BC? We now know it was due to an avulsion, a natural eastward re-routing of the big Mezcalapa-Grijalva River. See the work done on the Río Bari and Pajonal paleochannels by Mary Pohl, William F. Rust, Kevin Pope, and Robert Sharer.
Without its life-sustaining river, La Venta ceased to be a viable urban community. And where did the majority of its former inhabitants migrate to? The Tuxtla Mountains, Tres Zapotes, where they founded what we now call the Epi-Olmec culture. Since Hill Ramah is likely in the Tuxtlas, the Book of Mormon now seems more plausible, both spatially and temporally.
