There are several plains mentioned in the Book of Mormon which Mesotheorists are hard pressed to identify within the jungles of Mesoamerica. Plains however should be accounted for, and in the right places.
Because we know that the Sea that Divides the Land was ancient Lake Tonawanda, we know that it was thus called because it was a shallow inland freshwater sea encompassing Tonawanda Plain.
As it evaporated, the shores became flat “plains” which were well suited for battle and travel. In the south, this plain was called the Line Bountiful and covered a 1.5 day distance from the West Sea to the East Sea.
In the Jaredite Land Northward, the Plains of Agosh are adjacent to the north seashore:
12 And it came to pass that he fought with Lib, in which Lib did smite upon his arm that he was wounded; nevertheless, the army of Coriantumr did press forward upon Lib, that he fled to the borders upon the seashore.
13 And it came to pass that Coriantumr pursued him; and Lib gave battle unto him upon the seashore.
14 And it came to pass that Lib did smite the army of Coriantumr, that they fled again to the wilderness of Akish.
15 And it came to pass that Lib did pursue him until he came to the plains of Agosh. And Coriantumr had taken all the people with him as he fled before Lib in that quarter of the land whither he fled. (Ether 14)
In the Land Southward the south side plains were used for travel (Line Bountiful) and battle:
32 And now it was only the distance of a day and a half’s journey for a Nephite on the line Bountiful and the land Desolation, from the east to the west sea and thus the land of Nephi and the land of Zarahemla were nearly surrounded by water there being a small neck of land between the land northward and the land southward. (Alma 22)
20 And it came to pass they sent embassies to the army of the Lamanites, which protected the city of Mulek, to their leader, whose name was Jacob, desiring him that he would come out with his armies to meet them upon the plains between the two cities [Mulek & Bountiful]. But behold, Jacob, who was a Zoramite, would not come out with his army to meet them upon the plains. (Alma 52)
The two other plains are Nephihah and Heshlon. The Plains of Nephihah were adjacent to the City Nephihah which was along the Sea East in the southern quadrant, which is currently smooth, flat farm lands along the Genesee River Valley:
13 And it came to pass that the Nephites began the foundation of a city, and they called the name of the city Moroni; and it was by the east sea; and it was on the south by the line of the possessions of the Lamanites.
14 And they also began a foundation for a city between the city of Moroni and the city of Aaron, joining the borders of Aaron and Moroni; and they called the name of the city, or the land, Nephihah. (Alma 50)18 And it came to pass that when they had sent them away they pursued their march towards the land of Nephihah. And it came to pass that when they had come to the city of Nephihah, they did pitch their tents in the plains of Nephihah, which is near the city of Nephihah. (Alma 62)
Thus, the East Sea did not have rugged steep cliffs like you find in Mesoamerica, but rather flat plains. If said plains do not exist in the above mentioned locations, they aren’t Book of Mormon lands.
28 And it came to pass that Shared fought against him for the space of three days. And it came to pass that Coriantumr beat him, and did pursue him until he came to the plains of Heshlon. (Ether 13)