triviaink.com

Sorry 'bout that. Fixed now. What's a gandy?

con·cur·rent   /kənˈkɜrənt, -ˈkʌr-/ Show Spelled[kuhn-kur-uhnt, -kuhr-] Show IPA –adjective 1. occurring or existing simultaneously or side by side: concurrent attacks by land, sea, and air. 2. acting in conjunction; cooperating: the concurrent efforts of several legislators to pass the new law. 3. ...

I enjoyed the question and the search. I had to use some "strategy" to get the answer. HOWEVER "concurrent" is not the right word. The three series overlapped, but were not really concurrent.

Charlene, you question has MANY possible answers. A majority of prehistoric Native American peoples built earthen mounds for burial and other purposes.