The mound-builders were the civilized, noble, urban race that inhabited North America long ago. The savage red men invaded and wiped them out, leaving no trace of their civilization except for the large earthen mounds common in North America. In the 19th century, the hypothesis of the "mound-builders" as a "forgotten race" served the purpose of explaining how the impressive mounds could be built without giving any credit to the natives. The story of these noble people cut down by barbarians also helped justify the massacre of Native Americans, purportedly ancestors of savages that had themselves wiped out the previous inhabitants.
The romantic notion of the mound builders is preserved in the 1834 poem "The Prairies," by William Cullen Bryant.
. . . Are they here-
The dead of other days!—and did the dust
Of these fair solitudes once stir with life
And burn with passion? Let the mighty mounds
That overlook the rivers, or that rise
In the dim forest crowded with old oaks,
Answer. A race that long has passed away,
Built them;-a disciplined and populous race
Heaped, with long toil, the earth, while yet the Greek
Was hewing the Pentelicus to forms
Of symmetry, and rearing on its rock
The glittering Parthenon. These ample fields
Nourished their harvests, here their herds were fed,
When haply by their stalls the bison lowed,
And bowed his maned shoulder to the yoke.
All day this desert murmured with their toils,
Till twilight blushed and lovers walked, and wooed
In a forgotten language, and old tunes,
From instruments of unremembered form,
Gave soft winds a voice. The red man came-
The roaming hunter tribes, warlike and fierce,
And the mound-builders vanished from the earth.
The solitude of centuries untold
Has settled where they dwelt. The prairie wolf
Hunts in their meadows, and his fresh dug den
Yawns by my path. The gopher mines the ground
Where stood their swarming cities. All is gone—
All-save the piles of earth that hold their bones—
The platforms where they worshipped unknown gods—
The barriers which they builded from the soil
To keep the foe at bay-till o'er the walls
The wild beleaguerers broke, and, one by one,
The strong holds of the plain were forced, and heaped
With corpses. . .
The idea of the mound-builders as a lost, civilized race destroyed by savages is also preserved in the Book of Mormon. Written when the mound-builder hypothesis was current, the Book of Mormon recounts the history of a white, godly race that settled North America long ago, only to be overrun by savages. Now that the mound-builder hypothesis is laughable and embarrassing, the Latter Day Saints are a little more reticent about their version of it.