About twenty thousand years before, melting chunks of ice from the retreating Keewatin glacier had left the land mass of the northern Plains with thousands of deep, oddly shaped lakes called kettles, whose clear waters attracted settlers to their shores.
(melting chunks of ice from the retreating Keewatin glacier; had left; the land mass of the northern Plains; with thousands of deep, oddly shaped lakes; T:About twenty thousand years before)
melting chunks of ice from the retreating Keewatin glacier
had left
the land mass of the northern Plains with thousands of deep, oddly shaped lakes T:About twenty thousand years before