sentences:  Bedrock in this region is mainly Archean, with small areas of Lower Paleozoic and Upper Mesozoic sedimentary rocks along the western border.By late Wisconsinan times this bedrock had been covered by clayey glacial drift scoured and transported from sedimentary rocks of Manitoba.The bottomland is undissected and essentially flat, but imperceptibly declines from about 400 meters at the southern beaches of Lake Agassiz to 335 meters along the Rainy River.