The age of the beds has also been a mystery as rocks and fossils representing Precambrian time to the Ice Age are found here.  Klassen (1979) gives a Pre Wisconsin age for them or certainly a time of deposition before the last major ice advance during the Pleistocene.  But much of the gravel shows reworking and deposition from an older unit.  Thus they are geologically very young deposits and in some of the natural outcrops, show evidence of very recent reworking by modern streams.

The diversity of materials suggests a great number of sources, the larger contribution coming from the erosion of the Rocky Mountains and associated uplifts during the Tertiary (approximately the last 65 million years).  Tertiary rivers, in their meandering fashion, delivered these agate bearing gravels to a number of places in Montana, North Dakota, South Dakota, Saskatchewan, and to a number of places in Manitoba.  Also there are indications that some of the north flowing streams from the Black Hills of South Dakota have contributed some Tertiary volcanics to the Souris gravels.  The source that is the farthest from Souris, but a significant contributing province is the area of the Hudson Bay between the Belcher Islands and mainland Quebec.

Pieces of graywacke (a dirty sandstone) originate from the Omaroluik Formation of Precambrian age in this area.  Many other rocks of Precambrian and Paleozoic age are also found in Souris, transported by the glaciers from points to the northwest, north and area.  Many other rocks of Precambrian and Paleozoic age are also found in Souris, transported by the glaciers from points to the northwest, north and northeast of Souris.  It seems that all points of the compass have contributed material to the Souris gravels except from the due south-east.