The Cypress Hills Formation, a fluvial deposit of Oligocene age, caps the Cypress Hills in southeastern Alberta and adjacent parts of southwestern Saskatchewan. Study of primary directional structures, changes in grain size, and ancient stream gradients reveals a north-easterly direction of streamflow in the streams which transported and deposited the sediments.

The Cypress Hills Formation consists of varying amounts of sands and gravels. Near the Alberta-Saskatchewan border, gravel is the major constituent, whereas going downstream, toward the east, the formation becomes more sandy, and gravel is only a minor constituent.

The gravel component of the formation comprises mainly quarzites and chert. In addition, pebbles of trachyte and other porphyritic rocks are present. The sands consist mainly of quartz, with some feldspar and ferromagnesian minerals. The most likely source for the sediments of the Cypress Hills Formation are the Rocky Mountains in northwestern Montana. This source accounts, in addition to the quartzites and cherts, for the presence of porphyritic rocks.

South of the Cypress Hills upland, at much lower elevations, several isolated deposits of gravels and sands occur which are here grouped together in an informal stratigraphic unit, the "Redeposited Cypress Hills Formation." These deposits are lithologically similar to the Cypress Hills Formation, but their steep gradient to the south and the presence of directional structures indicating southerly flow, support the contention that they represent reworked material derived from the Cypress Hills Formation. They are overlain by Pleistocene deposits and are regarded as of middle Oligocene to late Pliocene age.

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