Birch Mountains kimberlite field shales interbedded with kimberlite at some of the diatremes are consistent with a Late Cretaceous age (Late Albian to Maastrichtian). The volcanic rocks of the BM field consist primarily of crater-facies pyroclastic kimberlite and resedimented kimberlite. They include massive layers, as well as bedded and graded layers of coarse lapilli alternating with layers of finer tuff. Lapilli and olivine phenocrysts are set in a grey-green matrix of serpentine, carbonate minerals (calcite, dolomite and magnesite) and clay minerals. Phlogopite, oxide minerals (ilmenite, perovskite and spinel), apatite and pyrite are also present. Diamonds and microdiamonds have been recovered from samples taken from
Lake Ellen Kimberlite till. Magnetic survey work done in 1956 depicts an elliptical positive anomaly long in an east–west direction and wide, which probably defines the limits of the pipe. The kimberlite is intruded into volcanic rocks of the Proterozoic Hemlock Formation. The exposed material is grayish green to reddish, iron stained highly weathered and consists of disaggregated rubbly fragments up to . It is generally accepted that the pipe was emplaced about 180 million years ago. The lake Ellen Kimberlite is popular with rock hounds as the classic indicator minerals (pyrope, magnesian ilmenite and chrome diopside) for kimberlite are easily found in