The lake is mostly underlain by a mix of volcanic and plutonic igneous rock. Most of it is the same Archean basalt greenstone belt that predominates in the Yellowknife area. Running through the central portion from north northeast, under the heritage centre and Legislative Assembly building, to south southwest is a narrow belt of dacite flanked by two gabbro sills (one of which forms the largest island), with some more dacite under the visitors centre and the southern of the two points of the western peninsula. Small diabase dykes run through various portions of the bedrock, with the largest forming the elevated area at the lakes southeast corner. Quartz intrusions are visible in some areas.
Two local faults run across the south end of the lake. The stream from Robinson's Pond runs along the Pud Fault, which continues across the lakebed to McNiven Beach. At the south end is the larger Kam Fault, which when it was active had the same potential for earthquakes as the San Andreas Fault in the U.S. state of California. It divides the basalt from an area of lighter granite and granodiorite between the lake and the hospital.
There are also visible signs of the lakes glacial origins. Just south of Somba Ke Park on the east shore, the rock has striations and scour marks in the northeast-to-southwest direction of the glaciers' advance. The fine sand on the lake bed is also glacial residue. In some areas around the lake edge, the ground is underlain by permafrost at an average depth of .
Where soil exists around the lake, it is mostly silt and clay as opposed to the sand of neighboring New Town. It does not support buildings well, nor does it give way to bedrock at a uniform depth. The military building and visitor's centre both encountered construction difficulties due to this.