Field observations of downslope bentonite erosion from a geosynthetic clay liner (GCL) covered by only a black geomembrane are reported for a composite liner left exposed without a protective soil cover for much longer than recommended by the GCL manufacturer. A new nondestructive, light-transmission technique developed to investigate bentonite erosion features in the field is presented. A classification system is developed to describe the type of erosion features observed. Results from five field investigations at the Queen’s University Environmental Liner Test Site (QUELTS II) are reported to quantify the onset, progression, and severity of downslope erosion for one particular geotextile-encased, needle-punched GCL for exposure periods of between 7 weeks and 1.3 years. The first significant erosion feature (type “E”with bentonite loss narrower than 2.5 cm) was observed after 6 months of exposure. Irrecoverable erosion features (type “EE” with bentonite loss wider than 2.5 cm) were observed after 12 months of exposure. These findings highlight the need to follow the manufacturer’s recommendations for timely covering of a composite liner with soil following liner installation.