Castle Lake Limnological Research Station

Castle Lake and the Limnological Research Laboratory is located southwest of Mt. Shasta City in the Shasta/Trinity National Forest of northern California.

Castle Lake was formed during the Pleistocene Era over 10,000 years ago when much of North America was covered with glaciers. A glacier carved out the basin in which Castle Lake lies today. Castle Lake is a typical glacier 'cirque' lake. Cirque is a French word meaning semicircle or amphitheater. Cirque lakes are the deepest against the steep rock wall--also known as the cirque face-- where the glacier did most of the eroding. The northeastern shore of the lake is a terminal moraine of boulders and gravel that form a natural dam.

The rock composition within the Castle Lake basin is generally very poor in the essential plant nutrients nitrogen and phosphorus, which partially explains the great clarity of Castle Lake and why it is only moderately productive.

Against the cirque face of Castle Lake, the waters are up to 110 feet deep (35 M). At the other end of the lake is the outlet where the lake's depth ranges from 10 to 15 feet (3 to 5 M).

Bedrock Geology

The Castle Lake basin primarily contains dense mafic and ultramafic rocks that solidified from magma underneath the oceanic plate 400 to 500 million years ago. Usually, as dense oceanic plates move towards lighter continental plates, they sink and re-melt. Surprisingly, these oceanic rocks were moved on top of the continental plate and were preserved. Oceanic rocks that are thrust on top of continental rocks instead of subducted are called ophiolites. After the ophiolite was in place on the continent, magma intruded into these oceanic rocks 250 to 300 million years ago forming diorite and granodiorite plutons. These plutons are more resistant to erosion than the oceanic gabbros. This explains why the diorite and granodiorite rocks are found on the ridges surrounding the Castle Lake basin.