We came to a 40-50 foot cliff that overlooked the Clark River Valley. Most of the cliff had alternating layers of pink and cream-colored sediments. These layers were from a few centimeters to half a meter thick. Both of these layers were composed of extremely fine-grained sediments. The pink layer was made of coarser-grained sediments. You could see alternating layers called laminations. The cream layer was so fine-grained it felt soft, like baby powder. If you touched it to your tongue, it stuck as it sucked the moisture right out. We knew it was the clay kaolinite when it did that. Near the bottom of the cliff, there were a few sandy, gravelly layers, each about 5 centimeters thick. Some of these layers had cross beds in them – the grains were arranged at an angle to the layer.
Note the pink (geologist pointing) and the cream units.
The fine-grained layers are called lacustrine deposits. Fine-grained sediments were suspended in a quiet lake. Like leaves that fall from trees, the sediments settled to the bottom of the still water. The pink-colored sediments, which may contain iron, were slightly larger and settled out first. The thin layers in the pink unit may be “varves.” Varves reflect seasonal changes in a lake. In the spring more water flows into the lake, carrying sediment that settles in the quiet environment. In the winter, perhaps the lake was covered with ice for part of the year. The water below the ice was all but motionless as the ice protected it from wind and storm and the finest material was able to settle to the lake floor. The varves are annual accumulations. The thicker cream-colored sediments also could only settle out when the lake was very still. We saw about 10 repetitions of this sequence.
Varves in pink unit.
There were also a few sandy, pebbly layers near the bottom of the cliff. These larger sediments could have come from a stream. To carry this size sediment, the water had to be moving; the cross beds also indicate that water was flowing, creating ripples or waves in the sediment. Perhaps when a stream flowed into the lake, it dropped its sediment load onto the lake deposits.
Gravelly sandy unit with cross beds.
One other feature was a few larger rocks embedded in the fine-grained layers. These may be dropstones that rode on icebergs out into the lake. When the ice melted, the stones fell into the soft deposits on the lake bottom.
All of the lake sediments were deposited about 15,000 to 12,000 years ago in Glacial Lake Missoula. Glacial Lake Missoula was an ice dammed lake. The repetition of the units suggests that the lake drained and refilled several times, perhaps when the dam lifted and broke. The coarsest material may reflect the draining or flooding. The cream and pink layers may reflect the quieter times when the lake was filling.
If a deposit like this were found on Mars, it would indicate the presence of a lake.