Why do some shale cliffs along the upper Amur River steam or catch fire?

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Topographic map of the Amur River basin and its drainage network, covering parts of northeastern China, eastern Russia, Mongolia, and North Korea. The map uses color shading to indicate elevation, shows major rivers and tributaries, surrounding mountain ranges, swamps, marshes, and lakes, and includes a locator inset of the region within Asia. Major cities, railways, and dams are also marked. — Amur River The Amur River basin and its drainage network.

Along the upper Amur River, specifically in the region below Yermakovo, the river flows past rocky precipices composed of complex layers of carbonaceous, clayey shales. These shales are characterized by their ability to ignite spontaneously, which causes them to steam continuously and occasionally burst into flames.1