An alternation is generally a cyclical sequence of repeating lithologies.....usually coarse sand units with intervening finer grained shale units. In the case of the Upper Angwa there are also coals present. Each cycle will consist of a basal sandstone, then a shale and then a coal at top. The cycle then repeats. What you're seeing is the result of laterally migrating fluvial channels (sands) with associated overbank shale and coal deposits.
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