Question

one log forms a natural bridge the log has a span of 40 feet which sentence combines the two sentences using a present participial phrase

Answer 1

One log spanning 40 feet forms a natural bridge.
The phrase spanning 40 feet starts with a present participle spanning, and it is a phrase (given that there is no finite verb), which means that it is a participial phrase. 

Answer 2

Forming a natural bridge, one log has a span of 40 feet. (Gradpoint)