Complex sentence: Skirmshing around the craters diminished and it was thought that the Germans were concentrating on consolidating new positions but on 18 March, the Germans were able to surprise the British with five mines, which had been quietly dug through the clay layer above the chalk.
Split into simpler sentences: Skirmshing around the craters diminished and it was thought that the Germans were concentrating on consolidating new positions. On 18 March, the Germans were able to surprise the British with five mines, which had been quietly dug through the clay layer above the chalk.