World Without End by Ken Follett

From the central boss at the highest point of the vault, he followed a rib all the way down again to the matching pier on the opposite corner of the crossing.
    As he did so, something odd happened. His vision seemed momentarily to blur, and it looked as if the east side of the transept moved.
    There was a low rumbling sound, so deep it was almost inaudible, and a tremor underfoot, as if a tree had fallen nearby.
    The singing faltered.
    In the chancel, a crack appeared in the south wall, right next to the pier Merthin had been looking at.
    He found himself turning toward Caris. Out of the corner of his eye he saw masonry falling in the choir and the crossing. Then there was nothing but noise: women screaming, men shouting, and the deafening crash of huge stones hitting the floor. It lasted a long moment. When silence descended, Merthin found he was holding Caris, his left arm around her shoulders pressing her to him, his right arm protectively covering her head, his body interposed between her and the place where a part of the great church lay in ruins.
     
    It was obviously a miracle that no one died.
    The worst of the damage was in the south aisle of the chancel, which had been empty of people during the service. The congregation was not admitted to the chancel, and the clergy had all been in the central part, called the choir. Several monks had had narrow escapes, which only heightened the talk of miracles, and others had bad cuts and bruises from flying chips of stone. The congregation suffered no more than a few scratches. Evidently, they had all been supernaturally protected by St. Adolphus, whose bones were preserved under the high altar, and whose deeds included many instances of curing the sick and saving people from death. However, it was generally agreed that God had sent the people of Kingsbridge a warning. What he was warning them about was not yet clear.
    An hour later, four men were inspecting the damage. Brother Godwyn, the cousin of Caris, was the sacrist, responsible for the church and all its treasures. Under him as matricularius, in charge of building operations and repairs, was Brother Thomas, who had been Sir Thomas Langley ten years ago. The contract for cathedral maintenance was held by Elfric, a carpenter by training and a general builder by trade. And Merthin tagged along as Elfric's apprentice.
    The east end of the church was divided by pillars into four sections, called bays. The collapse had affected the two bays nearest the crossing. The stone vaulting over the south aisle was destroyed completely in the first bay and partially in the second. There were cracks in the tribune gallery, and stone mullions had fallen from the windows of the clerestory.
    Elfric said: 'Some weakness in the mortar allowed the vault to crumble, and that in turn caused the cracks at higher levels.'
    That did not sound right to Merthin, but he lacked an alternative explanation.
    Merthin hated his master. He had first been apprenticed to Elfric's father, Joachim, a builder of wide experience who had worked on churches and bridges in London and Paris. The old man had delighted in explaining to Merthin the lore of the masons - what they called their 'mysteries,' which were mostly arithmetical formulas for building, such as the ratio between the height of a building and the depth of its foundations. Merthin liked numbers and lapped up everything Joachim could teach him.
    Then Joachim died, and Elfric took over. Elfric believed the main thing an apprentice had to learn was obedience. Merthin found this difficult to accept, and Elfric punished him with short rations, thin clothing, and outdoor work in frosty weather. To make matters worse, Elfric's chubby daughter Griselda, the same age as Merthin, was always well fed and warmly dressed.
    Three years ago Elfric's wife had died, and he had married Alice, the older sister of Caris. People thought Alice was the prettier sister, and it was true that she had more regular features, but