请帮忙理清一下这句话中的主句谓语动词,从句谓语动词以及非谓语动词的时态关系

请帮忙理清一下这句话中的主句谓语动词,从句谓语动词以及非谓语动词的时态关系
A group of people moving slowly down the Mackenzie River valley east of the Rockies into the general region of Southern Alberta, then eastward across the northern prairies reaching the wooded county around the upper Mississippi and the Western Great Lake, then in a southeastward movement following the Mississippi valley until some final settlement was reached in the Gulf states, would encounter a wide variety of physical environment.