BH250-143a
Title
BH250-143a
Subject
Mill Street Conglomerate
Description
Mill Street Conglomerate
Geologic significance: Great Unconformity
The Mill Street Conglomerate is a coarse, clast-supported sedimentary rock exposed near Taylors Falls. It consists predominantly of subrounded to well-rounded basalt cobbles and smaller basalt fragments. The spaces between the larger clasts are filled with granule- to sand-sized basalt debris and finer sediment. The poor sorting and coarse grain size indicate deposition in a high-energy environment capable of transporting and reworking cobbles and boulders.
The basalt clasts were eroded from the much older volcanic rocks of the Midcontinent Rift. Their rounded shapes record repeated abrasion, probably along a Cambrian shoreline as waves reworked the exposed basalt landscape. Variations in clast size and matrix abundance reflect changing water energy and local relief along that ancient coast.
At Taylors Falls, the conglomerate rests unconformably on approximately 1.1-billion-year-old basalt. This contact is part of the Great Unconformity and represents hundreds of millions of years of weathering, erosion, and missing geologic time before younger sediment accumulated. BH250-143a therefore preserves both the erosion of the ancient volcanic landscape and the return of sedimentation during the Paleozoic.
Geologic significance: Great Unconformity
The Mill Street Conglomerate is a coarse, clast-supported sedimentary rock exposed near Taylors Falls. It consists predominantly of subrounded to well-rounded basalt cobbles and smaller basalt fragments. The spaces between the larger clasts are filled with granule- to sand-sized basalt debris and finer sediment. The poor sorting and coarse grain size indicate deposition in a high-energy environment capable of transporting and reworking cobbles and boulders.
The basalt clasts were eroded from the much older volcanic rocks of the Midcontinent Rift. Their rounded shapes record repeated abrasion, probably along a Cambrian shoreline as waves reworked the exposed basalt landscape. Variations in clast size and matrix abundance reflect changing water energy and local relief along that ancient coast.
At Taylors Falls, the conglomerate rests unconformably on approximately 1.1-billion-year-old basalt. This contact is part of the Great Unconformity and represents hundreds of millions of years of weathering, erosion, and missing geologic time before younger sediment accumulated. BH250-143a therefore preserves both the erosion of the ancient volcanic landscape and the return of sedimentation during the Paleozoic.
Coverage
Taylors Falls Minnesota
GPS Coordinates: 45°23'53.03"N 92°39'34.41"W
GPS Coordinates: 45°23'53.03"N 92°39'34.41"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-143a. Housed at Carleton College in Minnesota.
Relation
Collection
Citation
Bereket Haileab, “BH250-143a,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/390.