NorthShore

Greetings! I am wondering if someone can help me identify the types of rock/minerals that compose the attached sample? The sample was found along the NE shore of Lake Winnibigoshish in north-central Minnesota. The sample was found near the shoreline (steeply sloping shoreline); the shoreline is almost exclusively sandy, and the lake is relatively shallow in this area (6' deep about 100 yards from shore). The sample itself measures about 4"x6"x2". Please let me know if you have a good idea of what I am looking at, and/or if you can recommend a good resource to learn more about the sample/composition. I appreciate your support and consideration! Best, Matt