# Relationship history for MSHA mine 1800688

Current hierarchy: **Merlo Charles J Jr → Frostburg Coal Company Inc → Shaft Md Prep Plant**

- Valid-time bounds: inclusive, date granularity
- Null end: open-ended in the upstream history row; not independent proof of present control
- Transaction-time observation: 2026-10-13T11:19:05.459773+00:00

## Valid-time edges

- **operator_operates_mine:** Frostburg Coal Company Inc → Shaft Md Prep Plant; 1981-08-01 to current/open; source msha-controller-operator-history
- **controller_controls_mine:** Merlo Charles J Jr → Shaft Md Prep Plant; 1981-08-01 to current/open; source msha-controller-operator-history
- **controller_controls_operator:** Merlo Charles J Jr → Frostburg Coal Company Inc; 1981-08-01 to current/open; source msha-controller-operator-history